Climate Change and...

Annotated Bibliography

Carbon Dynamics

Global Carbon Cycle

Alexis, M., Rasse, D., Rumpel, C., Bardoux, G., Pechot, N., Schmalzer, P., Drake, B., Mariotti, A. (2007). Fire impact on C and N losses and charcoal production in a scrub oak ecosystem. Biogeochemistry 82 (2): 201-216

ABSTRACT: Fire profoundly modifies the terrestrial C cycle of about 40% of the Earth’s land surface. The immediate effect of fire is that of a net loss of C as CO2 gas and soot particles to the atmosphere. Nevertheless, a proportion of the ecosystem biomass is converted into charcoal, which contains highly recalcitrant molecular structures that contribute to long-term C storage. The present study aimed to assess simultaneously losses to the atmosphere and charcoal production rates of C and N compounds as a result of prescription fire in a Florida scrub-oak ecosystem. Pre-fire and post-fire charred and unburned organic matter stocks were determined for vegetation leaves and stems, litter and soil in 20 sub-plots installed in a 30-ha area that was subjected to prescribed fire. Concentrations of C and N were determined, and fluxes among pools and to the atmosphere were derived from these measurements. Soil C and N stocks were unchanged by the fire. Post-fire standing dead biomass contained 30% and 12% of pre-fire vegetation C and N stocks, respectively. In litter, post-fire stocks contained 64% and 83% of pre-fire C and N stocks, respectively. Most of the difference in relative losses between vegetation and litter could be attributed to substantial litter fall of charred and unburned leaves during the fire event. Indeed, an estimated 21% of pre-fire vegetation leaf C was found in the post-fire litter, while the remaining 79% was lost to the atmosphere. About 3/4 of the fire-induced leaf litter fall was in the form of unburned tissue and the remainder was charcoal, which amounted to 5% of pre-fire leaf C stocks. Charcoal production ranged between 4% and 6% of the fire-affected biomass, i.e. the sum of charcoal production and atmospheric losses. This value is below the range of literature values for the transformation of plant tissue into stable soil organic matter through humification processes, which suggests that fire generates a smaller quantity of stable organic C than humification processes over decades and potentially centuries.

Alpert, P., Niyogi, D., Pielke, Sr., R.A., Eastman, J.L., Xue, Y.K., Raman, S. (2006). Evidence for carbon dioxide and moisture interactions from the leaf cell up to global scales: Perspective on human-caused climate change. Global and Planetary Change 54 (1-2): 202-208

ABSTRACT: It is of utmost interest to further understand the mechanisms behind the potential interactions or synergies between the greenhouse gases (GHG) forcing(s), particularly as represented by CO2 , and water processes and through different climatic scales down to the leaf scale. Toward this goal, the factor separation methodology introduced by Stein and Alpert [Stein U. and Alpert, P. 1993. Factor separation in numerical simulations, J. Atmos. Sci., 50, 2107–2115.] that allows an explicit separation of atmospheric synergies among different factors, is employed. Three independent experiments carried out recently by the present authors, are reported here, all strongly suggest the existence of a significant CO2 –water synergy in all the involved scales. The experiments employed a very wide range of up-to-date atmospheric models that complement the physics currently introduced in most Global Circulation Models (GCMs) for global climate change prediction.

Three modeling experiments that go from the small/micro scale (leaf scale and soil moisture) to mesoscale (land-use change and CO2 effects ) and to global scale (greenhouse gases and cloudiness) all show that synergies between water and CO2 are essential in predicting carbon assimilation, minimum daily temperature and the global Earth temperature, respectively. The study also highlights the importance of including the physics associated with carbon–water synergy which is mostly unresolved in global climate models suggesting that significant carbon–water interactions are not incorporated or at least well parameterized in current climate models. Hence, there is a need for integrative climate models. As shown in earlier studies, the climate involves physical, chemical and biological processes. To only include a subset of these processes limits the skill of local, regional and global models to simulate the real climate system.

In addition, our results provide explicit determination of the direct and the interactive effect of the CO2 response on the terrestrial biosphere response. There is also an implicit scale interactive effect that can be deduced from the multiscale effects discussed in the three examples. Processes at each scale-leaf, regional and global will all synergistically contribute to increase the feedbacks — which can decrease or increase the overall system's uncertainty depending on specific case/setup and needs to be examined in future coupled, multiscale studies.

Amundson, R. (2001). The carbon budget of soils. Annual Review of Earth and Planetary Science 29: 535-562

ABSTRACT: The global soil C reservoir, 1500 Gt of C (1 Gt = 1012 kg of C), is dynamic on decadal time scales and is sensitive to climate and human disturbance. At present, as a result of land use, soil C is a source of atmospheric CO2 in the tropics and possibly part of a sink in northern latitudes. Here I review the processes responsible for maintaining the global soil C reservoir and what is known about how it responds to direct and indirect human perturbations.

Armentano, T.V., Menges, E.S. (1986). Patterns of changes in the carbon balance of organic soil-wetlands of the temperate zone. Journal of Ecology 74 (3): p.755-774

ABSTRACT: (1) Organic soil-wetlands, particularly those in the temperate zone, under natural conditions, are net carbon sinks and hence are important links in the global cycling of carbon dioxide and other atmospheric gases. Human alteration of wetlands has brought about shifts in the balance of carbon movement between the wetlands and the atmosphere. Because previous analyses have not fully considered these shifts, disturbance of carbon storage in organic soil-wetlands of the temperate zone has been analysed for the last two centuries and considered in relation to other sources of atmospheric CO2 from the biosphere. (2) Storage before recent disturbance is estimated as 57 to 83 Mt of carbon per year, over two-thirds of this in boreal peatlands. The total storage rate, lower than previous estimates, reflects accumulation rates of carbon of only 0.20 t ha-1 yr-1 and less in the boreal zone where 90% of temperate organic soils are found. (3) Widespread drainage of organic soil-wetlands for agriculture has significantly altered the carbon balance. A computer model was used to track the consequent changes in the carbon balance of nine wetland regions. Drainage reduced or eliminated net carbon sinks, converting some wetlands into net carbon sources. Different regions thus can function as smaller carbon sinks, or as sources, depending on the extent of drainage. In either case a shift in carbon balance can be quantified. (4) The net carbon sink in Finland and the U.S.S.R. has been reduced by 21-33%, in Western European wetlands by nearly 50%, and in Central Europe the sink has been completely lost. Overall, by 1900 the temperate zone sink was reduced 28-38% by agricultural drainage alone. (5) By 1980 the total annual shift in carbon balance attributable to agricultural drainage was 63-85 Mt of carbon, 38% in Finland and U.S.S.R. wetlands, and 37% in Europe. Twenty-five percent of the shift occurred in North American wetlands south of the boreal zone. No apparent change occurred in boreal Canada and Alaskan wetlands. (6) Peat combustion for fuel released 32-39 Mt of carbon annually, nearly all in the U.S.S.R. A total of 590-700 Mt of carbon has been released from peat combustion since 1795, compared with a release of 4140-5600 Mt from agricultural drainage. (7) The aggregate shift in the carbon balance of temperate zone wetlands, when added to a far smaller shift from tropical wetlands, equalled 150-185 Mt of carbon in 1980 and 5711-6480 Mt since 1795. Despite occupying an area equivalent to only 2% of the world's tropical forest, the wetlands have experienced an annual shift in carbon balance 15-18% as great. Wetlands thus are seen on an area-specific basis to be concentrated sources of atmospheric CO2 which respond differently from those ecosystems assumed to have no net carbon exchange before disturbance.

Asner, G. P., Seastedt, T. R., Townsend, A. R. (1997). The decoupling of terrestrial carbon and nitrogen cycles. BioScience 47 (4): 226-234

FIRST PARAGRAPH: Global cycles of carbon (C) and nitrogen (N) are coupled through processes of terrestrial and marine biomass accumulation, decomposition, and storage. Alfred Redfield (1958) proposed that nearly constant carbon-to-nutrient ratios in marine phytoplankton and bacteria required that changes in one biogeochemical element be matched by changes in other essential elements. The "Redfield ratio" approach has proven valuable in understanding not only marine biogeochemistry (Broecker et al. 1979, Howarth 1988), but also carbon and nutrient cycles on land (Bolin and Cook 1983, Melillo and Gosz 1983, Reiners 1986, Rosswall 1981, Vitousek 1982). Both plant species diversity and their ability to produce varying amounts of structural material cause greater variation in carbon to nutrient ratios within terrestrial biomass than is found in the ocean (Vitousek et al. 1988). Yet nutrient limitation of net terrestrial primary production is still commonplace: In particular, the photosynthetic requirement for nitrogen, coupled with relatively low levels of available nitrogen in many terrestrial ecosystems, causes carbon uptake and storage on land to be tightly regulated by the nitrogen cycle (Vitousek and Howarth 1991).

Bala, G., Caldeira, K., Mirin, A., Wickett, M., Delire, C. (2005). Multicentury changes to the global climate and carbon cycle: Results from a coupled climate and carbon cycle model. Journal of Climate 18 (21): 4531-4544

ABSTRACT: A coupled climate and carbon (CO2 ) cycle model is used to investigate the global climate and carbon cycle changes out to the year 2300 that would occur if CO2 emissions from all the currently estimated fossil fuel resources were released to the atmosphere. By the year 2300, the global climate warms by about 8 K and atmospheric CO2 reaches 1423 ppmv. The warming is higher than anticipated because the sensitivity to radiative forcing increases as the simulation progresses. In this simulation, the rate of emissions peaks at over 30 Pg C yr-1 early in the twenty-second century. Even at the year 2300, nearly 50% of cumulative emissions remain in the atmosphere. Both soils and living biomass are net carbon sinks throughout the simulation. Despite having relatively low climate sensitivity and strong carbon uptake by the land biosphere, these model projections suggest severe long-term consequences for global climate if all the fossil fuel carbon is ultimately released into the atmosphere.

Bellamy, P. H., Loveland, P. J., Bradley, R. I., Lark, R. M., Kirk, G. J. D. (2005). Carbon losses from all soils across England and Wales 1978-2003. Nature 437 (7056): 245-248

ABSTRACT: More than twice as much carbon is held in soils as in vegetation or the atmosphere, and changes in soil carbon content can have a large effect on the global carbon budget. The possibility that climate change is being reinforced by increased carbon dioxide emissions from soils owing to rising temperature is the subject of a continuing debate. But evidence for the suggested feedback mechanism has to date come solely from small-scale laboratory and field experiments and modelling studies. Here we use data from the National Soil Inventory of England and Wales obtained between 1978 and 2003 to show that carbon was lost from soils across England and Wales over the survey period at a mean rate of 0.6% yr-1 (relative to the existing soil carbon content). We find that the relative rate of carbon loss increased with soil carbon content and was more than 2% yr-1 in soils with carbon contents greater than 100 g kg-1 . The relationship between rate of carbon loss and carbon content is irrespective of land use, suggesting a link to climate change. Our findings indicate that losses of soil carbon in England and Wales—and by inference in other temperate regions—are likely to have been offsetting absorption of carbon by terrestrial sinks.

Bird, M.I., Chivas, A.R., Head, J. (1996). A latitudinal gradient in carbon turnover in forest soils. Nature 381 (May 9)

ABSTRACT: Attempts to model the global carbon cycle, and anthropogenic modifications to carbon flow between the atmospheric, oceanic and terrestrial carbon reservoirs, commonly rely on values assumed for the13 C/12 C ratio and 'bomb-spike'14 C signature of carbon in each reservoir1,2 . A large proportion of the carbon in the terrestrial biosphere resides in the soil organic carbon (SOC) pool3 , most of which is derived from plants that assimilate carbon via the C3 photosynthetic pathway4 . Here we report measurements of the13 C and14 C signatures of particulate organic carbon from surface soils of C3 biomes from a global distribution of low-altitude, non-water-stressed locations. We find that there is currently a latitudinal gradient in the signature, with low-latitude soils being relatively depleted in13 C. The14 C signatures indicate that today's gradient is due to a latitudinal gradient in the residence time of the soil organic carbon, coupled with anthropogenic modifications to the13 C/12 C ratio of atmospheric CO2 (for example by fossil-fuel burning5 ). The long residence times (tens of years) of particulate organic carbon from high-latitude soils provide empirical evidence that if fluxes of carbon from vegetation to the soil increase, these soils have the capacity to act as a carbon sink on decadal timescales.

Bohn, H. L. (1982). Estimate of organic carbon in world soils: II. Soil Science Society Of America JournalSoil Sci So 46: 1118-1119

ABSTRACT: Soil organic carbon (C) is the largest carbon reservoir at the earth's surface, but its mass is the least certain. The completed FAO Soil Map of the World yielded these estimates: 22 x 1014 kg organic C in global soils, made up of 18 x 1014 kg C in mineral soils and 4 x 1014 kg C in the surface meter of peatlands.

Bondeau, A., Smith, P.C., Zaehle, S., Schaphoff, S., Lucht, W., Cramer, W., Gerten, D., Lotze-Campen, H., Müller, C., Reichstein, M., Smith, B. (2007). Modelling the role of agriculture for the 20th century global terrestrial carbon balance. Global Change Biology 13 (3): 679-706

ABSTRACT: In order to better assess the role of agriculture within the global climate-vegetation system, we present a model of the managed planetary land surface, Lund–Potsdam–Jena managed Land (LPJmL), which simulates biophysical and biogeochemical processes as well as productivity and yield of the most important crops worldwide, using a concept of crop functional types (CFTs). Based on the LPJ-Dynamic Global Vegetation Model, LPJmL simulates the transient changes in carbon and water cycles due to land use, the specific phenology and seasonal CO2 fluxes of agricultural-dominated areas, and the production of crops and grazing land. It uses 13 CFTs (11 arable crops and two managed grass types), with specific parameterizations of phenology connected to leaf area development. Carbon is allocated daily towards four carbon pools, one being the yield-bearing storage organs. Management (irrigation, treatment of residues, intercropping) can be considered in order to capture their effect on productivity, on soil organic carbon and on carbon extracted from the ecosystem. For transient simulations for the 20th century, a global historical land use data set was developed, providing the annual cover fraction of the 13 CFTs, rain-fed and/or irrigated, within 0.5° grid cells for the period 1901–2000, using published data on land use, crop distributions and irrigated areas. Several key results are compared with observations. The simulated spatial distribution of sowing dates for temperate cereals is comparable with the reported crop calendars. The simulated seasonal canopy development agrees better with satellite observations when actual cropland distribution is taken into account. Simulated yields for temperate cereals and maize compare well with FAO statistics. Monthly carbon fluxes measured at three agricultural sites also compare well with simulations. Global simulations indicate a ~24% (respectively ~10%) reduction in global vegetation (respectively soil) carbon due to agriculture, and 6–9 Pg C of yearly harvested biomass in the 1990s. In contrast to simulations of the potential natural vegetation showing the land biosphere to be an increasing carbon sink during the 20th century, LPJmL simulates a net carbon source until the 1970s (due to land use), and a small sink (mostly due to changing climate and CO2 ) after 1970. This is comparable with earlier LPJ simulations using a more simple land use scheme, and within the uncertainty range of estimates in the 1980s and 1990s. The fluxes attributed to land use change compare well with Houghton's estimates on the land use related fluxes until the 1970s, but then they begin to diverge, probably due to the different rates of deforestation considered. The simulated impacts of agriculture on the global water cycle for the 1990s are ~5% (respectively ~20%) reduction in transpiration (respectively interception), and ~44% increase in evaporation. Global runoff, which includes a simple irrigation scheme, is practically not affected.

Brown, S., Hall, C.A.S., Knabe, W., Raich, J., Trexler, M.C., Woomer, P. (1993). Tropical forests: their past, present, and potential future role in the terrestrial carbon budget. Water, Air, and Soil Pollution 70 (1-4): 71-94

ABSTRACT: In this paper we review results of research to summarize the state-of-knowledge of the past, present, and potential future roles of tropical forests in the global C cycle. In the pre-industrial period (ca. 1850), the flux from changes in tropical land use amounted to a small C source of about 0.06 Pg yr–1 . By 1990, the C source had increased to 1.7 ± 0.5 Pg yr–1 . The C pools in forest vegetation and soils in 1990 was estimated to be 159 Pg and 216 Pg, respectively. No concrete evidence is available for predicting how tropical forest ecosystems are likely to respond to CO2 enrichment and/or climate change. However, C sources from continuing deforestation are likely to overwhelm any change in C fluxes unless land management efforts become more aggressive. Future changes in land use under a business as usual scenario could release 41–77 Pg C over the next 60 yr. Carbon fluxes from losses in tropical forests may be lessened by aggressively pursued agricultural and forestry measures. These measures could reduce the magnitude of the tropical C source by 50 Pg by the year 2050. Policies to mitigate C losses must be multiple and concurrent, including reform of forestry, land tenure, and agricultural policies, forest protection, promotion of on-farm forestry, and establishment of plantations on non-forested lands. Policies should support improved agricultural productivity, especially replacing non-traditional slash-and-burn agriculture with more sustainable and appropriate approaches.

Canadell, Josep G., Pataki, Diane E., Gifford, Roger, Houghton, Richard A., Luo, Yiqi, Raupach, Michael R., Smith, Pete, Steffen, Will, Anonymous (2007). Saturation of the terrestrial carbon sink. 24: 59-78

FIRST PARAGRAPH: There is strong evidence that the terrestrial biosphere has acted as a net carbon (C) sink over the last two and half decades. Its strength is highly variable year-to-year ranging from 0.3 to 5.0 Pg C yr–1 ; an amount of significant magnitude compared to the emission of about 7 Pg C yr–1 from fossil fuel burning (Prentice et al. 2001; Schimel et al. 2001; Sabine et al. 2004). Uncertainties associated with C emissions from land-use change are large. On average, the terrestrial C sink is responsible for removing from the atmosphere approximately one third of the CO2 emitted from fossil fuel combustion, thereby slowing the build-up of atmospheric CO2 . The ocean sink is of similar magnitude (Sabine et al. 2004). Given the international efforts to stabilize atmospheric CO2 concentration and climate (i.e., Kyoto Protocol, C trading markets), the terrestrial C sink can be viewed as a subsidy to our global economy worth trillions of dollars. Because many aspects of the terrestrial C sink are amenable to purposeful management, its basis and dynamics need to be well understood.

Cao,M., Zhang,Q., Shugart,H. H. (2001). Dynamic responses of African ecosystem carbon cycling to climate change. Climate Research 17 (2): 183-193

ABSTRACT: Global climate change has been modifying ecosystem carbon cycling, which has produced feedbacks on climate by affecting the concentration of atmospheric CO2 . The importance of biospheric CO2 uptake or release to climate change has generated great interest in quantifying the dynamic responses of terrestrial ecosystem carbon cycling to climate change. However, less attention has been given to Africa, although it accounts for about one-fifth of the global net primary production and is one of the regions that have the greatest climate change. Here we use a biogeochemical model to simulate the dynamic variations in the carbon fluxes and stocks of African ecosystems caused by changes in climate and atmospheric CO2 from 1901 and 1995. We estimate that climate change reduces plant production and soil carbon stocks and causes net CO2 release, but the fertilization effect of increasing atmospheric CO2 on photosynthesis reverses the reduction and leads to carbon accumulation in vegetation. Therefore, the combined effect of climate change and increasing atmospheric CO2 causes net CO2 uptake, particularly in central Africa. The mean rate of the carbon sequestration in the period 1981-1995 is calculated to be 0.34 Gt C yr-1 . Nevertheless, Africa is not necessarily a significant carbon sink, because a large part of the carbon sequestration is offset by the carbon release arising from land use changes.

Cao, M. K., Prince, S. D., Shugart, H. H. (2002). Increasing terrestrial carbon uptake from the 1980s to the 1990s with changes in climate and atmospheric CO2 . Global Biogeochemical Cycles 16 (4): 1069, doi:10.1029/2001GB001553

ABSTRACT: Atmospheric measurements suggest that the terrestrial carbon sink increased from the 1980s to the 1990s, but the causes of the increase are not well understood yet. In this study we investigated the responses of global net primary production in (NPP), soil heterotrophic respiration (HR), and net ecosystem production (NEP) to atmospheric CO2 increases and climate variation in the period 1981-1998. Our results show that the unusual climate variability in this period associated with strong warming and El Niño caused high interannual variations in terrestrial ecosystem carbon fluxes; nevertheless NPP and NEP increased consistently from the 1980s to 1990s. Annual global NPP and HR varied with a similar magnitude and contributed about equally to the interannual variations in NEP. Global NEP fluctuated between -0.64 and 1.68 Gt C yr-1 with a mean value of 0.62 Gt C yr-1 , its decadal means increased from 0.23 Gt C yr-1 in the 1980s to 1.10 Gt C yr-1 in the 1990s. Total and vegetation carbon storage increased with increases of NPP, but soil carbon storage declined because of higher HR than litter inputs. The tropics (20°N-20°S) had higher mean NEP than the north (>20°N), however, they contributed similarly to the global NEP increase from the 1980s and 1990s. Our estimated terrestrial ecosystem carbon uptake, in response to climate variation and atmospheric CO2 increase, accounted for only about 15 to 30% of the total terrestrial carbon sink but contributed 73% of its increase from the 1980s to the 1990s.

Cao, M. K., Woodward, F. I. (1998). Net primary and ecosystem production and carbon stocks of terrestrial ecosystems and their responses to climate change. Global Change Biology 4 (2): 185-198

ABSTRACT: Evaluating the role of terrestrial ecosystems in the global carbon cycle requires a detailed understanding of carbon exchange between vegetation, soil, and the atmosphere. Global climatic change may modify the net carbon balance of terrestrial ecosystems, causing feedbacks on atmospheric CO2 and climate. We describe a model for investigating terrestrial carbon exchange and its response to climatic variation based on the processes of plant photosynthesis, carbon allocation, litter production, and soil organic carbon decomposition. The model is used to produce geographical patterns of net primary production (NPP), carbon stocks in vegetation and soils, and the seasonal variations in net ecosystem production (NEP) under both contemporary and future climates. For contemporary climate, the estimated global NPP is 57.0 Gt C y–1 , carbon stocks in vegetation and soils are 640 Gt C and 1358 Gt C, respectively, and NEP varies from –0.5 Gt C in October to 1.6 Gt C in July. For a doubled atmospheric CO2 concentration and the corresponding climate, we predict that global NPP will rise to 69.6 Gt C y–1 , carbon stocks in vegetation and soils will increase by, respectively, 133 Gt C and 160 Gt C, and the seasonal amplitude of NEP will increase by 76%. A doubling of atmospheric CO2 without climate change may enhance NPP by 25% and result in a substantial increase in carbon stocks in vegetation and soils. Climate change without CO2 elevation will reduce the global NPP and soil carbon stocks, but leads to an increase in vegetation carbon because of a forest extension and NPP enhancement in the north. By combining the effects of CO2 doubling, climate change, and the consequent redistribution of vegetation, we predict a strong enhancement in NPP and carbon stocks of terrestrial ecosystems. This study simulates the possible variation in the carbon exchange at equilibrium state. We anticipate to investigate the dynamic responses in the carbon exchange to atmospheric CO2 elevation and climate change in the past and future.

Cao, M. K., Woodward, F. I. (1998). Dynamic responses of terrestrial ecosystem carbon cycling to global climate change. Nature 393 (6682): 249-252

ABSTRACT: Terrestrial ecosystems and the climate system are closely coupled, particularly by cycling of carbon between vegetation, soils and the atmosphere. It has been suggested1,2 that changes in climate and in atmospheric carbon dioxide concentrations have modified the carbon cycle so as to render terrestrial ecosystems as substantial carbon sinks3,4 ; but direct evidence for this is very limited5,6 . Changes in ecosystem carbon stocks caused by shifts between stable climate states have been evaluated7,8 , but the dynamic responses of ecosystem carbon fluxes to transient climate changes are still poorly understood. Here we use a terrestrial biogeochemical model9 , forced by simulations of transient climate change with a general circulation model10 , to quantify the dynamic variations in ecosystem carbon fluxes induced by transient changes in atmospheric CO2 and climate from 1861 to 2070. We predict that these changes increase global net ecosystem production significantly, but that this response will decline as the CO2 fertilization effect becomes saturated and is diminished by changes in climatic factors. Thus terrestrial ecosystem carbon fluxes both respond to and strongly influence the atmospheric CO2 increase and climate change.

Catovsky, S., Bradford, M. A., Hector, A. (2002). Biodiversity and ecosystem productivity: implications for carbon storage. Oikos 97 (3): 443-448

ABSTRACT: Recent experiments have found that Net Primary Productivity (NPP) can often be a positive saturating function of plant species and functional diversity. These findings raised the possibility that more diverse ecosystems might store more carbon as a result of increased photosynthetic inputs. However, carbon inputs will not only remain in plant biomass, but will be translocated to the soil via root exudation, fine root turnover, and litter fall. Thus, we must consider not just plant productivity (NPP), but also net productivity of the whole ecosystem (NEP), which itself measures net carbon storage. We currently know little about how plant diversity could influence soil processes that return carbon back to the atmosphere, such as heterotrophic respiration and decomposition of organic matter. Nevertheless, it is clear that any effects on such processes could make NPP a poor predictor of whole-ecosystem productivity, and potentially the ability of the ecosystem to store carbon. We examine the range of mechanisms by which plant diversity could influence net ecosystem productivity, incorporating processes involved with carbon uptake (productivity), loss (autotrophic and heterotrophic respiration), and residence time within the system (decomposition rate). Understanding the relationship between plant diversity and ecosystem carbon dynamics must be made a research priority if we wish to provide information relevant to global carbon policy decisions. This goal is entirely feasible if we utilize some basic methods for measuring the major fluxes of carbon into and out of the ecosystem.

Cornelissen, J. H. C., van Bodegom, P. M., Aerts, R., Callaghan, T. V., van Logtestijn, R. S. P., Alatalo, J., Stuart C. F., Gerdol, R., Gudmundsson, J., Gwynn-Jones, D., Hartley, A. E., Hik, D. S., Hofgaard, A., Jonsdottir, I. S., Karlsson, S., Klein, J. A., Laundre, J., Magnusson, B., Michelsen, A., Molau, U., Onipchenko, V. G., Quested, H. M., Sandvik, S. M., Schmidt, I. K., Shaver, G. R., Solheim, B., Soudzilovskaia, N. A., Stenstrom, A., Tolvanen, A., Totland, O., Wada, N., Welker, J. M., Zhao, X. (2007). Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes. Ecology Letters 10 (7): 619-627

ABSTRACT: Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth-form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.

Dargaville, R., Ciais, P., Baker, D., Rödenbeck, C., Rayner, P. (2006). Estimating high latitude carbon fluxes with inversions of atmospheric CO2. Mitigation and Adaptation Strategies for Global Change 11 (4): 769-782

ABSTRACT: Atmospheric inversions have proven to be useful tools, showing for example the likely existence of a large terrestrial carbon sink in the northern mid-latitudes. However, as we go to smaller spatial scales the uncertainties in the inversions increase rapidly, and the task of finding the distribution of the sink between North America, Europe and Asia has been shown to be very difficult. The uncertainty in the fluxes due to network selection, transport model error and inversion set up tends to be too high for studying either net annual fluxes or interannual variability on spatial scales such as the North American Boreal or Eurasian Boreal regions. We discuss the path forward; to couple together the atmospheric inversions with process based terrestrial carbon models, creating carbon data assimilation systems. Such systems are being developed now and could prove to be very powerful. The multi-disciplinary nature of the data assimilation system requires information from flux towers, soil and above ground biomass inventories, remote sensed fields, atmospheric CO2 concentrations and climate data as well as model development and will need a massive community effort if it will succeed.

Davi, H., Dufrene, E., Francois, C., Le Maire, G., Loustau, D., Bosc, A., Rambal, S., Granier, A., Moors, E. (2006). Sensitivity of water and carbon fluxes to climate changes from 1960 to 2100 in European forest ecosystems. Agricultural and Forest Meteorology 141 (1): 35-56

ABSTRACT: The effects of climate changes on carbon and water fluxes are quantified using a physiologically multi-layer, process-based model containing a carbon allocation model and coupled with a soil model (CASTANEA). The model is first evaluated on four EUROFLUX sites using eddy covariance data, which provide estimates of carbon and water fluxes at the ecosystem scale. It correctly reproduces the diurnal fluxes and the seasonal pattern. Thereafter simulations were conducted on six French forest ecosystems representative of three climatic areas (oceanic, continental and Mediterranean areas) dominated by deciduous species (Fagus sylvatica ,Quercus robur ), coniferous species (Pinus pinaster ,Pinus sylvestris ) or sclerophyllous evergreen species (Quercus ilex ). The model is driven by the results of a meteorological model (ARPEGE) following the B2 scenario of IPCC. From 1960 to 2100, the average temperature increases by 3.1 °C (30%) and the rainfall during summer decreases by 68 mm (-27%). For all the sites, between the two periods, the simulations predict on average a gross primary production (GPP) increase of 513 g(C) m-2 (+38%). This increase is relatively steep until 2020, followed by a slowing down of the GPP rise due to an increase of the effect of water stress. Contrary to GPP, the ecosystem respiration (Reco) raises at a constant rate (350 g(C) m-2 i.e. 31% from 1960 to 2100). The dynamics of the net ecosystem productivity (GPP minus Reco) is the consequence of the effect on both GPP and Reco and differs per site. The ecosystems always remain carbon sinks; however the sink strength globally decreases for coniferous (-8%), increases for sclerophyllous evergreen (+34%) and strongly increases for deciduous forest (+67%) that largely benefits by the lengthening of the foliated period. The separately quantified effects of the main variables (temperature, length of foliated season, CO2 fertilization, drought effect), show that the magnitude of these effects depends on the species and the climatic zone.

R. K. Dixon, A. M. Solomon, S. Brown, R. A. Houghton, M. C. Trexier, J. Wisniewski (1994). Carbon pools and flux of global forest ecosystems. Science 263 (5144): 185-190

ABSTRACT: Forest systems cover more than 4.1 x 109 hectares of the Earth's land area. Globally, forest vegetation and soils contain about 1146 petagrams of carbon, with approximately 37 percent of this carbon in low-latitude forests, 14 percent in mid-latitudes, and 49 percent at high latitudes. Over two-thirds of the carbon in forest ecosystems is contained in soils and associated peat deposits. In 1990, deforestation in the low latitudes emitted 1.6 ± 0.4 petagrams of carbon per year, whereas forest area expansion and growth in mid- and high-latitude forest sequestered 0.7 ± 0.2 petagrams of carbon per year, for a net flux to the atmosphere of 0.9 ± 0.4 petagrams of carbon per year. Slowing deforestation, combined with an increase in forestation and other management measures to improve forest ecosystem productivity, could conserve or sequester significant quantities of carbon. Future forest carbon cycling trends attributable to losses and regrowth associated with global climate and land-use change are uncertain. Model projections and some results suggest that forests could be carbon sinks or sources in the future.

Eliseev, A. V., Mokhov, I. I. (2007). Carbon cycle-climate feedback sensitivity to parameter changes of a zero-dimensional terrestrial carbon cycle scheme in a climate model of intermediate complexity. Theoretical and Applied Climatology 89 (1): 9-24

ABSTRACT: A series of sensitivity runs have been performed with a coupled climate–carbon cycle model. The climatic component consists of the climate model of intermediate complexity IAP RAS CM. The carbon cycle component is formulated as a simple zero-dimensional model. Its terrestrial part includes gross photosynthesis, and plant and soil respirations, depending on temperature viaQ 10 -relationships (Lenton, 2000). Oceanic uptake of anthropogenic carbon is formulated is a bi-linear function of tendencies of atmospheric concentration of CO2 and globally averaged annual mean sea surface temperature. The model is forced by the historical industrial and land use emissions of carbon dioxide for the second half of the 19th and the whole of the 20th centuries, and by the emission scenario SRES A2 for the 21st century. For the standard set of the governing parameters, the model realistically captures the main features of the Earth’s observed carbon cycle. A large number of simulations have been performed, perturbing the governing parameters of the terrestrial carbon cycle model. In addition, the climate part is perturbed, either by zeroing or artificially increasing the climate model sensitivity to the doubling of the atmospheric CO2 concentration. Performing the above mentioned perturbations, it is possible to mimic most of the range found in the C4MIP simulations. In this way, a wide range of the climate–carbon cycle feedback strengths is obtained, differing even in the sign of the feedback. If the performed simulations are subjected to the constraints of a maximum allowed deviation of the simulated atmospheric CO2 concentration (p CO2( ) ) from the observed values and correspondence between simulated and observed terrestrial uptakes, it is possible to narrow the corresponding uncertainty range. Among these constraints, consideringp CO2( ) and uptakes are both important. However, the terrestrial uptakes constrain the simulations more effectively than the oceanic ones. These constraints, while useful, are still unable to rule out both extremely strong positive and modest negative climate–carbon cycle feedback.

Eliseev, A.V., Mokhov, I.I., Karpenko, A.A. (2007). Climate and carbon cycle variations in the 20th and 21st centuries in a model of intermediate complexity. Izvestiya - Atmospheric and Ocean Physics 43 (1): 1-14

ABSTRACT: The climate model of intermediate complexity developed at the Oboukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS CM), has been supplemented by a zero-dimensional carbon cycle model. With the carbon dioxide emissions prescribed for the second half of the 19th century and for the 20th century, the model satisfactorily reproduces characteristics of the carbon cycle over this period. However, with continued anthropogenic CO2 emissions (SRES scenarios A1B, A2, B1, and B2), the climate-carbon cycle feedback in the model leads to an additional atmospheric CO2 increase (in comparison with the case where the influence of climate changes on the carbon exchange between the atmosphere and the underlying surface is disregarded). This additional increase is varied in the range 67–90 ppmv depending on the scenario and is mainly due to the dynamics of soil carbon storage. The climate-carbon cycle feedback parameter varies nonmonotonically with time. Positions of its extremes separate characteristic periods of the change in the intensity of anthropogenic emissions and of climate variations. By the end of the 21st century, depending on the emission scenario, the carbon dioxide concentration is expected to increase to 615–875 ppmv and the global temperature will rise by 2.4–3.4 K relative to the preindustrial value. In the 20th–21st centuries, a general growth of the buildup of carbon dioxide in the atmosphere and ocean and its reduction in terrestrial ecosystems can be expected. In general, by the end of the 21st century, the more aggressive emission scenarios are characterized by a smaller climate-carbon cycle feedback parameter, a lower sensitivity of climate to a single increase in the atmospheric concentration of carbon dioxide, a larger fraction of anthropogenic emissions stored in the atmosphere and the ocean, and a smaller fraction of emissions in terrestrial ecosystems.

Erbrecht, T., Lucht, W. (2006). Impacts of large-scale climatic disturbances on the terrestrial carbon cycle. Carbon Balance and Management 1 (2): 1-7

ABSTRACT: Background: The amount of carbon dioxide in the atmosphere steadily increases as a consequence of anthropogenic emissions but with large interannual variability caused by the terrestrial biosphere. These variations in the CO2 growth rate are caused by large-scale climate anomalies but the relative contributions of vegetation growth and soil decomposition is uncertain. We use a biogeochemical model of the terrestrial biosphere to differentiate the effects of temperature and precipitation on net primary production (NPP) and heterotrophic respiration (Rh) during the two largest anomalies in atmospheric CO2 increase during the last 25 years. One of these, the smallest atmospheric year-to-year increase (largest land carbon uptake) in that period, was caused by global cooling in 1992/93 after the Pinatubo volcanic eruption. The other, the largest atmospheric increase on record (largest land carbon release), was caused by the strong El Niño event of 1997/98.

Results: We find that the LPJ model correctly simulates the magnitude of terrestrial modulation of atmospheric carbon anomalies for these two extreme disturbances. The response of soil respiration to changes in temperature and precipitation explains most of the modelled anomalous CO2 flux.

Conclusion: Observed and modelled NEE anomalies are in good agreement, therefore we suggest that the temporal variability of heterotrophic respiration produced by our model is reasonably realistic. We therefore conclude that during the last 25 years the two largest disturbances of the global carbon cycle were strongly controlled by soil processes rather then the response of vegetation to these large-scale climatic events.

Falloon, P., Jones, C. D., Cerri, C. E. P., Al-Adamat, R., Kamoni, P., Bhattacharyya, T., Easter, M., Paustian, K., Killian, K., Coleman, K., Milne, E. (2007). Climate change and its impact on soil and vegetation carbon storage in Kenya, Jordan, India and Brazil: Soil carbon stocks at regional scales - Assessment of Soil Organic Carbon Stocks and Change at National Scale, Final Project Presentation, The United Nations Environment Programme, Nairobi, Kenya, 23-24 May 2005. Agriculture, Ecosystems & Environment 122 (1): 114-124

ABSTRACT: The terrestrial biosphere is an important global carbon (C) sink, with the potential to drive large positive climate feedbacks. Thus a better understanding of interactions between land use change, climate change and the terrestrial biosphere is crucial in planning future land management options. Climate change has the potential to alter terrestrial C storage since changes in temperature, precipitation and carbon dioxide (CO2 ) concentrations could affect net primary production (NPP), C inputs to soil, and soil C decomposition rates. Climate change could also act as a driver for land use change, thus further altering terrestrial C fluxes. The net balance of these different effects varies considerably between regions and hence the case studies presented in this paper (the GEFSOC project countries Kenya, Jordan, Brazil, and India) provide a unique opportunity to study climate impacts on terrestrial C storage. This paper first presents predicted changes in climate for the four case study countries from a coupled climate-C cycle Global Circulation Model (HadCM3LC), followed by predicted changes in vegetation type, NPP and soil C storage. These very coarse assessments provide an initial estimate of large-scale effects. A more detailed study of climate impacts on soil C storage in the Brazilian Amazon is provided as an example application of the GEFSOC system. Interestingly in the four cases studied here precipitation seems to control the sign of the soil C changes under climate change with wetter conditions resulting in higher soil C stocks and drier conditions in lower soil C stocks, presumably because increased NPP in wetter conditions here will override any increase in respiration. In contrast, globally, it seems to be temperature that controls changes in C stocks under climate change. Even if there is a slight increase in precipitation globally, a decrease in C stocks is predicted—in other words, the regional response to precipitation differs from the global response. The reason for this may be that whilst temperature increases under climate change were predicted everywhere, the nature of precipitation changes varies greatly between regions.

Fearnside, P. M., Imbrozio Barbosa, R. (1998). Soil carbon changes from conversion of forest to pasture in Brazilian Amazonia. Forest Ecology and Management 108 (1-2): 147-166

ABSTRACT: Soils in Brazilian Amazonia may contain up to 136 Gt of carbon to a depth of 8 m, of which 47 Gt are in the top meter. The current rapid conversion of Amazonian forest to cattle pasture makes disturbance of this carbon stock potentially important to the global carbon balance and net greenhouse gas emissions. Information on the response of soil carbon pools to conversion to cattle pasture is conflicting. Some of the varied results that have been reported can be explained by effects of soil compaction, clay content and seasonal changes. Most studies have compared roughly simultaneous samples taken at nearby sites with different use histories (i.e., `chronosequences'); a clear need exists for longitudinal studies in which soil carbon stocks and related parameters are monitored over time at fixed locations. Whether pasture soils are a net sink or a net source of carbon depends on their management, but an approximation of the fraction of pastures under 'typical' and 'ideal' management practices indicates that pasture soils in Brazilian Amazonia are a net carbon source, with the upper 8 m releasing an average of 12.0 t C/ha in land maintained as pasture in the equilibrium landscape that is established in the decades following deforestation. Considering the equilibrium landscape as a whole, which is dominated by pasture and secondary forest derived from pasture, the average net release of soil carbon is 8.5 t C/ha, or 11.7x106 t C for the 1.38x106 ha cleared in 1990. Only 3% of the calculated emission comes from below 1 m depth, but the ultimate contribution from deep layers may be substantially greater. The land area affected by soil C losses under pasture is not restricted to the portion of the region maintained under pasture in the equilibrium landscape, but also the portion under secondary forests derived from pasture. Pasture effects from deforestation in 1990 represent a net committed emission from soils of 9.2x106 t C, or 79% of the total release from soils from deforestation in that year. Soil emissions from Amazonian deforestation represent a quantity of carbon approximately 20% as large as Brazil's annual emission from fossil fuels.

Field, C. B., Lobell, D. B., Peters, H. A., Chiariello, N. R. (2007). Feedbacks of terrestrial ecosystems to climate change. Annual Review of Environment and Resources 32: 1-29

ABSTRACT: Most modeling studies on terrestrial feedbacks to warming over the twenty-first century imply that the net feedbacks are negative-that changes in ecosystems, on the whole, resist warming, largely through ecosystem carbon storage. Although it is clear that potentially important mechanisms can lead to carbon storage, a number of less well-understood mechanisms, several of which are rarely or incompletely modeled, tend to diminish the negative feedbacks or lead to positive feedbacks. At high latitudes, negative feedbacks from forest expansion are likely to be largely or completely compensated by positive feedbacks from decreased albedo, increased carbon emissions from thawed permafrost, and increased wildfire. At low latitudes, negative feedbacks to warming will be decreased or eliminated, largely through direct human impacts. With modest warming, net feedbacks of terrestrial ecosystems to warming are likely to be negative in the tropics and positive at high latitudes. Larger amounts of warming will generally push the feedbacks toward the positive.

Fontaine, S., Barot, S., Barre, P., Bdioui, N., Mary, B., Rumpel, C. (2007). Stability of organic carbon in deep soil layers controlled by fresh carbon supply. Nature 450 (7167): 277-280

ABSTRACT: The world's soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon. Supplementary material at: http://www.nature.com/nature/journal/v450/n7167/extref/nature06275-s1.pdf

Godbold, D.L., Brunner, I. (2007). The platform for European root science, COST action E38: An introduction and overview. Plant Biosystems 141 (3): 390-393

ABSTRACT: Globally, forests cover 4 billion ha or 30% of the Earth's land surface and account for more that 75% of carbon stored in terrestrial ecosystem. However, 20 - 40% of the forest biomass is roots. Roots play a key role in acquisition of water and nutrients from the soil, the transfer of carbon to soil, as well as providing physical stabilisation. In temperate forests of Europe, average biomass of trees is estimated to be ca. 220 t ha-1 , of which 52 t ha-1 are coarse roots and 2.4 t ha-1 are fine roots. Thus, forests and their soils belong to the planets largest reservoirs of carbon. As an outcome of a recently established European platform for scientists working on woody roots, COST action E38, a series of papers has been initiated in order to review the current knowledge on processes in and of roots of woody plants and to identify possible knowledge gaps. These reviews concentrate on aspects of roots as indicators of environmental change, biomass of fine roots, and modelling of course root systems. The reviews of roots as indicators of environmental change cover a number of aspects including, specific root length, the calcium to aluminium ratio, root electrolyte leakage, and ectomycorrhiza community composition.

Hashimoto, H., Nemani, R. R., White, M. A., Jolly, W. M., Piper, S. C., Keeling, C. D., Myneni, R. B., Running, S. W. (2004). El Niño–Southern Oscillation–induced variability in terrestrial carbon cycling. Journal of Geophysical Research 109 (D23110): doi:10.1029/2004JD004959

ABSTRACT: We examined the response of terrestrial carbon fluxes to climate variability induced by the El Niño–Southern Oscillation (ENSO). We estimated global net primary production (NPP) from 1982 to 1999 using a light use efficiency model driven by satellite-derived canopy parameters from the Advanced Very High Resolution Radiometer and climate data from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project. We estimated a summed heterotrophic respiration and fire carbon flux as the residual between NPP and the terrestrial net carbon flux inferred from an atmospheric inversion model, excluding the impacts of land use change. We propose that for global applications this approach may be more robust than traditional, biophysically based approaches of simulating heterotrophic respiration. NPP interannual variability was significantly related to ENSO, particularly at lower latitudes (22.5°N–22.5°S) but was weakly related to global temperature. Global heterotrophic respiration and fire carbon fluxes were strongly correlated with global temperature (7.9 pgC/°C). Our results confirm the dependence of global heterotrophic respiration and fire carbon fluxes on interannual temperature variability and strongly suggest that ENSO-mediated NPP variability influences the atmospheric CO2 growth rate.

Houghton, R.A., Hobbie, J.E., Melillo, J.M., Moore, B., Peterson, B.J., Shaver, G.R., Woodwell, G.M. (1983). Changes in the carbon content of terrestrial biota and soils between 1860 and 1980: A net release of CO2 to the atmosphere. Ecological Monographs 53 (3): 235-262

ABSTRACT: Changes in land use over the past two centuries have caused a significant release of CO2 to the atmosphere from the terrestrial biota and soils. An analysis of this release is based on amounts of organic carbon within an ecosystem following changes such as harvest of forests; it is also based on rates of changes, such as conversion of forest to agriculture, deduced from agricultural and forestry statistics. A model is used to calculate the net amount of carbon stored or released each year by the biota and soils of 69 regional ecosystems. Some of the changes, such as afforestation, the growth of harvested forests, and buildup of soil organic matter, result in a storage of carbon; others, such as harvest of forests and increase in pasture and agricultural areas, result in a loss of carbon to the atmosphere. According to this analysis, there has been a net release of carbon from terrestrial ecosystems worldwide since at least 1860. Until ~1960, the annual release was greater than release of carbon from fossil fuels. The total net release of carbon from terrestrial ecosystems since 1860 is estimated to have been 180 x 1015 g (a range of estimates is 135—228 x 1015 g). The estimated net release of carbon in 1980 was 1.8—4.7 x 1015 g; for the 22 yr since 1958 the release of C was 38—76 x 1015 g. The ranges reflect the differences among various estimates of forest biomass, soil carbon, and agricultural clearing. Improvements in the data on the clearing of tropical forests alone would reduce the range of estimates for 1980 by almost 60%. Estimates of the other major terms in the global carbon budget, the atmospheric increase in CO2 , the fossil fuel release of CO2 , and the oceanic uptake of CO2 , are all subject to uncertainties. The combined errors in these estimates are large enough that the global carbon budget appears balanced if the low estimate for the biotic release of carbon given above is used (1.8 x 1015 g released in 1980) with the higher estimates of oceanic uptake. If higher estimates for biotic release are used, then the carbon budget does not balance, and the estimates of oceanic uptake or of other factors require revision.

House,J. I., Heimann,M., Prentice,I. C., Ramankutty,N., Houghton,R. A. (2003). Reconciling apparent inconsistencies in estimates of terrestrial CO2 sources and sinks. Tellus B 55 (2): 345-363

ABSTRACT: The magnitude and location of terrestrial carbon sources and sinks remains subject to large uncertainties. Estimates of terrestrial CO2 fluxes from ground-based inventory measurements typically find less carbon uptake than inverse model calculations based on atmospheric CO2 measurements, while a wide range of results have been obtained using models of different types. However, when full account is taken of the processes, pools, time scales and geographic areas being measured, the different approaches can be understood as complementary rather than inconsistent, and can provide insight as to the contribution of various processes to the terrestrial carbon budget. For example, quantitative differences between atmospheric inversion model estimates and forest inventory estimates in northern extratropical regions suggest that carbon fluxes to soils (often not accounted for in inventories), and into non-forest vegetation, may account for about half of the terrestrial uptake. A consensus of inventory and inverse methods indicates that, in the 1980s, northern extratropical land regions were a large net sink of carbon, and the tropics were approximately neutral (albeit with high uncertainty around the central estimate of zero net flux). The terrestrial flux in southern extratropical regions was small. Book-keeping model studies of the impacts of land-use change indicated a large source in the tropics and almost zero net flux for most northern extratropical regions; similar land use change impacts were also recently obtained using process-based models. The difference between book-keeping land-use change model studies and inversions or inventories was previously interpreted as a "missing" terrestrial carbon uptake. Land-use change studies do not account for environmental or many management effects (which are implicitly included in inventory and inversion methods). Process-based model studies have quantified the impacts of CO2 fertilisation and climate change in addition to land use change, and found that these environmental effects are in the right order of magnitude to account for the "missing" terrestrial carbon uptake. Despite recent carbon losses due to fire and insect attack in Canada and Russia, the northern extratropical regions generally have been a net carbon sink, only partially due to land-use changes such as abandonment of agricultural land. In the tropics, inventory data and flux measurements in extant forests support the existence of an environmental or management sink that counterbalances the effect of deforestation. Woody encroachment in savannas may also be a significant (but as yet poorly quantified) cause of tropical carbon uptake.

Humphreys, E. R., Lafleur, P. M., Flanagan, L. B., Hedstrom, N., Syed, K. H., Glenn, A. J., Granger, R. (2006). Summer carbon dioxide and water vapor fluxes across a range of northern peatlands. Journal of Geophysical Research -- Biogeosciences 111 (G04011)

ABSTRACT: Northern peatlands are a diverse group of ecosystems varying along a continuum of hydrological, chemical, and vegetation gradients. These ecosystems contain about one third of the global soil carbon pool, but it is uncertain how carbon and water cycling processes and response to climate change differ among peatland types. This study examines midsummer CO2 and H2 O fluxes measured using the eddy covariance technique above seven northern peatlands including a low-shrub bog, two open poor fens, two wooded moderately rich fens, and two open extreme-rich fens. Gross ecosystem production and ecosystem respiration correlated positively with vegetation indices and with each other. Consequently, 24-hour net ecosystem CO2 exchange was similar among most of the sites (an average net carbon sink of 1.5 ± 0.2 g C m−2 d−1 ) despite large differences in water table depth, water chemistry, and plant communities. Evapotranspiration was primarily radiatively driven at all sites but a decline in surface conductance with increasing water vapor deficit indicated physiological restrictions to transpiration, particularly at the peatlands with woody vegetation and less at the peatlands with 100%Sphagnum cover. Despite these differences, midday evapotranspiration ranged only from 0.21 to 0.34 mm h−1 owing to compensation among the factors controlling evapotranspiration. Water use efficiency varied among sites primarily as a result of differences in productivity and plant functional type. Although peatland classification includes a great variety of ecosystem characteristics, peatland type may not be an effective way to predict the magnitude and characteristics of midsummer CO2 and water vapor exchanges.

Ise, T., Moorcroft, P. R. (2006). The global-scale temperature and moisture dependencies of soil organic carbon decomposition: an analysis using a mechanistic decomposition model. Biogeochemistry 80 (3): 217-231

ABSTRACT: Since the decomposition rate of soil organic carbon (SOC) varies as a function of environmental conditions, global climate change is expected to alter SOC decomposition dynamics, and the resulting changes in the amount of CO2 emitted from soils will feedback onto the rate at which climate change occurs. While this soil feedback is expected to be significant because the amount of SOC is substantially more than the amount of carbon in the atmosphere, the environmental dependencies of decomposition at global scales that determine the magnitude of the soil feedback have remained poorly characterized. In this study, we address this issue by fitting a mechanistic decomposition model to a global dataset of SOC, optimizing the model’s temperature and moisture dependencies to best match the observed global distribution of SOC. The results of the analysis indicate that the temperature sensitivity of decomposition at global scales (Q10 =1.37) is significantly less than is assumed by many terrestrial ecosystem models that directly apply temperature sensitivity from small-scale studies, and that the maximal rate of decomposition occurs at higher moisture values than is assumed by many models. These findings imply that the magnitude of the soil decomposition feedback onto rate of global climate change will be less sensitive to increases in temperature, and modeling of temperature and moisture dependencies of SOC decomposition in global-scale models should consider effects of scale.

Jain, A. K. (2007). Global estimation of CO emissions using three sets of satellite data for burned area. Atmospheric Environment 41 (33): 6931-6940

ABSTRACT: Using three sets of satellite data for burned areas together with the tree cover imagery and a biogeochemical component of the Integrated Science Assessment Model (ISAM) the global emissions of CO and associated uncertainties are estimated for the year 2000. The available fuel load (AFL) is calculated using the ISAM biogeochemical model, which accounts for the aboveground and surface fuel removed by land clearing for croplands and pasturelands, as well as the influence on fuel load of various ecosystem processes (such as stomatal conductance, evapotranspiration, plant photosynthesis and respiration, litter production, and soil organic carbon decomposition) and important feedback mechanisms (such as climate and fertilization feedback mechanism). The ISAM estimated global total AFL in the year 2000 was about 687 Pg AFL. All forest ecosystems account for about 90% of the global total AFL. The estimated global CO emissions based on three global burned area satellite data sets (GLOBSCAR, GBA, and Global Fire Emissions Database version 2 (GFEDv2)) for the year 2000 ranges between 320 and 390 Tg CO. Emissions from open fires are highest in tropical Africa, primarily due to forest cutting and burning. The estimated overall uncertainty in global CO emission is about +/-65%, with the highest uncertainty occurring in North Africa and Middle East region (+/-99%). The results of this study suggest that the uncertainties in the calculated emissions stem primarily from the area burned data.

Janzen, H. H. (2004). Carbon Cycling in Earth Systems - a Soil Science Perspective. 104 (3): 399-417

ABSTRACT: The carbon cycle binds together earth’s ecosystems and their inhabitants. My intent is to review the global carbon cycle, examine how humans have modified it, and contemplate (from a soil science bias) the new questions that await us on a changing earth. These thoughts are proffered, not to propose a way forward, but to invite conversation about opportunities that await us.

Terrestrial ecosystems hold a lot of carbon—about 500 Pg C in plant biomass, and 2000 Pg C in soil organic matter. Oceans contain even more. And the atmosphere, now with about 785 Pg C, connects all of these pools. The flows of carbon between the pools, and their feedbacks, have kept atmospheric CO2 reasonably constant for millennia. But humans have increasingly distorted the balance, by changing land use and by injecting fossil C back into the cycle. Consequently, atmospheric CO2 has increased recently by more than 3 Pg C per year and, by century’s end, its concentration may be twice pre-industrial levels, or more.

The changing carbon cycle poses new questions for scientists. Now we will be asked, not how things are, but how they will be. For example: How will changes in CO2 alter flows of carbon through biological carbon stocks? Can we manage ecosystems to hold more carbon? Are current carbon stores vulnerable should the earth warm, or water cycles shift, or nitrogen flows be altered? What will the C cycle look like a century from now; and will it then still provide all that we expect from it? These and other new questions may elicit from us fresh insights and approaches.We may learn to look more broadly at the C cycle, seeing all the ‘ecosystem services’ (not just C sequestration). We may insist on studies yielding deeper understanding of the C cycle, relevant beyond current issues. We may further emphasize ‘time’ in our studies, looking more at flows and changes than at describing what is—and looking long enough to see even subtle shifts.We may learn to follow C beyond the usual boundaries set by arbitrary disciplines. And we may come to see, more than before, how the carbon cycle weaves through our fields and skies and forests—and find new ways to reveal its grandeur to those who have not yet seen it. And then, it may happen that
our successors, a century from now, will look back, almost in envy, at the urgent, enticing questions we were given to solve.

Johnston, C. A., Groffman, P., Breshears, D. D., Cardon, Z. G., Currie, W., Emanuel, W., Gaudinski, J., Jackson, R. B., Lajtha, K., Nadelhoffer, K., Nelson, D., Post, W. M., Retallack, G., Wielopolski, L. (2004). Carbon cycling in soil. Frontiers in Ecology and the Environment 2 (10): 522-528

ABSTRACT: As yet, nobody knows what effects climate change will have on soil carbon reserves, or how those changes will affect the global carbon cycle. Soils are the primary terrestrial repository for carbon, so minor changes in the balance between belowground carbon storage and release could have major impacts on greenhouse gases. Soil fauna, roots, fungi, and microbes interact with mineral and organic matter to process soil carbon. Studies have been hampered by the difficulty of observing processes beneath the earth's surface, but advances in science and technology are improving our ability to understand belowground ecosystems.

Jones, C., McConnell, C., Coleman, K., Cox, P., Falloon, P., Jenkinson, D., Powlson, D. (2005). Global climate change and soil carbon stocks; predictions from two contrasting models for the turnover of organic carbon in soil. Global Change Biology 11 (1): 154-166

ABSTRACT: Enhanced release of CO2 to the atmosphere from soil organic carbon as a result of increased temperatures may lead to a positive feedback between climate change and the carbon cycle, resulting in much higher CO2 levels and accelerated global warming. However, the magnitude of this effect is uncertain and critically dependent on how the decomposition of soil organic C (heterotrophic respiration) responds to changes in climate. Previous studies with the Hadley Centre's coupled climate–carbon cycle general circulation model (GCM) (HadCM3LC) used a simple, single-pool soil carbon model to simulate the response. Here we present results from numerical simulations that use the more sophisticated 'RothC' multipool soil carbon model, driven with the same climate data.

The results show strong similarities in the behaviour of the two models, although RothC tends to simulate slightly smaller changes in global soil carbon stocks for the same forcing. RothC simulates global soil carbon stocks decreasing by 54 Gt C by 2100 in a climate change simulation compared with an 80 Gt C decrease in HadCM3LC. The multipool carbon dynamics of RothC cause it to exhibit a slower magnitude of transient response to both increased organic carbon inputs and changes in climate. We conclude that the projection of a positive feedback between climate and carbon cycle is robust, but the magnitude of the feedback is dependent on the structure of the soil carbon model.

Kimball, J. S., Zhao, M., McGuire, A. D., Heinsch, F. A., Clein, J., Calef, M., Jolly, W. M., Kang, S., Euskirchen, S. E., McDonald, K. C., Running, S. W. (2007). Recent Climate-Driven Increases in Vegetation Productivity for the Western Arctic: Evidence of an Acceleration of the Northern Terrestrial Carbon Cycle. Earth Interactions 11 (4): 1-30

ABSTRACT: Northern ecosystems contain much of the global reservoir of terrestrial carbon that is potentially reactive in the context of near-term climate change. Annual variability and recent trends in vegetation productivity across Alaska and northwest Canada were assessed using a satellite remote sensing–based production efficiency model and prognostic simulations of the terrestrial carbon cycle from the Terrestrial Ecosystem Model (TEM) and BIOME–BGC (BioGeoChemical Cycles) model. Evidence of a small, but widespread, positive trend in vegetation gross and net primary production (GPP and NPP) is found for the region from 1982 to 2000, coinciding with summer warming of more than 1.8°C and subsequent relaxation of cold temperature constraints to plant growth. Prognostic model simulation results were generally consistent with the remote sensing record and also indicated that an increase in soil decomposition and plant-available nitrogen with regional warming was partially responsible for the positive productivity response. Despite a positive trend in litter inputs to the soil organic carbon pool, the model results showed evidence of a decline in less labile soil organic carbon, which represents approximately 75% of total carbon storage for the region. These results indicate that the regional carbon cycle may accelerate under a warming climate by increasing the fraction of total carbon storage in vegetation biomass and more rapid turnover of the terrestrial carbon reservoir.

Lal, R. (2007). Soil carbon stocks under present and future climate with specific reference to European ecoregions. Nutrient Cycling in Agroecosystems 81 (2): 113-127

ABSTRACT; World soils and terrestrial ecosystems have been a source of atmospheric abundance of CO2 ever since settled agriculture began about 10–13 millennia ago. The amount of CO2 -C emitted into the atmosphere is estimated at 136 ± 55 Pg from terrestrial ecosystems, of which emission from world soils is estimated at 78 ± 12 Pg. Conversion of natural to agricultural ecosystems decreases soil organic carbon (SOC) pool by 30–50% over 50–100 years in temperate regions, and 50–75% over 20–50 years in tropical climates. The projected global warming, with estimated increase in mean annual temperature of 4–6°C by 2100, may have a profound impact on the total soil C pool and its dynamics. The SOC pool may increase due to increase in biomass production and accretion into the soil due to the so-called “CO2 fertilization effect”, which may also enhance production of the root biomass. Increase in weathering of silicates due to increase in temperature, and that of the formation of secondary carbonates due to increase in partial pressure of CO2 in soil air may also increase the total C pool. In contrast, however, SOC pool may decrease because of: (i) increase in rate of respiration and mineralization, (ii) increase in losses by soil erosion, and (iii) decrease in protective effects of stable aggregates which encapsulate organic matter. Furthermore, the relative increase in temperature projected to be more in arctic and boreal regions, will render Cryosols under permafrost from a net sink to a net source of CO2 if and when permafrost thaws. Thus, SOC pool of world soils may decrease with increase in mean global temperature. In contrast, the biotic pool may increase primarily because of the CO2 fertilization effect. The magnitude of CO2 fertilization effect may be constrained by lack of essential nutrients (e.g., N, P) and water. The potential of SOC sequestration in agricultural soils of Europe is 70–190 Tg C yr−1 . This potential is realizable through adoption of recommended land use and management, and restoration of degraded soils and ecosystems including wetlands.

Leifeld, J. (2006). Soils as sources and sinks of greenhouse gases. : 23-44

ABSTRACT: Soils annually emit between 6.8 and 7.9 Gt CO2 equivalents, mainly as CH4 from intact peatlands and from rice agriculture; as N2 O from unmanaged and managed soils; and as CO2 from land-use change. Methane emissions attributable to other wetlands add another 1.6–3.8 Gt CO2 equivalents. From a global standpoint, N2 O from unmanaged soils and CH4 from peatlands and other wetlands make soils naturally net greenhouse gas emitters. In addition, the storage of carbon in soils and the fluxes of CH4 and N2 O have been changed by anthropogenic effects towards emission rates 52 to 72% above those under natural conditions before the dawn of intensive agriculture and land-use change. Land-use changes on mineral soils induced most of the recorded losses of soil organic matter (SOM), but there is evidence that proper agricultural management of soil resources is able to recover some of these losses and to maintain soil functions. However, the discrepancy between so-called ‘sequestration potentials’ and the measures already adopted is amazingly large. Globally, only about 5% of the cropped areas is managed according to practices such as no tillage or organic farming. The contribution of soil loss by erosion, desertification and sealing to global oxidative SOM losses is uncertain; however, in the case of soil erosion, it is considered to be a major factor in global SOM decline. Mitigation options calculated for SOM restoration, reduced CH4 and N2O emissions are able to alleviate mean annual emissions by 1.2 to 2.9 Gt CO2 equivalents, mainly as a result of carbon sequestration, which is the most efficient measure for the next few decades. In the longer term, however, the large potential for reducing CH4 and N2 O emissions outweigh the finite capacity of soils to recover C. Integrated assessment of net greenhouse-gas fluxes is key for evaluating management practices aimed at reducing overall emissions. From the viewpoint of climate change and taking into consideration the mean fluxes of CO2 , CH4 and N2 O, peatland protection is more favourable than peatland cultivation in the long term. The most important gaps in our understanding appear to be with regard to estimating fluxes along with soil erosion and desertification processes, in the extent of peatland cultivation; the role of black carbon formation, natural ‘background’ sequestration rates of undisturbed soils; and the net response of soils, particularly in cold regions, to global warming. With regard to the societal perception of soil contributing to the global cycling of greenhouse gases, it is important to emphasize that significant proportions of the emissions are inevitably linked to intensive agriculture.

Leifeld, Jens, Fuhrer, Juerg (2005). The temperature response of CO2 production from bulk soils and soil fractions is related to soil organic matter quality. Biogeochemistry 75 (3): 433-453

ABSTRACT: The projected increase in global mean temperature could accelerate the turnover of soil organic matter (SOM). Enhanced soil CO2 emissions could feedback on the climate system, depending on the balance between the sensitivity to temperature of net carbon fixation by vegetation and SOM decomposition. Most of the SOM is stabilised by several physico-chemical mechanisms within the soil architecture, but the response of this quantitatively important fraction to increasing temperature is largely unknown. The aim of this study was to relate the temperature sensitivity of decomposition of physical and chemical soil fractions (size fractions, hydrolysis residues), and of bulk soil, to their quality and turnover time. Soil samples were taken from arable and grassland soils from the Swiss Central Plateau, and CO2 production was measured under strictly controlled conditions at 5, 15, 25, and 35 °C by using sequential incubation. Physico-chemical properties of the samples were characterised by measuring elemental composition, surface area,14 C age, and by using DRIFT spectroscopy. CO2 production rates per unit (g) organic carbon (OC) strongly varied between samples, in relation to the difference in the biochemical quality of the substrates. The temperature response of all samples was exponential up to 25 °C, with the largest variability at lower temperatures. Q10 values were negatively related to CO2 production over the whole temperature range, indicating higher temperature sensitivity of SOM of lower quality. In particular, hydrolysis residues, representing a more stabilised SOM pool containing older C, produced less CO2 g−1 OC than non-hydrolysed fractions or bulk samples at lower temperatures, but similar rates at ≥25 °C, leading to higher Q10 values than in other samples. Based on these results and provided that they apply also to other soils it is suggested that because of the higher sensitivity of passive SOM the overall response of SOM to increasing temperatures might be higher than previously expected from SOM models. Finally, surface area measurements revealed that micro-aggregation rather than organo-mineral association mainly contributes to the longer turnover time of SOM isolated by acid hydrolysis.

Lenton, T., Williamson, M., Edwards, N., Marsh, R., Price, A., Ridgwell, A., Shepherd, J., Cox, S., The GENIE team (2006). Millennial timescale carbon cycle and climate change in an efficient Earth system model. Climate Dynamics 26 (7-8): 687-711

ABSTRACT: A new Earth system model, GENIE-1, is presented which comprises a 3-D frictional geostrophic ocean, phosphate-restoring marine biogeochemistry, dynamic and thermodynamic sea-ice, land surface physics and carbon cycling, and a seasonal 2-D energy-moisture balance atmosphere. Three sets of model climate parameters are used to explore the robustness of the results and for traceability to earlier work. The model versions have climate sensitivity of 2.8–3.3°C and predict atmospheric CO2 close to present observations. Six idealized total fossil fuel CO2 emissions scenarios are used to explore a range of 1,100–15,000 GtC total emissions and the effect of rate of emissions. Atmospheric CO2 approaches equilibrium in year 3000 at 420–5,660 ppmv, giving 1.5–12.5°C global warming. The ocean is a robust carbon sink of up to 6.5 GtC year−1 . Under ‘business as usual’, the land becomes a carbon source around year 2100 which peaks at up to 2.5 GtC year−1 . Soil carbon is lost globally, boreal vegetation generally increases, whilst under extreme forcing, dieback of some tropical and sub-tropical vegetation occurs. Average ocean surface pH drops by up to 1.15 units. A Greenland ice sheet melt threshold of 2.6°C local warming is only briefly exceeded if total emissions are limited to 1,100 GtC, whilst 15,000 GtC emissions cause complete Greenland melt by year 3000, contributing 7 m to sea level rise. Total sea-level rise, including thermal expansion, is 0.4–10 m in year 3000 and ongoing. The Atlantic meridional overturning circulation shuts down in two out of three model versions, but only under extreme emissions including exotic fossil fuel resources.

Luo, Y., Su, B., Currie, W.S., Dukes, J,S., Finzi, A., Hartwig, U., Hungate, B. A., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D. R., Field, C. B. (2004). Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54 (8): 731-739

ABSTRACT: A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental results and to help researchers integrate models and data, direct tests of the PNL hypothesis remain a great challenge to the research community.

Luyssaert, S., Inglima, I., Jung, M., Richardson, A.D., Reichstein, M., Papale, D., Piao, S.L., Schulze, E. -D., Wingate, L., Matteucci, G., Aubinet, M., Beer, C., Bernhofer, C., Black, K.G., Bonal, D., Bonnefond, J. -M., Chambers, J., Ciais, P., Cook, B. (2007). CO2 balance of boreal, temperate, and tropical forests derived from a global database.. Global Change Biology 13 (12): 2509-2537

ABSTRACT: Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome-specific carbon budgets; to re-examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 °C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO2 balance required the introduction of substantial biome-specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non-CO2 carbon fluxes are not presently being adequately accounted for.

McGuire, A.D., Melillo, J.M., Kicklighter, D.W., Joyce, L. A. (1995). Equilibrium responses of soil carbon to climate change: empirical and process-based estimates. Journal of Biogeography 22 (4/5): 785-796

ABSTRACT: We use a new version of the Terrestrial Ecosystem Model (TEM), which has been parameterized to control for reactive soil organic carbon (SOC) across climatic gradients, to evaluate the sensitivity of SOC to a 1°C warming in both empirical and process-based analyses. In the empirical analyses we use the steady state SOC estimates of TEM to derive SOC-response equations that depend on temperature and volumetric soil moisture, and extrapolate them across the terrestrial biosphere at 0.5° spatial resolution. For contemporary climate and atmospheric CO2 , mean annual temperature explains 34.8% of the variance in the natural logarithm of TEM-estimated SOC. Because the inclusion of mean annual volumetric soil moisture in the regression explains an additional 19.6%, a soil mosture term in an equation of SOC response should improve estimates. For a 1°C warming, the globally derived empirical model estimates a terrestrial SOC loss of 22.6 x 1015 g (Pg), with 77.9% of the loss in extra-tropical ecosystems. To explore whether loss estimates SOC are affected by the spatial scale at which the response equations are derived equations for each of the eighteen ecosystems considered in this study. The sensitivity of terrestrial SOC estimated by summing the losses predicted by each of the ecosystem empirical models is greater (27.9 Pg per °C) than that estimated by the global empirical model; the 12.2 Pg loss (43.7%) in tropical ecosystems suggests that they may be more sensitive to warming. The global process-based loss of SOC estimated by TEM in response to a 1°C warming (26.3 Pg) is similar to the sum of the ecosystem empirical losses, but the 13.6 Pg loss (51.7%) in extra-tropical ecosystems suggests that they may be slightly less sensitive to warming. For the modelling of SOC responses, these results suggest that soil moisture is useful to incorporate in empirical models of SOC response and that globally derived empirical models may conceal regional sensitivity of SOC to warming. The analyses in this study suggest that the maximum loss of SOC to the atmosphere per °C warming is less than 2% of the terrestrial soil carbon inventory. Because the NPP response to elevated CO2 has the potential to compensate for this loss, the scenario of warming enhancing soil carbon loss to further enhance warming is unlikely in the absence of land use or changes in vegetation distribution.

McGuire, A.D., Melillo, J.M., Kicklighter, D.W., Pan, Y., Xiao, X., Helfrich, J., Moore, B., III, Vorosmarty, C.J., Schloss, A.L. (1997). Equilibrium responses of global net primary production and carbon storage to doubled atmospheric carbon dioxide: Sensitivity to changes in vegetation nitrogen concentration. Global Biogeochemical Cycles 11 (2): 173-189

ABSTRACT: We ran the terrestrial ecosystem model (TEM) for the globe at 0.5° resolution for atmospheric CO2 concentrations of 340 and 680 parts per million by volume (ppmv) to evaluate global and regional responses of net primary production (NPP) and carbon storage to elevated CO2 for their sensitivity to changes in vegetation nitrogen concentration. At 340 ppmv, TEM estimated global NPP of 49.0 1015 g (Pg) C yr−1 and global total carbon storage of 1701.8 Pg C; the estimate of total carbon storage does not include the carbon content of inert soil organic matter. For the reference simulation in which doubled atmosphericCO2 was accompanied with no change in vegetation nitrogen concentration, global NPP increased 4.1 Pg C yr−1 (8.3%), and global total carbon storage increased 114.2 Pg C. To examine sensitivity in the global responses of NPP and carbon storage to decreases in the nitrogen concentration of vegetation, we compared doubled CO2 responses of the reference TEM to simulations in which the vegetation nitrogen concentration was reduced without influencing decomposition dynamics ("lower N" simulations) and to simulations in which reductions in vegetation nitrogen concentration influence decomposition dynamics ("lower N+D" simulations). We conducted three lower N simulations and three lower N+D simulations in which we reduced the nitrogen concentration of vegetation by 7.5, 15.0, and 22.5%. In the lower N simulations, the response of global NPP to doubled atmospheric CO2 increased approximately 2 Pg C yr−1 for each incremental 7.5% reduction in vegetation nitrogen concentration, and vegetation carbon increased approximately an additional 40 Pg C, and soil carbon increased an additional 30 Pg C, for a total carbon storage increase of approximately 70 Pg C. In the lower N+D simulations, the responses of NPP and vegetation carbon storage were relatively insensitive to differences in the reduction of nitrogen concentration, but soil carbon storage showed a large change. The insensitivity of NPP in the N+D simulations occurred because potential enhancements in NPP associated with reduced vegetation nitrogen concentration were approximately offset by lower nitrogen availability associated with the decomposition dynamics of reduced litter nitrogen concentration. For each 7.5% reduction in vegetation nitrogen concentration, soil carbon increased approximately an additional 60 Pg C, while vegetation carbon storage increased by only approximately 5 Pg C. As the reduction in vegetation nitrogen concentration gets greater in the lower N+D simulations, more of the additional carbon storage tends to become concentrated in the north temperate-boreal region in comparison to the tropics. Other studies with TEM show that elevated CO2 more than offsets the effects of climate change to cause increased carbon storage. The results of this study indicate that carbon storage would be enhanced by the influence of changes in plant nitrogen concentration on carbon assimilation and decomposition rates. Thus changes in vegetation nitrogen concentration may have important implications for the ability of the terrestrial biosphere to mitigate increases in the atmospheric concentration of CO2 and climate changes associated with the increases.

Patra, P. K., Ishizawa, M., Maksyutov, S., Nakazawa, T., Inoue, G. (2005). Role of biomass burning and climate anomalies for land-atmosphere carbon fluxes based on inverse modeling of atmospheric CO2 . Global Biogeochemical Cycles 19 (3): doi:10.1029/2004GB002258

ABSTRACT: A Time-dependent inverse (TDI) model is used to estimate carbon dioxide (CO2 ) fluxes for 64 regions of the globe from atmospheric measurements in the period January 1994 to December 2001. The global land anomalies agree fairly well with earlier results. Large variability in CO2 fluxes are recorded from the land regions, which are typically controlled by the available water for photosynthesis, and air temperature and soil moisture dependent heterotrophic respiration. For example, the anomalous CO2 emissions during the 1997/1998 El Niño period are estimated to be about 1.27 ± 0.22, 2.06 ± 0.37, and 1.17 ± 0.20 Pg-C yr−1 from tropical regions in Asia, South America, and Africa, respectively. The CO2 flux anomalies for boreal Asia region are estimated to be 0.83 ± 0.19 and 0.45 ± 0.14 Pg-C yr−1 of CO2 during 1996 and 1998, respectively. Comparison of inversion results with biogeochemical model simulations provide strong evidence that biomass burning (natural and anthropogenic) constitutes the major component in land-atmosphere carbon flux anomalies. The net biosphere-atmosphere carbon exchanges based on the biogeochemical model used in this study are generally lower than those estimated from TDI model results, by about 1.0 Pg-C yr−1 for the periods and regions of intense fire. The correlation and principal component analyses suggest that changes in meteorology (i.e., rainfall and air temperature) associated with the El Niño Southern Oscillation are the most dominant controlling factors of CO2 flux anomaly in the tropics, followed by the Indian Ocean Dipole Oscillation. Our results indicate that the Arctic and North Atlantic Oscillations are closely linked with CO2 flux variability in the temperate and high-latitude regions.

Pepper, D. A., Del Grosso, S. J., Mcmurtrie, R. E., Parton, W. J. (2005). Simulated carbon sink response of shortgrass steppe, tallgrass prairie and forest ecosystems to rising [CO2 ], temperature and nitrogen input. Global Biogeochemical Cycles 19 (GB1004): doi:10.1029/2004GB002226

ABSTRACT: The response of plant ecosystems to environmental change will determine whether the terrestrial biosphere will remain a substantial carbon sink or become a source during the next century. We use two ecosystem models, the Generic Decomposition And Yield model (G'DAY) and the daily time step version of the Century model (DAYCENT), to simulate net ecosystem productivity (NEP) for three contrasting ecosystems (shortgrass steppe in Colorado, tallgrass prairie in Kansas, and Norway spruce in Sweden) with varying degrees of water, temperature, and nutrient limitation, to determine responses to gradual increases in atmospheric CO2 concentration ([CO2 ]), temperature, and nitrogen input over 100 years. Using G'DAY, under rising [CO2 ], there is evidence of C sink “saturation,” defined here as positive NEP reaching an upper limit and then declining toward zero, at all three sites (due largely to increased N immobilization in soil organic matter) but a positive C sink is sustained throughout the 100 years. DAYCENT also predicts a sustained C sink at all three sites under rising [CO2 ], with evidence of C sink saturation for the Colorado grassland and the C sink levels off after 80 years for the Kansas grassland. Warming reduces soil C and the C sink in both grassland ecosystems but increases the C sink in the forest. Warming increases decomposition and soil N mineralization, which stimulates net primary productivity (NPP) at all sites except when inducing water limitation. At the forest site some of the enhanced N release is allocated to a woody biomass pool with a low N:C ratio so that warming enhances NEP without increased N input at the forest site, but not at the grassland sites. Responses to combinations of treatments are generally additive for DAYCENT but more interactive for G'DAY, especially under combined rising [CO2 ] and warming at the strongly water- and N-limited shortgrass steppe. Increasing N input alleviates C sink saturation and enhances NEP, NPP, and soil C at all sites. At the water-limited grassland sites the effect of rising [CO2 ] on growth is greatest during the drier seasons. Key sensitivities in the simulations of NEP are identified and include NPP sensitivity to gradual increase in [CO2], N immobilization as a long-term feedback, and the presence or not of plant biomass pools with low N:C ratio.

Raich, J.W., Schleisinger, W.H. (1992). The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate. Tellus Series B Chemical and Physical Meteorology 44 (2): 81-99

ABSTRACT: We review measured rates of soil respiration from terrestrial and wetland ecosystems to define the annual global CO2 flux from soils, to identify uncertainties in the global flux estimate, and to investigate the influences of temperature, precipitation, and vegetation on soil respiration rates. The annual global CO2 flux from soils is estimated to average (± S.D.) 68 ± 4 PgC/ yr, based on extrapolations from biome land areas. Relatively few measurements of soil respiration exist from arid, semi-arid, and tropical regions; these regions should be priorities for additional research. On a global scale, soil respiration rates are positively correlated with mean annual air temperatures and mean annual precipitation. There is a close correlation between mean annual net primary productivity (NPP) of different vegetation biomes and their mean annual soil respiration rates, with soil respiration averaging 24% higher than mean annual NPP. This difference represents a minimum estimate of the contribution of root respiration to the total soil CO2 efflux. Estimates of soil C turnover rates range from 500 years in tundra and peaty wetlands to 10 years in tropical savannas. We also evaluate the potential impacts of human activities on soil respiration rates, with particular focus on land use changes, soil fertilization, irrigation and drainage, and climate changes. The impacts of human activities on soil respiration rates are poorly documented, and vary among sites. Of particular importance are potential changes in temperatures and precipitation. Based on a review of in situ measurements, the Q10 value for total soil respiration has a median value of 2.4. Increased soil respiration with global warming is likely to provide a positive feedback to the greenhouse effect.

Schlesinger, W. H., Andrews, J. A. (2000). Soil respiration and the global carbon cycle. Biogeochemistry 48 (1): 7-20.

ABSTRACT: Soil respiration is the primary path by which CO2 fixed by land plants returns to the atmosphere. Estimated at approximately 75 × 1015 g C/yr, this large natural flux is likely to increase due changes in the Earth's condition. The objective of this paper is to provide a brief scientific review for policymakers who are concerned that changes in soil respiration may contribute to the rise in CO2 in Earth's atmosphere. Rising concentrations of CO2 in the atmosphere will increase the flux of CO2 from soils, while simultaneously leaving a greater store of carbon in the soil. Traditional tillage cultivation and rising temperature increase the flux of CO2 from soils without increasing the stock of soil organic matter. Increasing deposition of nitrogen from the atmosphere may lead to the sequestration of carbon in vegetation and soils. The response of the land biosphere to simultaneous changes in all of these factors is unknown, but a large increase in the soil carbon pool seems unlikely to moderate the rise in atmospheric CO2 during the next century.

Topp, E., Pattey, E. (1997). Soils as sources and sinks for atmospheric methane. Canadian Journal of Soil Science 77 (2): 167-178

ABSTRACT: Methane is considered to be a significant greenhouse gas. Methane is produced in soils as the end product of the anaerobic decomposition of organic matter. In the absence of oxygen, methane is very stable, but under aerobic conditions it is mineralized to carbon dioxide by methanotrophic bacteria. Soil methane emissions, primarily from natural wetlands, landfills and rice paddies, are estimated to represent about half of the annual global methane production. Oxidation of atmospheric methane by well-drained soils accounts for about 10% of the global methane sink. Whether a soil is a net source or sink for methane depends on the relative rates of methanogenic and methanotrophic activity. A number of factors including pH, Eh, temperature and moisture content influence methane transforming bacterial populations and soil fluxes. Several techniques are available for measuring methane fluxes. Flux estimation is complicated by spatial and temporal variability. Soil management can impact methane transformations. For example, landfilling of organic matter can result in significant methane emissions, whereas some cultural practices such as nitrogen fertilization inhibit methane oxidation by agricultural soils.

Trettin, C.C., Jurgensen, M.F., Kimble, J.M., Heath, L.S., Birdsey, R.A., Lal, R. (2003). Carbon cycling in wetland forest soils. CRC Press: 311-331

ABSTRACT; Wetlands comprise a small proportion (i.e., 2 to 3%) of earth's terrestrial surface, yet they contain a significant proportion of the terrestrial carbon (C) pool. Soils comprise the largest terrestrial C pool (ca. 1550 Pg C in upper 100 cm; Eswaran et al., 1993; Batjes, 1996), and wetlands contain the single largest component, with estimates ranging between 18 and 30% of the total soil C. In addition to being an important C pool, wetlands contribute approximately 22% of the annual global methane emissions (Bartlett and Harris, 1993; Matthews and Fung, 1987). Despite the importance of wetlands in the global C budget, they are typically omitted from large-scale assessments because of scale, inadequate models, and limited information on C turnover and temporal dynamics.

Trumbore, Susan E. (1997). Potential responses of soil organic carbon to global environmental change. Proceedings Of The National Academy Of Sciences Of The United States Of America 94 (16): 8284-8291.

ABSTRACT: Recent improvements in our understanding of the dynamics of soil carbon have shown that 20–40% of the approximately 1,500 Pg of C stored as organic matter in the upper meter of soils has turnover times of centuries or less. This fast-cycling organic matter is largely comprised of undecomposed plant material and hydrolyzable components associated with mineral surfaces. Turnover times of fast-cycling carbon vary with climate and vegetation, and range from <20 years at low latitudes to >60 years at high latitudes. The amount and turnover time of C in passive soil carbon pools (organic matter strongly stabilized on mineral surfaces with turnover times of millennia and longer) depend on factors like soil maturity and mineralogy, which, in turn, reflect long-term climate conditions. Transient sources or sinks in terrestrial carbon pools result from the time lag between photosynthetic uptake of CO2 by plants and the subsequent return of C to the atmosphere through plant, heterotrophic, and microbial respiration. Differential responses of primary production and respiration to climate change or ecosystem fertilization have the potential to cause significant interrannual to decadal imbalances in terrestrial C storage and release. Rates of carbon storage and release in recently disturbed ecosystems can be much larger than rates in more mature ecosystems. Changes in disturbance frequency and regime resulting from future climate change may be more important than equilibrium responses in determining the carbon balance of terrestrial ecosystems.

Soil carbon inventories and turnover rates are influenced by climate, vegetation, parent material, topography, and time, the fundamental state factors outlined by Jenny (1, 2). Studies attempting to understand the influence of a specific factor (e.g., temperature or moisture) on soil properties have found it useful to identify a suite of soils for which the factor in question varies whereas the others are held constant (1–4). This approach has been used successfully to look at the role of temperature (3, 5) and time (6–10) on the turnover of soil C. Ecosystem models such as century (11, 12), casa (13), or the Rothamsted model (14, 15) predict the sensitivity of soil C inventory and turnover to climate, vegetation, and parent material, but as yet few data exist to test these predictions. Parameterizations of decomposition used in these models are based on empirical fits to specific calibration sites and may not include enough basic understanding of the interaction between plant substrates and the soil environment to make successful predictions in different environments (16).

The reservoir of soil carbon has been proposed as both a significant source and sink of atmospheric CO2 . A soil source results when net decomposition exceeds C inputs to the soil, either as a result of human activities such as clearing forests for agriculture (17, 18) or because of increased decomposition rates due to global warming (12, 14, 19, 20). Net sinks of C in soils are postulated from the difference between net ecosystem C uptake and tree growth rates (21) or from presumed increases in net C inputs from CO2 or N-fertilization of plants (19, 20, 22–24). In both cases, the magnitude and timing of the response depends on the amount of carbon in pools that respond quickly to changes in climate and vegetation, and to the time lag between fixation of C by plants and its subsequent release to the atmosphere during decomposition.

This paper will describe recent approaches used to study soil C dynamics, and preliminary applications of these tools to the problems of soil C response to global environmental changes. The results indicate the importance of the global soil C pool to the global C cycle on interrannual to century time scales and suggest profitable areas for future research.

Miko U. F. Kirschbaum (2006). The temperature dependence of organic-matter decomposition—still a topic of debate. Soil Biology and Biochemistry 38 (9): 2510-2518

ABSTRACT: The temperature dependence of organic matter decomposition is of considerable ecosphysiological importance, especially in the context of possible climate-change feedback effects. It effectively controls whether, or how much, carbon will be released with global warming, and to what extent that release of carbon constitutes a dangerous positive feedback effect that leads to further warming.

The present paper is an invited contribution in a series of Citation Classics based on a review paper of the temperature dependence of organic matter decomposition that was published in 1995. It discusses the context and main findings of the 1995 study, the progress has been made since then and what issues still remain unresolved.

Despite the continuation of much further experimental work and repeated publication of summary articles, there is still no scientific consensus on the temperature dependence of organic matter decomposition. It is likely that this lack of consensus is largely due to different studies referring to different experimental conditions where confounding factors play a greater or lesser role.

Substrate availability is particularly important. If it changes during the course of measurements, it can greatly confound the derived apparent temperature dependence. This confounding effect is illustrated through simulations and examples of experimental work drawn from the literature. The paper speculates that much of the current disagreement between studies might disappear if different studies would ensure that they are all studying the same system attributes, and if confounding factors were always considered and, if possible, eliminated.

R. A. Houghton, F. Hall, S. J. Goetz (2009). Importance of biomass in the global carbon cycle. Journal of Geophysical Research 114 (G00E03): doi:10.1029/2009JG000935

ABSTRACT: Our knowledge of the distribution and amount of terrestrial biomass is based almost entirely on ground measurements over an extremely small, and possibly biased sample, with many regions still unmeasured. Our understanding of changes in terrestrial biomass is even more rudimentary, although changes in land use, largely tropical deforestation, are estimated to have reduced biomass, globally. At the same time, however, the global carbon balance requires that terrestrial carbon storage has increased, albeit the exact magnitude, location, and causes of this residual terrestrial sink are still not well quantified. A satellite mission capable of measuring aboveground woody biomass could help reduce these uncertainties by delivering three products. First, a global map of aboveground woody biomass density would halve the uncertainty of estimated carbon emissions from land use change. Second, an annual, global map of natural disturbances could define the unknown but potentially large proportion of the residual terrestrial sink attributable to biomass recovery from such disturbances. Third, direct measurement of changes in aboveground biomass density (without classification of land cover or carbon modeling) would indicate the magnitude and distribution of at least the largest carbon sources (from deforestation and degradation) and sinks (from woody growth). The information would increase our understanding of the carbon cycle, including better information on the magnitude, location, and mechanisms responsible for terrestrial sources and sinks of carbon. This paper lays out the accuracy, spatial resolution, and coverage required for a satellite mission that would generate these products.

M. Heimann, M. Reichsteins (2008). Terrestrial ecosystem carbon dynamics and climate feedbacks. Nature 451 (17January 2008): 289-292

FIRST PARAGRAPH: It has only been recognized relatively recently that biological processes can control and steer the Earth system in a globally significant way. Terrestrial ecosystems constitute a major player in this respect: they can release or absorb globally relevant greenhouse gases such as carbon dioxide (CO2 ), methane and nitrous oxide, they emit aerosols and aerosol precursors, and they control exchanges of energy, water and momentum between the atmosphere and the land surface. Ecosystems themselves are subject to local climatic conditions, implying a multitude of climate–ecosystem feedbacks that might amplify or dampen regional and global climate change. Of these feedbacks, that between the carbon cycle and climate has recently received much attention. Large quantities of carbon are stored in living vegetation and soil organic matter, and liberation of this carbon into the atmosphere as CO2 or methane would have a serious impact on global climate. By definition, the carbon balance of an ecosystem at any point in time is the difference between its carbon gains and losses. Terrestrial ecosystems gain carbon through photosynthesis and lose it primarily as CO2 through respiration in autotrophs (plants and photosynthetic bacteria) and heterotrophs (fungi, animals and some bacteria), although losses of carbon as volatile organic compounds, methane or dissolved carbon (that is, non-CO2 losses) could also be significant. Quantifying and predicting these carbon-cycle–climate feedbacks is difficult, however, because of the limited understanding of the processes by which carbon and associated nutrients are transformed or recycled within ecosystems, in particular within soils, and exchanged with the overlying atmosphere.

Muller, C., Eickhout, B., Zaehle, S., Bondeau, A., Cramer, W., Lucht, W. (2007). Effects of changes in CO2 , climate, and land use on the carbon balance of the land biosphere during the 21st century. Journal of Geophysical Research-Biogeosciences 112 (G2): 2032

ABSTRACT: We studied the effects of climate and land-use change on the global terrestrial carbon cycle for the 21st century. Using the process-based land biosphere model (LPJmL), we mechanistically simulated carbon dynamics for natural and managed lands (agriculture and forestry) and for land-use change processes. We ran LPJmL with twelve different dynamic land-use patterns and corresponding climate and atmospheric CO2 projections. These input data were supplied from the IMAGE 2.2 implementations of the IPCC-SRES storylines for the A2, B1, and B2 scenarios. Each of these SRES scenarios was implemented under four different assumptions on spatial climate patterns in IMAGE 2.2, resulting in twelve different Earth System projections. Our selection of SRES scenarios comprises deforestation and afforestation scenarios, bounding a broad range of possible land-use change. Projected land-use change under different socio-economic scenarios has profound effects on the terrestrial carbon balance: While climate change and CO2 fertilization cause an additional terrestrial carbon uptake of 105-225 PgC, land-use change causes terrestrial carbon losses of up to 445 PgC by 2100, dominating the terrestrial carbon balance under the A2 and B2 scenarios. Our results imply that the potential positive feedback of the terrestrial biosphere on anthropogenic climate change will be strongly affected by land-use change. Spatiotemporally explicit projections of land-use change and the effects of land management on terrestrial carbon dynamics need additional attention in future research.

Potter, C., Klooster, S., Tan, P., Steinbach, M., Kumar, V., Genovese, V. (2005). Variability in terrestrial carbon sinks over two decades. Part III: South America, Africa, and Asia. Earth Interactions 9: 29

ABSTRACT: Seventeen years (1982 - 98) of net carbon flux predictions for Southern Hemisphere continents have been analyzed, based on a simulation model using satellite observations of monthly vegetation cover. The NASA Carnegie Ames Stanford Approach (CASA) model was driven by vegetation-cover properties derived from the Advanced Very High Resolution Radiometer and radiative transfer algorithms that were developed for the Moderate Resolution Imaging Spectroradiometer ( MODIS). The terrestrial ecosystem flux for atmospheric CO2 for the Amazon region of South America has been predicted between a biosphere source of - 0.17 Pg C per year ( in 1983) and a biosphere sink of + 0.64 Pg C per year (in 1989). The areas of highest variability in net ecosystem production (NEP) fluxes across all of South America were detected in the south-central rain forest areas of the Amazon basin and in southeastern Brazil. Similar levels of variability were recorded across central forested portions of Africa and in the southern horn of East Africa, throughout Indonesia, and in eastern Australia. It is hypothesized that periodic droughts and wildfires associated with four major El Niño events during the 1980s and 1990s have held the net ecosystem carbon sink for atmospheric CO2 in an oscillating pattern of a 4-6-yr cycle, despite observations of increasing net plant carbon fixation over the entire 17-yr time period.

Raupach, M. R., Rayner, P. J., Barrett, D. J., Defries, R. S., Heimann, M., Ojima, D. S., Quegan, S., Schmullius, C. C. (2005). Model-data synthesis in terrestrial carbon observation: methods, data requirements and data uncertainty specifications. Global Change Biology 11 (3): 378-397

ABSTRACT: Systematic, operational, long-term observations of the terrestrial carbon cycle (including its interactions with water, energy and nutrient cycles and ecosystem dynamics) are important for the prediction and management of climate, water resources, food resources, biodiversity and desertification. To contribute to these goals, a terrestrial carbon observing system requires the synthesis of several kinds of observation into terrestrial biosphere models encompassing the coupled cycles of carbon, water, energy and nutrients. Relevant observations include atmospheric composition (concentrations of CO2 and other gases); remote sensing; flux and process measurements from intensive study sites; in situ vegetation and soil monitoring; weather, climate and hydrological data; and contemporary and historical data on land use, land use change and disturbance (grazing, harvest, clearing, fire). A review of model-data synthesis tools for terrestrial carbon observation identifies 'nonsequential' and 'sequential' approaches as major categories, differing according to whether data are treated all at once or sequentially. The structure underlying both approaches is reviewed, highlighting several basic commonalities in formalism and data requirements. An essential commonality is that for all model-data synthesis problems, both nonsequential and sequential, data uncertainties are as important as data values themselves and have a comparable role in determining the outcome. Given the importance of data uncertainties, there is an urgent need for soundly based uncertainty characterizations for the main kinds of data used in terrestrial carbon observation. The first requirement is a specification of the main properties of the error covariance matrix. As a step towards this goal, semi-quantitative estimates are made of the main properties of the error covariance matrix for four kinds of data essential for terrestrial carbon observation: remote sensing of land surface properties, atmospheric composition measurements, direct flux measurements, and measurements of carbon stores.

Silver, W. L. (1998). The potential effects of elevated CO2 and climate change on tropical forests soils and biogeochemical cycling. Climatic Change 39 (2-3): 337-361

ABSTRACT: Tropical forests are responsible for a large proportion of the global terrestrial C flux annually for natural ecosystems. Increased atmospheric CO2 and changes in climate are likely to affect the distribution of C pools in the tropics and the rate of cycling through vegetation and soils. In this paper, I review the literature on the pools and fluxes of carbon in tropical forests, and the relationship of these to nutrient cycling and climate. Tropical moist and humid forests have the highest rates of annual net primary productivity and the greatest carbon flux from soil respiration globally. Tropical dry forests have lower rates of carbon circulation, but may have greater soil organic carbon storage, especially at depths below 1 meter. Data from tropical elevation gradients were used to examine the sensitivity of biogeochemical cycling to incremental changes in temperature and rainfall. These data show significant positive correlations of litterfall N concentrations with temperature and decomposition rates. Increased atmospheric CO2 and changes in climate are expected to alter carbon and nutrient allocation patterns and storage in tropical forest. Modeling and experimental studies suggest that even a small increase in temperature and CO2 concentrations results in more rapid decomposition rates, and a large initial CO2 efflux from moist tropical soils. Soil P limitation or reductions in C:N and C:P ratios of litterfall could eventually limit the size of this flux. Increased frequency of fires in dry forest and hurricanes in moist and humid forests are expected to reduce the ecosystem carbon storage capacity over longer time periods

Tao, Z. N., Jain, A. K. (2005). Modeling of global biogenic emissions for key indirect greenhouse gases and their response to atmospheric CO2 increases and changes in land cover and climate. Journal of Geophysical Research 110 (D21309): doi:10.1029/2005JD005874

ABSTRACT: [1] Natural emissions of nonmethane volatile organic compounds (NMVOCs) play a crucial role in the oxidation capacity of the lower atmosphere and changes in concentrations of major greenhouse gases (GHGs), particularly methane and tropospheric ozone. In this study, we integrate a global biogenic model within a terrestrial ecosystem model to investigate the vegetation and soil emissions of key indirect GHGs, e. g., isoprene, monoterpene, other NMVOCs (OVOC), CO, and NOx. The combination of a high-resolution terrestrial ecosystem model with satellite data allows investigation of the potential changes in net primary productivity (NPP) and resultant biogenic emissions of indirect GHGs due to atmospheric CO2 increases and changes in climate and land use practices. Estimated global total annual vegetation emissions for isoprene, monoterpene, OVOC, and CO are 601, 103, 102, and 73 Tg C, respectively. Estimated NOx emissions from soils are 7.51 Tg N. The land cover changes for croplands generally lead to a decline of vegetation emissions for isoprene OVOC, whereas temperature and atmospheric CO2 increases lead to higher vegetation emissions. The modeled global mean isoprene emissions show relatively large seasonal variations over the previous 20 years from 1981 to 2000 (as much as 31% from year to year). Savanna and boreal forests show large seasonal variations, whereas tropical forests with high plant productivity throughout the year show small seasonal variations. Results of biogenic emissions from 1981 to 2000 indicate that the CO2 fertilization effect, along with changes in climate and land use, causes the overall up-trend in isoprene and OVOC emissions over the past 2 decades. This relationship suggests that future emission scenario estimations for NMVOCs should account for effects of CO2 and climate in order to more accurately estimate local, regional, and global chemical composition of the atmosphere, the global carbon budget, and radiation balance of the Earth-atmosphere system.

Torn, M. S., Trumbore, S. E., Chadwick, O. A., Vitousek, P. M., Hendricks, D. M. (1997). Mineral control of soil organic carbon storage and turnover. Nature 389 (6647): 170-173

ABSTRACT: A large source of uncertainty in present understanding of the global carbon cycle is the distribution and dynamics of the soil organic carbon reservoir. Most of the organic carbon in soils is degraded to inorganic forms slowly, on timescales from centuries to millennia. Soil minerals are known to play a stabilizing role, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long-residence-time organic carbon is not well known. Here we use radiocarbon analyses to explore interactions between soil mineralogy and soil organic carbon along two natural gradients - of soil-age and of climate - in volcanic soil environments. During the first approximates 150,000 years of soil development, the volcanic parent material weathered to metastable, non-crystalline minerals. Thereafter, the amount of non-crystalline minerals declined, and more stable crystalline minerals accumulated. Soil organic carbon content followed a similar trend, accumulating to a maximum after 150,000 years, and then decreasing by 50% over the next four million years. A positive relationship between noncrystalline minerals and organic carbon was also observed in soils through the climate gradient, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere-ecosystem carbon fluxes during long-term soil development; this conclusion should be generalizable at least to other humid environments

Van Oost, K., Quine, T. A., Govers, G., De Gryze, S., Six, J., Harden, J. W., Ritchie, J. C., McCarty, G. W., Heckrath, G., Kosmas, C., Giraldez, J. V., da Silva, J. R. Marques, Merckx, R. (2007). The impact of agricultural soil erosion on the global carbon cycle. Science 318 (5850): 626-629

ABSTRACT: Agricultural soil erosion is thought to perturb the global carbon cycle, but estimates of its effect range from a source of 1 petagram per year1 to a sink of the same magnitude. By using Caesium-137 and carbon inventory measurements from a large-scale survey, we found consistent evidence for an erosion-induced sink of atmospheric carbon equivalent to approximately 26% of the carbon transported by erosion. Based on this relationship, we estimated a global carbon sink of 0.12 (range 0.06 to 0.27) petagrams of carbon per year resulting from erosion in the world's agricultural landscapes. Our analysis directly challenges the view that agricultural erosion represents an important source or sink for atmospheric CO2 .

P. A. Beedlow, D. T. Tingey, D.L. Phillips, W. E. Hogsett, D. M. Olszyk (2004). Rising atmospheric CO2 and carbon sequestration in forests. Frontiers in Ecology and Environment 2 (6): 315-322

ABSTRACT: Rising CO2 concentrations in the atmosphere could alter Earth's climate system, but it is thought that higher concentrations may improve plant growth through a process known as the “fertilization effect”. Forests are an important part of the planet's carbon cycle, and sequester a substantial amount of the CO2 released into the atmosphere by human activities. Many people believe that the amount of carbon sequestered by forests will increase as CO2 concentrations rise. However, an increasing body of research suggests that the fertilization effect is limited by nutrients and air pollution, in addition to the well documented limitations posed by temperature and precipitation. This review suggests that existing forests are not likely to increase sequestration as atmospheric CO2 increases. It is imperative, therefore, that we manage forests to maximize carbon retention in above- and belowground biomass and conserve soil carbon.

P. M. Cox, R. A. Betts, A. Betts, C. D. Jones, S. A. Spall, I. J. Totterdell (2002). Modelling vegetation and the carbon cycle as interactive elements of the climate system. International Geophysics 83: 259-279

ABSTRACT: The climate system and the global carbon cycle are tightly coupled. Atmospheric carbon in the form of the radiatively active gases, carbon dioxide and methane, plays a significant role in the natural greenhouse effect. The continued increase in the atmospheric concentrations of these gases, due to human emissions, is predicted to lead to significant climatic change over the next 100 years. The best estimates suggest that more than half of the current anthropogenic emissions of carbon dioxide are being absorbed by the ocean and by land ecosystems (Schimel et al., 1995). In both cases the processes involved are sensitive to the climatic conditions. Temperature affects the solubility of carbon dioxide in sea water and the rate of terrestrial and oceanic biological processes. In addition, vegetation is known to respond directly to increased atmospheric CO2 through increased photosynthesis and reduced transpiration (Sellers et al., 1996a; Field et al., 1995), and may also change its structure and distribution in response to any associated climate change (Betts et al., 1997). Thus there is great potential for the biosphere to produce a feedback on the climatic change due to given human emissions.

Despite this, simulations carried out with General Circulation Models (GCMs) have generally neglected the coupling between the climate and the biosphere. Indeed, vegetation distributions have been static and atmospheric concentrations of CO2 have been prescribed based on results from simple carbon cycle models, which neglect the effects of climate change (Enting et al., 1994). This chapter describes the inclusion of vegetation and the carbon cycle as interactive elements in a GCM. The coupled climate-carbon cycle model is able to reproduce key aspects of the observations, including the global distribution of vegetation types, seasonal and zonal variations in ocean primary production, and the interannual variability in atmospheric CO2 . A transient simulation carried out with this model suggests that previously-neglected climate-carbon cycle feedbacks could significantly accelerate atmospheric CO2 rise and climate change over the twenty-first century.

W. Cramer, A. Bondeau, F. I. Woodward, I. C. Prentice, R. A. Betts, V. Brovkin, P. M. Cox, V. Fisher, J. A. Foley, A. D. Friend, C. Kucharik, M. R. Lomas, N. Ramankutty, S. Sitch, B. Smith, A. White, C. Young-Molling (2001). Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models. Global Change Biology 7 (4): 357-373

ABSTRACT: The possible responses of ecosystem processes to rising atmospheric CO2 concentration and climate change are illustrated using six dynamic global vegetation models that explicitly represent the interactions of ecosystem carbon and water exchanges with vegetation dynamics. The models are driven by the IPCC IS92a scenario of rising CO2 (Wigley et al. 1991), and by climate changes resulting from effective CO2 concentrations corresponding to IS92a, simulated by the coupled ocean atmosphere model HadCM2-SUL. Simulations with changing CO2 alone show a widely distributed terrestrial carbon sink of 1.4–3.8 Pg C y−1 during the 1990s, rising to 3.7–8.6 Pg C y−1 a century later. Simulations including climate change show a reduced sink both today (0.6–3.0 Pg C y−1 ) and a century later (0.3–6.6 Pg C y−1 ) as a result of the impacts of climate change on NEP of tropical and southern hemisphere ecosystems. In all models, the rate of increase of NEP begins to level off around 2030 as a consequence of the 'diminishing return' of physiological CO2 effects at high CO2 concentrations. Four out of the six models show a further, climate-induced decline in NEP resulting from increased heterotrophic respiration and declining tropical NPP after 2050. Changes in vegetation structure influence the magnitude and spatial pattern of the carbon sink and, in combination with changing climate, also freshwater availability (runoff). It is shown that these changes, once set in motion, would continue to evolve for at least a century even if atmospheric CO2 concentration and climate could be instantaneously stabilized. The results should be considered illustrative in the sense that the choice of CO2 concentration scenario was arbitrary and only one climate model scenario was used. However, the results serve to indicate a range of possible biospheric responses to CO2 and climate change. They reveal major uncertainties about the response of NEP to climate change resulting, primarily, from differences in the way that modelled global NPP responds to a changing climate. The simulations illustrate, however, that the magnitude of possible biospheric influences on the carbon balance requires that this factor is taken into account for future scenarios of atmospheric CO2 and climate change.

S. Del Grosso, W. Parton, T. Stohlgren, D. Zheng, D. Bachelet, S. Prince, K. Hibbard, R. Olson (2008). Global net primary production predicted from vegetation class, precipitation, and temperature. Ecology 89 (8): 2117-2126

ABSTRACT: Net primary production (NPP), the difference between CO2 fixed by photosynthesis and CO2 lost to autotrophic respiration, is one of the most important components of the carbon cycle. Our goal was to develop a simple regression model to estimate global NPP using climate and land cover data. Approximately 5600 global data points with observed mean annual NPP, land cover class, precipitation, and temperature were compiled. Precipitation was better correlated with NPP than temperature, and it explained much more of the variability in mean annual NPP for grass- or shrub-dominated systems (r2 = 0.68) than for tree-dominated systems (r2 = 0.39). For a given precipitation level, tree-dominated systems had significantly higher NPP (100–150 g C·m−2 ·yr−1 ) than non-tree-dominated systems. Consequently, previous empirical models developed to predict NPP based on precipitation and temperature (e.g., the Miami model) tended to overestimate NPP for non-tree-dominated systems. Our new model developed at the National Center for Ecological Analysis and Synthesis (the NCEAS model) predicts NPP for tree-dominated systems based on precipitation and temperature; but for non-tree-dominated systems NPP is solely a function of precipitation because including a temperature function increased model error for these systems. Lower NPP in non-tree-dominated systems is likely related to decreased water and nutrient use efficiency and higher nutrient loss rates from more frequent fire disturbances. Late 20th century aboveground and total NPP for global potential native vegetation using the NCEAS model are estimated to be 28 Pg and 46 Pg C/yr, respectively. The NCEAS model estimated an 13% increase in global total NPP for potential vegetation from 1901 to 2000 based on changing precipitation and temperature patterns.

R.A. Houghton (2007). Balancing the global carbon budget. Annual Review of Earth and Planetary Sciences 35: 313-347

ABSTRACT: The global carbon budget is, of course, balanced. The conservation of carbon and the first law of thermodynamics are intact. “Balancing the carbon budget” refers to the state of the science in evaluating the terms of the global carbon equation. The annual increases in the amount of carbon in the atmosphere, oceans, and land should balance the emissions of carbon from fossil fuels and deforestation. Balancing the carbon budget is not the real issue, however. The real issue is understanding the processes responsible for net sources and sinks of carbon. Such understanding should lead to more accurate predictions of future concentrations of CO2 and more accurate predictions of the rate and extent of climatic change. The recent past may be insufficient for prediction, however. Oceanic and terrestrial sinks that have lessened the rate of growth in atmospheric CO2 until now may diminish as feedbacks between the carbon cycle and climate become more prominent.

R. Lal (2004). Soil carbon sequestration to mitigate climate change. Geoderma 123 (1-2): 1-22

ABSTRACT: The increase in atmospheric concentration of CO2 by 31% since 1750 from fossil fuel combustion and land use change necessitates identification of strategies for mitigating the threat of the attendant global warming. Since the industrial revolution, global emissions of carbon (C) are estimated at 270±30 Pg (Pg=petagram=1015 G=1 billion ton) due to fossil fuel combustion and 136±55 Pg due to land use change and soil cultivation. Emissions due to land use change include those by deforestation, biomass burning, conversion of natural to agricultural ecosystems, drainage of wetlands and soil cultivation. Depletion of soil organic C (SOC) pool have contributed 78±12 Pg of C to the atmosphere. Some cultivated soils have lost one-half to two-thirds of the original SOC pool with a cumulative loss of 30–40 Mg C/ha (Mg=megagram=106 G=1 ton). The depletion of soil C is accentuated by soil degradation and exacerbated by land misuse and soil mismanagement. Thus, adoption of a restorative land use and recommended management practices (RMPs) on agricultural soils can reduce the rate of enrichment of atmospheric CO2 while having positive impacts on food security, agro-industries, water quality and the environment. A considerable part of the depleted SOC pool can be restored through conversion of marginal lands into restorative land uses, adoption of conservation tillage with cover crops and crop residue mulch, nutrient cycling including the use of compost and manure, and other systems of sustainable management of soil and water resources. Measured rates of soil C sequestration through adoption of RMPs range from 50 to 1000 kg/ha/year. The global potential of SOC sequestration through these practices is 0.9±0.3 Pg C/year, which may offset one-fourth to one-third of the annual increase in atmospheric CO2 estimated at 3.3 Pg C/year. The cumulative potential of soil C sequestration over 25–50 years is 30–60 Pg. The soil C sequestration is a truly win–win strategy. It restores degraded soils, enhances biomass production, purifies surface and ground waters, and reduces the rate of enrichment of atmospheric CO2 by offsetting emissions due to fossil fuel.

Luo, Y. (2007). Terrestrial carbon-cycle feedback to climate warming. Annual Review of Ecology, Evolution, and Systematics 38: 683-712

ABSTRACT: The coupled carbon-climate models reported in the literature all demonstrate a positive feedback between terrestrial carbon cycles and climate warming. A primary mechanism underlying the modeled positive feedback is the kinetic sensitivity of photosynthesis and respiration to temperature. Field experiments, however, suggest much richer mechanisms driving ecosystem responses to climate warming, including extended growing seasons, enhanced nutrient availability, shifted species composition, and altered ecosystem-water dynamics. The diverse mechanisms likely define more possibilities of carbon-climate feedbacks than projected by the kinetics-based models. Nonetheless, experimental results are so variable that we have not generated the necessary insights on ecosystem responses to effectively improve global models. To constrain model projections of carbon-climate feedbacks, we need more empirical data from whole ecosystem warming experiments across a wide range of biomes, particularly in tropic regions, and closer interactions between models and experiments.

Nemani, R., M. White, P. Thornton, K. Nishida, S. Reddy, J. Jenkins, S. Running (2002). Recent trends in hydrologic balance have enhanced the terrestrial carbon sink in the United States. Geophysical Research Letters 29 (10): doi:10.1029/2002GL014867

ABSTRACT: Climate data show significant increases in precipitation and humidity over the U.S. since 1900, yet the role of these hydro-climatic changes on the reported U.S. carbon sink is incompletely understood. Using a prognostic terrestrial ecosystem model, we simulated 1900–1993 continental U.S. carbon fluxes and found that increased growth by natural vegetation was associated with increased precipitation and humidity, especially during the 1950–1993 period. CO2 trends and warmer temperatures had a lesser effect. Two thirds of the increase in observed forest growth rates could be accounted for by observed climatic changes, including the confluence of earlier springs and wetter autumns leading to a lengthening of the vegetation carbon uptake period. However, regional differences in precipitation trends produced differing regional carbon sink responses. The strong coupling between carbon and hydrologic cycles implies that global carbon budget studies, currently dominated by temperature analyses, should consider changes in the hydrologic cycle.

I. Y. Fung, S. C. Doney, K. Lindsay, J. John (2005). Evolution of carbon sinks in a changing climate. Proceedings of the National Academy of Sciences 102 (32): 11201-11206

ABSTRACT: Climate change is expected to influence the capacities of the land and oceans to act as repositories for anthropogenic CO2 and hence provide a feedback to climate change. A series of experiments with the National Center for Atmospheric Research–Climate System Model 1 coupled carbon–climate model shows that carbon sink strengths vary with the rate of fossil fuel emissions, so that carbon storage capacities of the land and oceans decrease and climate warming accelerates with faster CO2 emissions. Furthermore, there is a positive feedback between the carbon and climate systems, so that climate warming acts to increase the airborne fraction of anthropogenic CO2 and amplify the climate change itself. Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses. Analysis of our results in the context of comparable models suggests that destabilization of the tropical land sink is qualitatively robust, although its degree is uncertain.

D.A. Semenov (2008). Biological contribution to global climate dynamics on a “geological” timescale. Ecological Modelling 212 (1-2): 171-177

ABSTRACT: A model consisting of two blocks (equation) is proposed for the analytical study of the “biosphere—climate” system over a great period of time. The first equation describes the balance of carbon dioxide in the atmosphere and represents the biological block of the model. The second equation is the equation of energy balance or the physical block of the system. The model is based on the most general concepts of living matter and the evolution process. A possible interpretation of some events and phenomena in the Earth history is given in terms of the model.

J. A. Aitkenhead, W. H. McDowell (2000). Soil C:N ratio as a predictor of annual riverine DOC flux at local and global scales. Global Biogeochemical Cycles 14 (1): 127-138

ABSTRACT; Dissolved organic carbon (DOC) is important in a wide variety of chemical, physical, and biological processes in surface waters. We examined the relationship between DOC flux and soil C:N ratio on a biome basis. DOC fluxes for 164 rivers were subdivided into 15 biome types including tropical rain forest, coniferous forests, peatland, deciduous forests, mixed forests, and grasslands. A database of soil C:N ratios was constructed and subdivided into biome types. At a global scale, mean soil C:N ratio of a biome accounts for 99.2% of the variance in annual riverine DOC flux among biomes. The relationship between soil C:N ratio and DOC flux at the biome scale was used to predict annual riverine DOC flux at the watershed scale for three test watersheds not included in the original model. Predicted flux of each watershed was within 4.5% of the actual DOC flux. Using the C:N model, we estimated the total export of carbon from land to the oceans to be 3.6×1014 g yr−1 . This empirical model should be useful in predicting changes in DOC flux under changing climatic conditions.

S. T. Gower (2003). Patterns and mechanisms of the forest carbon cycle. Annual Review of Environment and Resources 28: 169-204

ABSTRACT: Forests are an important source for fiber and fuel for humans and contain the majority of the total terrestrial carbon (C). The amount of C stored in the vegetation and soil are strongly influenced by environmental constraints on annual C uptake and decomposition and time since disturbance. Increasing concentrations of atmospheric carbon dioxide (CO2 ), nitrogen deposition, and climate warming induced by greater greenhouse gas (GHG) concentrations in the atmosphere influence C accumulation rates of forests, but their effects will likely differ in direction and magnitude among forest ecosystems. The net interactive effect of global change on the forest C cycle is poorly understood. The growing demand for wood fiber and fuel by humans and the ongoing anthropogenic perturbations of the climate have changed the natural disturbance regimes (i.e., frequency and intensity); these changes influence the net exchange of CO2 between forests and the atmosphere. To date, the role of forest products in the global C cycle have largely been ignored, and important emissions associated with the production, transport, and utilization of the forest products have been excluded, leading to erroneous conclusions about net C storage in forest products.

M. Berthelot, P. Friedlingstein, P. Ciais, J.-L. Dufresne, P. Monfray (2005). How uncertainties in future climate change predictions translate into future terrestrial carbon fluxes. Global Change Biology 11 (6): 959-970

ABSTRACT: We forced a global terrestrial carbon cycle model by climate fields of 14 ocean and atmosphere general circulation models (OAGCMs) to simulate the response of terrestrial carbon pools and fluxes to climate change over the next century. These models participated in the second phase of the Coupled Model Intercomparison Project (CMIP2), where a 1% per year increase of atmospheric CO2 was prescribed. We obtain a reduction in net land uptake because of climate change ranging between 1.4 and 5.7 Gt C yr−1 at the time of atmospheric CO2 doubling. Such a reduction in terrestrial carbon sinks is largely dominated by the response of tropical ecosystems, where soil water stress occurs. The uncertainty in the simulated land carbon cycle response is the consequence of discrepancies in land temperature and precipitation changes simulated by the OAGCMs. We use a statistical approach to assess the coherence of the land carbon fluxes response to climate change. The biospheric carbon fluxes and pools changes have a coherent response in the tropics, in the Mediterranean region and in high latitudes of the Northern Hemisphere. This is because of a good coherence of soil water content change in the first two regions and of temperature change in the high latitudes of the Northern Hemisphere.

Then we evaluate the carbon uptake uncertainties to the assumptions on plant productivity sensitivity to atmospheric CO2 and on decomposition rate sensitivity to temperature. We show that these uncertainties are on the same order of magnitude than the uncertainty because of climate change. Finally, we find that the OAGCMs having the largest climate sensitivities to CO2 are the ones with the largest soil drying in the tropics, and therefore with the largest reduction of carbon uptake.

Houghton, R. A. (2000). Interannual variability in the global carbon cycle. Journal of Geophysical Research 105 (D15): 20,121 - 20,130

ABSTRACT: The annual growth rate of atmospheric CO2 has varied between 1 and 5 Pg C yr−1 over the last decades. Most of this variation is associated with terrestrial and oceanic exchanges of carbon which seem to vary independently. Three processes contribute to the annual flux of carbon from terrestrial ecosystems: changes in land use, natural disturbances, and metabolic changes caused by variations in climate. Because rates of land-use change are often not available on an annual basis, estimates of the flux of carbon attributable to land use change may underestimate year-to-year variability. Limited data reviewed here suggest that the interannual variability of this flux is generally small for two reasons. First, although rates of land use change may vary substantially from year to year at a local scale, variability is generally less at regional and global scales because high rates of deforestation in one area do not necessarily coincide with high rates in other areas. Second, less than 50% of the carbon lost to the atmosphere as a result of land use change is lost in the year of disturbance; the rest is released in subsequent years. The interannual variability of the flux of carbon from land use change is thus less variable than rates of land use change and probably accounts, globally, for less than 5–10% of the observed variation in the annual growth rate of CO2 in the atmosphere. Natural disturbances are estimated to account for a similar fraction of the variation. The most important contributor appears to be the effect of short-term changes in climate (temperature and precipitation) on terrestrial metabolism. Over the period 1980–1995, year-to-year differences in the flux of carbon from terrestrial metabolism have almost been as large as variations in the growth rate of atmospheric CO2 .

J. L. Sarmiento, M. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. M. Fletcher, S. Pacala, K. Rodgers (2009). Trends and regional distributions of land and ocean carbon sinks. Biogeosciences Discussions 6 (6): 10583-10624

ABSTRACT: We show here a new estimate of the variability and long-term trends in the net land carbon sink from 1960 onwards calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. The net land carbon sink appears to have increased by −0.88 (−0.77 to −1.04) Pg C yr−1 after ~1988/1989 from a relatively constant mean of −0.27 Pg C yr−1 before then to −1.15 Pg C yr−1 thereafter (the sign convention is negative out of the atmosphere). This result is significant at the 1% critical level. The increase in net land uptake is partially compensated by a reduction in the expected oceanic uptake, which we estimate from model simulations as about 0.35 (0.26 to 0.49) Pg C yr−1 . This implies that the atmospheric growth rate must have decreased by about −0.53 (−0.51 to −0.55) Pg C yr−1 (equivalent to −0.25 ppm yr−1 ) below what would have been projected if the ocean uptake had continued to grow at the rate expected from a constant climate model and if the net land uptake had continued at its pre-1988/1989 level. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the northern hemisphere land is a major sink of atmospheric CO2 , but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

R. G. Zepp, Erickson, D.J., III, N. D. Paul, B. Sulzberger (2007). Interactive effects of solar UV radiation and climate change on biogeochemical cycling. Photochemical & Photobiological Sciences 6 (3): 286-300

ABSTRACT: This report assesses research on the interactions of UV radiation (280–400 nm) and global climate change with global biogeochemical cycles at the Earth's surface. The effects of UV-B (280–315 nm), which are dependent on the stratospheric ozone layer, on biogeochemical cycles are often linked to concurrent exposure to UV-A radiation (315–400 nm), which is influenced by global climate change. These interactions involving UV radiation (the combination of UV-B and UV-A) are central to the prediction and evaluation of future Earth environmental conditions. There is increasing evidence that elevated UV-B radiation has significant effects on the terrestrial biosphere with implications for the cycling of carbon, nitrogen and other elements. The cycling of carbon and inorganic nutrients such as nitrogen can be affected by UV-B-mediated changes in communities of soil organisms, probably due to the effects of UV-B radiation on plant root exudation and/or the chemistry of dead plant material falling to the soil. In arid environments direct photodegradation can play a major role in the decay of plant litter, and UV-B radiation is responsible for a significant part of this photodegradation. UV-B radiation strongly influences aquatic carbon, nitrogen, sulfur and metals cycling that affect a wide range of life processes. UV-B radiation changes the biological availability of dissolved organic matter to microorganisms, and accelerates its transformation into dissolved inorganic carbon and nitrogen, including carbon dioxide and ammonium. The coloured part of dissolved organic matter (CDOM) controls the penetration of UV radiation into water bodies, but CDOM is also photodegraded by solar UV radiation. Changes in CDOM influence the penetration of UV radiation into water bodies with major consequences for aquatic biogeochemical processes. Changes in aquatic primary productivity and decomposition due to climate-related changes in circulation and nutrient supply occur concurrently with exposure to increased UV-B radiation, and have synergistic effects on the penetration of light into aquatic ecosystems. Future changes in climate will enhance stratification of lakes and the ocean, which will intensify photodegradation of CDOM by UV radiation. The resultant increase in the transparency of water bodies may increase UV-B effects on aquatic biogeochemistry in the surface layer. Changing solar UV radiation and climate also interact to influence exchanges of trace gases, such as halocarbons (e.g., methyl bromide) which influence ozone depletion, and sulfur gases (e.g., dimethylsulfide) that oxidize to produce sulfate aerosols that cool the marine atmosphere. UV radiation affects the biological availability of iron, copper and other trace metals in aquatic environments thus potentially affecting metal toxicity and the growth of phytoplankton and other microorganisms that are involved in carbon and nitrogen cycling. Future changes in ecosystem distribution due to alterations in the physical and chemical climate interact with ozone-modulated changes in UV-B radiation. These interactions between the effects of climate change and UV-B radiation on biogeochemical cycles in terrestrial and aquatic systems may partially offset the beneficial effects of an ozone recovery.

J. Grace (2004). Understanding and managing the global carbon cycle. Journal of Ecology 92 (2): 189-202

ABSTRACT: 1 Biological carbon sinks develop in mature ecosystems that have high carbon storage when these systems are stimulated to increase productivity, so that carbon gains by photosynthesis run ahead of carbon losses by heterotrophic respiration, and the stocks of carbon therefore increase. This stimulation may occur through elevated CO2 concentration, nitrogen deposition or by changes in climate.
2 Sinks also occur during the 'building' phase of high carbon ecosystems, for example following establishment of forests by planting.
3 New methods have been developed to identify biological carbon sinks: ground based measurements using eddy covariance coupled with inventory methods, atmospheric methods which rely on repeated measurement of carbon dioxide concentrations in a global network, and mathematical models which simulate the processes of production, storage and decomposition of organic matter. There is broad agreement among the results from these methods: carbon sinks are currently found in tropical, temperate and boreal forests as well as the ocean.
4 However, on a global scale the effect of the terrestrial sinks (absorbing 2–3 billion tonnes of carbon per year) is largely offset by deforestation in the tropics (losing 1–2 billion tonnes of carbon per year).
5 The Kyoto Protocol provides incentives for the establishment of sinks. Unfortunately, it does not provide an incentive to protect existing mature ecosystems which constitute both stocks of carbon and (currently) carbon sinks.
6 Incentives would be enhanced, if protection and nature conservation were to be part of any international agreement relating to carbon sinks.

P. Falkowski, R. J. Scholes, E. Boyle, J. Canadell, D. Canfield, J. Elser, N. Gruber, K. Hibbard, P. Högberg, S. Linder, F. T. Mackenzie, Moore, B., III, T. Pedersen, Y. Rosenthal, S. Seitzinger, V. Smetacek, W. Steffen (2000). The global carbon cycle: a test of our knowledge of Earth as a system. Science 290 (5490): 291-296

ABSTRACT: Motivated by the rapid increase in atmospheric CO2 due to human activities since the Industrial Revolution, several international scientific research programs have analyzed the role of individual components of the Earth system in the global carbon cycle. Our knowledge of the carbon cycle within the oceans, terrestrial ecosystems, and the atmosphere is sufficiently extensive to permit us to conclude that although natural processes can potentially slow the rate of increase in atmospheric CO2 , there is no natural "savior" waiting to assimilate all the anthropogenically produced CO2 in the coming century. Our knowledge is insufficient to describe the interactions between the components of the Earth system and the relationship between the carbon cycle and other biogeochemical and climatological processes. Overcoming this limitation requires a systems approach.

B. Bolin, B. R.Döös, J. Jäger, R.A. Warrick (1986). The greenhouse, climatic change and ecosystems. Island Press: 574 pp.

EXECUTIVE SUMMARY (partial): The amounts of some trace gases in the atmosphere, notably carbon dioxide (CO2 ), nitrous oxide (N2 O), methane (CH4 ), chlorofluorocarbons and tropospheric ozone, have been increasing. All of these gases are transparent to incoming short-wave radiation, but they absorb and emit long-wave radiation and are thus able to influence the Earth's climate. They are referred to in this report as greenhouse gases.

Increased concentrations of CO2 and other greenhouse gases lead to a warming of the Earth's surface and the lower atmosphere. The resulting changes in climate and their impacts (e.g. on sea level, agriculture and forestry) can be estimated without associating the origin of the warming to anyone of these gases specifically. It is, however, necessary to study the effects of these greenhouse gases separately in order to estimate their relative contributions to the warming at any given time and, consequently, to develop strategies for reducing their possible harmful effects.

A review of previous assessments of the CO2 problem shows that there are agreements on some basic issues. The net emissions of CO2 from the biota (due to deforestation and land use changes) in themselves will be insufficient to cause a significant change of climate, while fossil fuel reserves are large enough for climatic changes to occur if these reserves continue to be exploited at a high rate in the future.

Generally it has also been agreed that regional patterns of climatic change cannot yet be predicted. Thus, the ways in which higher CO2 concentrations and given changes in climate would affect ecosystems and human activities cannot be predicted either. This is presumably one of the main reasons why there has been substantial disagreement among previous studies regarding recommendations for future action.

Falkowski, P., R. J. Scholes, E. Boyle, J. Canadell, D. Canfield, J. Elser, N. Gruber, K. Hibbard, P. Högberg, S. Linder, F. T. Mackenzie, B. Moore, III, T. Pedersen, Y. Rosenthal, S. Seitzinger, V. Smetacek, W. Steffen (2000). The global carbon cycle: a test of of knowledge of earth as a system. Science 290 (5490): 291-296

ABSTRACT: Motivated by the rapid increase in atmospheric CO2 due to human activities since the Industrial Revolution, several international scientific research programs have analyzed the role of individual components of the Earth system in the global carbon cycle. Our knowledge of the carbon cycle within the oceans, terrestrial ecosystems, and the atmosphere is sufficiently extensive to permit us to conclude that although natural processes can potentially slow the rate of increase in atmospheric CO2 , there is no natural "savior" waiting to assimilate all the anthropogenically produced CO2 in the coming century. Our knowledge is insufficient to describe the interactions between the components of the Earth system and the relationship between the carbon cycle and other biogeochemical and climatological processes. Overcoming this limitation requires a systems approach.

M. Cao, F. I. Woodward (1998). Dynamic responses of terrestrial ecosystem carbon cycling to global climate change. Nature 393 (21 May): 249-252

ABSTRACT: Terrestrial ecosystems and the climate system are closely coupled, particularly by cycling of carbon between vegetation, soils and the atmosphere. It has been suggested1,2 that changes in climate and in atmospheric carbon dioxide concentrations have modified the carbon cycle so as to render terrestrial ecosystems as substantial carbon sinks3,4 ; but direct evidence for this is very limited5,6 . Changes in ecosystem carbon stocks caused by shifts between stable climate states have been evaluated7,8 , but the dynamic responses of ecosystem carbon fluxes to transient climate changes are still poorly understood. Here we use a terrestrial biogeochemical model9 , forced by simulations of transient climate change with a general circulation model10 , to quantify the dynamic variations in ecosystem carbon fluxes induced by transient changes in atmospheric CO2 and climate from 1861 to 2070. We predict that these changes increase global net ecosystem production significantly, but that this response will decline as the CO2 fertilization effect becomes saturated and is diminished by changes in climatic factors. Thus terrestrial ecosystem carbon fluxes both respond to and strongly influence the atmospheric CO2 increase and climate change.

A. Ridgwell, R. E. Zeebe (2005). The role of the global carbonate cycle in the regulation and evolution of the Earth system. Earth and Planetary Science Letters 234 (3-4): 299-315

ABSTRACT: We review one of the most ancient of all the global biogeochemical cycles and one which reflects the profound geochemical and biological changes that have occurred as the Earth system has evolved through time—that of calcium carbonate (CaCO3 ). In particular, we highlight a Mid-Mesozoic Revolution in the nature and location of carbonate deposition in the marine environment, driven by the ecological success of calcareous plankton. This drove the creation of a responsive deep-sea sedimentary sink of CaCO3 . The result is that biologically driven carbonate deposition provides a significant buffering of ocean chemistry and of atmospheric CO2 in the modern system. However, the same calcifying organisms that under-pin the deep-sea carbonate sink are now threatened by the continued atmospheric release of fossil fuel CO2 and increasing acidity of the surface ocean. We are not yet in a position to predict what the impact on CaCO3 production will be, or how the uptake of fossil fuel CO2 by the ocean will be affected. This uncertainty in the future trajectory of atmospheric CO2 that comes from incomplete understanding of the marine carbonate cycle is cause for concern.

D. M. Liverman, K. L. O'Brien (1991). Global warming and climate change in Mexico. Global Environmental Change 1 (5): 351-364

ABSTRACT: Climate models suggest that global warming could bring warmer, drier conditions to Mexico. Although precipitation increases are projected by some models, in most cases they do not compensate for increases in potential evaporation. Thus, soil moisture and water availability may decrease over much of Mexico with serious consequences for rainfed and irrigated agriculture, urban and industrial water supplies, hydropower and ecosystems. However, the assessment of global warming impacts in Mexico is an uncertain task because the projections of different models vary widely, particularly for precipitation, and because they perform poorly in reproducing the observed climate of Mexico.

P. Meir, P. Cox, J. Grace (2006). The influence of terrestrial ecosystems on climate. Trends in Ecology & Evolution 21 (5): 254-260

ABSTRACT: Terrestrial ecosystems influence climate by affecting how much solar energy is absorbed by the land surface and by exchanging climatically important gases with the atmosphere. Recent model analyses show widespread qualitative agreement that terrestrial ecological processes will have a net positive feedback effect on 21st-century global warming, and, therefore, cannot be ignored in climate-change projections. However, the quantitative uncertainty in the net feedback is large. The uncertainty in 21st-century carbon dioxide emissions resulting from terrestrial carbon cycle–climate feedbacks is second in magnitude only to the uncertainty in anthropogenic emissions. We estimate that this translates into an uncertainty in global warming owing to the land surface of 1.5°C by 2100. We also emphasise the need to improve our understanding of terrestrial ecological processes that influence land–atmosphere interactions at relatively long timescales (decadal–century) as well as at shorter intervals (e.g. hourly).

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