Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: Tropical forest ecosystems are large stores of carbon which supply millions of people with life support requirements. Currently tropical forests are undergoing massive deforestation. Here, I address the possible impact of global change conditions, including elevated CO2 , temperature rise, and nitrogen deposition on forest structure and dynamics. Tropical forests may be particularly susceptible to climate change for the following reasons: (1) Phenological events (such as flowering and fruiting) are highly tuned to climatic conditions. Thus a small change in climate can have a major impact on the forest, its biological diversity and its role in the carbon cycle. (2) There are strong coevolutionary interactions, such as pollination seed dispersal, with a high degree of specialization, i.e., only certain animals can effect these activities for certain species. Global change can decouple these tight coevolutionary interactions. (3) Because of high species diversity per unit area, species of the tropical rain forest must have narrow niches. Thus changes in global climate can eliminate species and therefore reduce biological diversity. (4) Deforestation and other forms of disturbance may have significant feedback on hydrology both regionally and globally. The predicted decline in the rainfall in the Amazon Basin and the intensification of the Indian monsoon can have a large effect on water availability and floods which are already devastating low-lying areas. It is concluded that tropical forests may be very sensitive to climate change. Under climatic change conditions their structure and function may greatly change, their integrity may be violated and their services to people may be greatly modified. Because they are large stores of great biological diversity, they require immediate study before it is too late. The study requires the collaboration of scientists with a wide range of backgrounds and experiences including biologists, climate modellers, atmospheric scientists, economists, human demographers and sociologists in order to carry out holistic and urgently needed work. Global climatic change brings a great challenge to science and to policy makers.
ABSTRACT: Vegetation change is involved in climate change through both forcing and feedback processes. Emissions of CO2 from past net deforestation are estimated to have contributed approximately 0.22 0.51 Wm-2 to the overall 1.46 Wm-2 radiative forcing by anthropogenic increases in CO2 up to the year 2000. Deforestation-induced increases in global mean surface albedo are estimated to exert a radiative forcing of 0 to -0.2 Wm-2 , and dust emissions from land use may exert a radiative forcing of between approximately +0.1 and -0.2 Wm-2 . Changes in the fluxes of latent and sensible heat due to tropical deforestation are simulated to have exerted other local warming effects which cannot be quantified in terms of a Wm-2 radiative forcing, with the potential for remote effects through changes in atmospheric circulation. With tropical deforestation continuing rapidly, radiative forcing by surface albedo change may become less useful as a measure of the forcing of climate change by changes in the physical properties of the land surface. Although net global deforestation is continuing, future scenarios used for climate change prediction suggest that fossil fuel emissions of CO2 may continue to increase at a greater rate than land use emissions and therefore continue to increase in dominance as the main radiative forcing. The CO2 rise may be accelerated by up to 66% by feedbacks arising from global soil carbon loss and forest dieback in Amazonia as a consequence of climate change, and Amazon forest dieback may also exert feedbacks through changes in the local water cycle and increases in dust emissions.
ABSTRACT: Changes in wetland ecosystems in the northern latitudes may have feedback effects on greenhouse gases. An approach for predicting the climate-change impact of northern wetlands is outlined.
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.
ABSTRACT: Ecosystems influence climate through multiple pathways, primarily by changing the energy, water, and greenhouse-gas balance of the atmosphere. Consequently, efforts to mitigate climate change through modification of one pathway, as with carbon in the Kyoto Protocol, only partially address the issue of ecosystem–climate interactions. For example, the cooling of climate that results from carbon sequestration by plants may be partially offset by reduced land albedo, which increases solar energy absorption and warms the climate. The relative importance of these effects varies with spatial scale and latitude. We suggest that consideration of multiple interactions and feedbacks could lead to novel, potentially useful climate-mitigation strategies, including greenhouse-gas reductions primarily in industrialized nations, reduced desertification in arid zones, and reduced deforestation in the tropics. Each of these strategies has additional ecological and societal benefits. Assessing the effectiveness of these strategies requires a more quantitative understanding of the interactions among feedback processes, their consequences at local and global scales, and the teleconnections that link changes occurring in different regions.
ABSTRACT: The temperature sensitivity of soil organic carbon decomposition is critical for predicting future climate change because soils store 2-3 times the amount of atmospheric carbon. Of particular controversy is the question, whether temperature sensitivity differs between young or labile and old or more stable carbon pools. Ambiguities in experimental methodology have so far limited corroboration of any particular hypothesis. Here, we show in a clear-cut approach that differences in temperature sensitivity between young and old carbon are negligible. Using the change in stable isotope composition in transitional systems from C3 to C4 vegetation, we were able to directly distinguish the temperature sensitivity of carbon differing several decades in age. This method had several advantages over previously followed approaches. It allowed to identify release of much older carbon, avoided un-natural conditions of long-term incubations and did not require arguable curve-fitting. Our results demonstrate that feedbacks of the carbon cycle on climate change are driven equally by young and old soil organic carbon.
ABSTRACT: Much research focuses on how the terrestrial biosphere influences climate through changes in surface albedo (reflectivity), stomatal conductance and leaf area index (LAI). By using a fully-coupled GCM (HadCM3LC), our research objective was to induce an increase in the growth of global vegetation to isolate the effect of increased LAI on atmospheric exchange of heat and moisture. Our Control simulation had a mean global net primary production (NPP) of 56.3 Gt Cyr−1 which is half that of our scenario value of 115.1 GtCyr−1 . LAI and latent energy (QE ) were simulated to increase globally, except in areas around Antarctica. A highly productive biosphere promotes mid-latitude mean surface cooling of ~2.5°C in the summer, and surface warming of ~1.0°C in the winter. The former response is primarily the result of reduced Bowen ratio (i.e. increased production of QE ) in combination with small increases in planetary albedo. Response in winter temperature is likely due to decreased planetary albedo that in turn permits a greater amount of solar radiation to reach the Earth’s surface. Energy balance calculations show that between 75° and 90°N latitude, an additional 2.4 Wm−2 of surface heat must be advected into the region to maintain energy balance, and ultimately causes high northern latitudes to warm by up to 3°C. We postulate that large increases in QE promoted by increased growth of terrestrial vegetation could contribute to greater surface-to-atmosphere exchange and convection. Our high growth simulation shows that convective rainfall substantially increases across three latitudinal bands relative to Control; in the tropics, across the monsoonal belt, and in mid-latitude temperate regions. Our theoretical research has implications for applied climatology; in the modeling of past “hot-house” climates, in explaining the greening of northern latitudes in modern-day times, and for predicting future changes in surface temperature with continued increases in atmospheric CO2 .
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.
ABSTRACT: The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate1 . About half of the current emissions are being absorbed by the ocean and by land ecosystems2 , but this absorption is sensitive to climate3, 4 as well as to atmospheric carbon dioxide concentrations5 , creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change6 . Here we present results from a fully coupled, three-dimensional carbon–climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr-1 is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models2 , resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.
ABSTRACT: Significantly more carbon is stored in the world's soils—including peatlands, wetlands and permafrost—than is present in the atmosphere. Disagreement exists, however, regarding the effects of climate change on global soil carbon stocks. If carbon stored belowground is transferred to the atmosphere by a warming-induced acceleration of its decomposition, a positive feedback to climate change would occur. Conversely, if increases of plant-derived carbon inputs to soils exceed increases in decomposition, the feedback would be negative. Despite much research, a consensus has not yet emerged on the temperature sensitivity of soil carbon decomposition. Unravelling the feedback effect is particularly difficult, because the diverse soil organic compounds exhibit a wide range of kinetic properties, which determine the intrinsic temperature sensitivity of their decomposition. Moreover, several environmental constraints obscure the intrinsic temperature sensitivity of substrate decomposition, causing lower observed 'apparent' temperature sensitivity, and these constraints may, themselves, be sensitive to climate.
J.-L. Dufresne, L. Fairhead, H. Le Treut, M. Berthelot, L. Bopp, P. Ciais, P. Friedlingstein, P. Monfray (2002). On the magnitude of positive feedback between future climate change and the carbon cycle. Geophysical Research Letters 29 (10): 1405, doi:10.1029/2001GL013777
ABSTRACT: We use an ocean-atmosphere general circulation model coupled to land and ocean carbon models to simulate the evolution of climate and atmospheric CO2 from 1860 to 2100. Our model reproduces the observed global mean temperature changes and the growth rate of atmospheric CO2 for the period 1860–2000. For the future, we simulate that the climate change due to CO2 increase will reduce the land carbon uptake, leaving a larger fraction of anthropogenic CO2 in the atmosphere. By 2100, we estimate that atmospheric CO2 will be 18% higher due to the climate change impact on the carbon cycle. Such a positive feedback has also been found by Cox et al.  . However, the amplitude of our feedback is three times smaller than the one they simulated. We show that the partitioning between carbon stored in the living biomass or in the soil, and their respective sensitivity to increased CO2 and climate change largely explain this discrepancy.
ABSTRACT: Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and may cause positive feedback to warming as CO2 is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15°C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO2 with radiocarbon (14 C) ages ranging between ~21 and 24 ka bp was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q10 value of 1.9±0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions ofΔ14 CO2 values in respiration CO2 and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO2 in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5°C, about 1 Pg C will be initially released from labile C pools, followed by respiration of ~40 Pg C to the atmosphere over a period of four decades.
ABSTRACT: Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton leads to formation of ~45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.
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.
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.
ABSTRACT: Differences in simulations of climate feedbacks are sources of significant divergence in climate models' temperature response to anthropogenic forcing. Snow albedo feedback is particularly critical for climate change prediction in heavily-populated northern hemisphere land masses. Here we show its strength in current models exhibits a factor-of-three spread. These large intermodel variations in feedback strength in climate change are nearly perfectly correlated with comparably large intermodel variations in feedback strength in the context of the seasonal cycle. Moreover, the feedback strength in the real seasonal cycle can be measured and compared to simulated values. These mostly fall outside the range of the observed estimate, suggesting many models have an unrealistic snow albedo feedback in the seasonal cycle context. Because of the tight correlation between simulated feedback strength in the seasonal cycle and climate change, eliminating the model errors in the seasonal cycle will lead directly to a reduction in the spread of feedback strength in climate change. Though this comparison to observations may put the models in an unduly harsh light because of uncertainties in the observed estimate that are difficult to quantify, our results map out a clear strategy for targeted observation of the seasonal cycle to reduce divergence in simulations of climate sensitivity.
Heath, J., Ayres, E., Possell, M., Bardgett, R. D., Black, H. I. J., Grant, H., Ineson, P., Kerstiens, G. (2005). Rising atmospheric CO2 reduces sequestration of root-derived soil carbon. Science 309 (5741): 1711-1713
ABSTRACT: Forests have a key role as carbon sinks, which could potentially mitigate the continuing increase in atmospheric carbon dioxide concentration and associated climate change. We show that carbon dioxide enrichment, although causing short-term growth stimulation in a range of European tree species, also leads to an increase in soil microbial respiration and a marked decline in sequestration of root-derived carbon in the soil. These findings indicate that, should similar processes operate in forest ecosystems, the size of the annual terrestrial carbon sink may be substantially reduced, resulting in a positive feedback on the rate of increase in atmospheric carbon dioxide concentration.
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.
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.
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.
A. R. Keyser, J. S. Kimball, R. R. Nemani, S. W. Running (2000). Simulating the effects of climate change on the carbon balance of North American high-latitude forests. Global Change Biology 6 (S1): 185-195
ABSTRACT: The large magnitude of predicted warming at high latitudes and the potential feedback of ecosystems to atmospheric CO2 concentrations make it important to quantify both warming and its effects on high-latitude carbon balance. We analysed long-term, daily surface meteorological records for 13 sites in Alaska and north-western Canada and an 82-y record of river ice breakup date for the Tanana River in interior Alaska. We found increases in winter and spring temperature extrema for all sites, with the greatest increases in spring minimum temperature, average 0.47 °C per 10 y, and a 0.7-day per 10 y advance in ice breakup on the Tanana River. We used the climate records to drive an ecosystem process model, BIOME_BGC, to simulate the effects of climate change on the carbon and water balances of boreal forest ecosystems. The growing season has lengthened by an average of 2.6 days per 10 y with an advance in average leaf onset date of 1.10 days per 10 y. This advance in the start of the active growing season correlates positively with progressively earlier ice breakup on the Tanana River in interior Alaska. The advance in the start of the growing season resulted in a 20% increase in net primary production for both aspen (Populus tremuloides ) and white spruce (Picea glauca ) stands. Aspen had a greater mean increase in maintenance respiration than spruce, whereas spruce had a greater mean increase in evapotranspiration. Average decomposition rates also increased for both species. Both net primary production and decomposition are enhanced in our simulations, suggesting that productive forest types may not experience a significant shift in net carbon flux as a result of climate warming.
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.
ABSTRACT: Ecosystem carbon cycle feedbacks to climate change comprise one of the largest remaining sources of uncertainty in global model predictions of future climate. Both direct climate effects on carbon cycling and indirect effects via climate-induced shifts in species composition may alter ecosystem carbon balance over the long term. In the short term, climate effects on carbon cycling may be mediated by ecosystem species composition. We used an elevational climate and tree species composition gradient in Rocky Mountain subalpine forest to quantify the sensitivity of all major ecosystem carbon stocks and fluxes to these factors. The climate sensitivities of carbon fluxes were species-specific in the cases of relative aboveground productivity and litter decomposition, whereas the climate sensitivity of dead wood decay did not differ between species, and total annual soil CO2 flux showed no strong climate trend. Lodgepole pine relative productivity increased with warmer temperatures and earlier snowmelt, while Engelmann spruce relative productivity was insensitive to climate variables. Engelmann spruce needle decomposition decreased linearly with increasing temperature (decreasing litter moisture), while lodgepole pine and subalpine fir needle decay showed a hump-shaped temperature response. We also found that total ecosystem carbon declined by 50% with a 2.8°C increase in mean annual temperature and a concurrent 63% decrease in growing season soil moisture, primarily due to large declines in mineral soil and dead wood carbon. We detected no independent effect of species composition on ecosystem C stocks. Overall, our carbon flux results suggest that, in the short term, any change in subalpine forest net carbon balance will depend on the specific climate scenario and spatial distribution of tree species. Over the long term, our carbon stock results suggest that with regional warming and drying, Rocky Mountain subalpine forest will be a net source of carbon to the atmosphere.
S. Lavorel, M. D. Flannigan, E. F. Lambin, M. C. Scholes (2007). Vulnerability of land systems to fire: Interactions among humans, climate, the atmosphere, and ecosystems. Mitigation and Adaptation Strategies for Global Change 12 (1): 33-53
ABSTRACT: Fires are critical elements in the Earth System, linking climate, humans, and vegetation. With 200–500 Mha burnt annually, fire disturbs a greater area over a wider variety of biomes than any other natural disturbance. Fire ignition, propagation, and impacts depend on the interactions among climate, vegetation structure, and land use on local to regional scales. Therefore, fires and their effects on terrestrial ecosystems are highly sensitive to global change. Fires can cause dramatic changes in the structure and functioning of ecosystems. They have significant impacts on the atmosphere and biogeochemical cycles. By contributing significantly to greenhouse gas (e.g., with the release of 1.7–4.1 Pg of carbon per year) and aerosol emissions, and modifying surface properties, they affect not only vegetation but also climate. Fires also modify the provision of a variety of ecosystem services such as carbon sequestration, soil fertility, grazing value, biodiversity, and tourism, and can hence trigger land use change. Fires must therefore be included in global and regional assessments of vulnerability to global change. Fundamental understanding of vulnerability of land systems to fire is required to advise management and policy. Assessing regional vulnerabilities resulting from biophysical and human consequences of changed fire regimes under global change scenarios requires an integrated approach. Here we present a generic conceptual framework for such integrated, multidisciplinary studies. The framework is structured around three interacting (partially nested) subsystems whose contribute to vulnerability. The first subsystem describes the controls on fire regimes (exposure). A first feedback subsystem links fire regimes to atmospheric and climate dynamics within the Earth System (sensitivity), while the second feedback subsystem links changes in fire regimes to changes in the provision of ecological services and to their consequences for human systems (adaptability). We then briefly illustrate how the framework can be applied to two regional cases with contrasting ecological and human context: boreal forests of northern America and African savannahs.
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.
ABSTRACT: The concept that nitrogen (N) availability can limit plant productivity is well established based on (1) N fertilization that stimulates productivity and (2) increases in productivity along gradients of soil fertility. Nitrogen limitations to plant productivity are regulated by processes such as mineralization, immobilization, and plant physiological adjustments. However, this production-centric perspective might not fully explain patterns in carbon (C) sequestration in terrestrial ecosystems. Carbon sequestration involves both plant and soil pools. The plant pool, which is the main concern of production research, can be much smaller than the soil pool. To quantify terrestrial C sequestration, therefore, we have to develop an ecosystem perspective to examine how C and N interact in both plant and soil pools.
Due to fossil fuel burning and deforestation, atmospheric CO2 concentration has increased by approximately 35% since the Industrial Revolution. In general, elevated CO2 enhances photosynthesis and stimulates initial C sequestration in terrestrial ecosystems. How sustainable the CO2 -induced C sequestration can be depends, in part, on ecosystem N availability and supply. Thus, the interdependence of C and N cycles is an issue that is not only interesting to ecologists, but also has important implications for global change policy.
Increased C influx into an ecosystem under elevated CO2 generally requires more N to support plant growth than is required at ambient CO2 and, in turn, sequesters N into long-lived plant biomass and soil organic matter pools. This N sequestration can decrease soil N availability for plant uptake and lead to progressive N limitation (PNL) over time. The PNL hypothesis states that N sequestration in long-term organic matter pools will, without new N input and/or decreases in N losses, lead to a decline in mineral N availability over time at elevated CO2 compared to ambient CO2 . On the other hand, increased plant N demand and/or sequestration could induce changes in N supply. When elevated CO2 increases N use efficiency (NUE) and stimulates N transfer from the soil organic pools with narrow C:N ratios to plants with broad C:N ratios, PNL may be delayed. If additional C input at elevated CO2 stimulates capital gain of N through fixation, decreased losses, increased forage for soil N, or any combinations of them, PNL may not occur. If it does, CO2 -induced C sequestration in ecosystems declines over time. In short, N will constrain C sequestration over time unless additional C input at elevated CO2 stimulates N gain in ecosystems.
This Special Feature consists of six papers that examine various aspects of PNL against field data collected from ecosystems that have been exposed to elevated CO2 treatments. The first two papers show sustained CO2 stimulation of net primary production (NPP) in forest ecosystems. Norby and Iversen present data from a sweetgum forest stand in Oak Ridge, Tennessee, that has been exposed to free-air CO2 enrichment (FACE) for six years. The sustained CO2 stimulation of NPP was associated primarily with increased N uptake, since NUE did not change significantly under elevated CO2 . Sufficient N supply from soil at Oak Ridge may help delay or even avoid PNL as elevated CO2 substantially stimulated root growth to explore N sources in deeper soil layers. At the Duke Forest FACE site, Finzi and colleagues demonstrate that the CO2 stimulation of NPP was sustained at 18–24% during the first six years of the experiment. Sustained NPP stimulation occurred together with significantly more N uptake by trees and higher NUE at elevated than at ambient CO2 . Their mass balance analysis shows that significantly more N accumulated at elevated CO2 in plants and in forest floor litter. The forest ecosystem accrued N capital at an average rate of 12 g N·m−2·yr−1, perhaps due to N uptake from deeper in the soil profile.
However, PNL of plant growth and C sequestration appears to occur in a scrub-oak ecosystem, Florida (Hungate et al.), and a C3/C4 grassland, Texas (Gill et al.) in response to elevated CO2 . Initial CO2 stimulation of plant biomass growth was supported by more N uptake from soil in the scrub-oak ecosystem. As N was accumulated in plant biomass and litter layers in the O horizon in years 4–7, soil N availability progressively declined, as did the CO2 stimulation of plant growth. Initially, PNL of plant growth was avoided by increased N uptake from the soil and alleviated later through increased NUE. Elevated CO2 did not change total ecosystem N content but caused a redistribution of N from the mineral soil to plants and litter. In the Texas grassland, increased CO2 along a gradient from 200 to 560μmol/mol also caused reallocation of N from soil to plant and from more recalcitrant to more labile fractions within the soil. The N reallocation alleviates PNL and allows plant production to increase with increasingCO2 . However, it does not support much long-term C sequestration in the soil at elevatedCO2 , since the C gained from increased plant production can be rapidly lost through decomposition.Results at experimental sites are often highly variable. To synthesize results from multiple sites, Luo et al. conducted a meta-analysis of data from 104 published papers and found significant increases in C and N contents on average in all the plant and soil pools under elevatedCO2 . The net N accumulation in plant and soil pools at least helps prevent complete down-regulation of, and likely supports, long-termCO2 stimulation of C sequestration. The net C and N accumulations under elevatedCO2 are consistent with C and N dynamics during succession over hundreds to millions of years, suggesting that ecosystems may have intrinsic capabilities to stimulate N accumulation by C input. Johnson reviews the early nutrient cycling literature related to PNL during forest stand development and more recent studies on C and N interactions under elevatedCO2 . In general, trees can“mine” N from soils over the long term, but PNL will constrain CO2 stimulation of plant growth unless external inputs of N are increased by N fixation or atmospheric deposition.
The six papers in this Special Feature provide experimental evidence on ecosystem C and N interactions but do not fully resolve the issue of whether N constrains the C cycle or additional C input stimulates the N cycle in response to elevated CO2 . Against the backdrop of diverse responses in nature, the challenge is how we can incorporate the diverse mechanisms of C and N interactions into models to predict future C sequestration. In the end, we hope this Special Feature will stimulate research to test the PNL hypothesis further and advance our understanding of the biogeochemical coupling of C and N cycles.
ABSTRACT: The latest report by the Intergovernmental Panel on Climate Change (IPCC) predicts a 1.4–5.8 °C average increase in the global surface temperature over the period 1990 to 2100 (ref. 1). These estimates of future warming are greater than earlier projections, which is partly due to incorporation of a positive feedback. This feedback results from further release of greenhouse gases from terrestrial ecosystems in response to climatic warming2, 3, 4 . The feedback mechanism is usually based on the assumption that observed sensitivity of soil respiration to temperature under current climate conditions would hold in a warmer climate5 . However, this assumption has not been carefully examined. We have therefore conducted an experiment in a tall grass prairie ecosystem in the US Great Plains to study the response of soil respiration (the sum of root and heterotrophic respiration) to artificial warming of about 2 °C. Our observations indicate that the temperature sensitivity of soil respiration decreases—or acclimatizes—under warming and that the acclimatization is greater at high temperatures. This acclimatization of soil respiration to warming may therefore weaken the positive feedback between the terrestrial carbon cycle and climate.
Matthews, H. D., M. Eby, T. Ewen, P. Friedlingstein, B. J. Hawkins (2007). What determines the magnitude of carbon cycle-climate feedbacks?. Global Biogeochemical Cycles 21 (GB2012): doi:10.1029/2006GB002733
ABSTRACT: Positive feedbacks between climate change and the carbon cycle have the potential to amplify the growth of atmospheric carbon dioxide and accelerate future climate warming. However, both the magnitude of and the processes which drive future carbon cycle-climate feedbacks remain highly uncertain. In this study, we use a coupled climate-carbon model to investigate how the response of vegetation photosynthesis to climate change contributes to the overall strength of carbon cycle-climate feedbacks. We find that the feedback strength is particularly sensitive to the model representation of the photosynthesis-temperature response, with lesser sensitivity to the parameterization of soil moisture and nitrogen availability. In all simulations, large feedbacks are associated with a climatic suppression of terrestrial primary productivity and consequent reduction of terrestrial carbon uptake. This process is particularly evident in the tropics and can explain a large part of the range of carbon cycle-climate feedbacks simulated by different coupled climate-carbon models.
ABSTRACT: Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO2 ), nitrous oxide (N2 O), and methane (CH4 ). This study reports the impact of year-round SM status on trace gas fluxes in three semiarid vegetation zones, mesquite (30 g organic C kg–1 soil), open/forb (6 g organic C kg–1 soil), and sacaton (18 g organic C kg–1 soil) from July 2002–September 2003 in southeastern Arizona. Carbon dioxide and N2 O emissions were highly dependent on available SM and T. During the heavy rains of the 2002 monsoon (238 mm total rainfall), large differences in soil C content did not correlate with variations in CO2 production, as efflux averaged 235.6 ± 39.5 mg CO2 m–2 h–1 over all sites. In 2003, limited monsoon rain (95 mm total rainfall) reduced CO2 emissions by 19% (mesquite), 40% (open), and 30% (sacaton), compared with 2002. Nitrous oxide emissions averaged 21.1 ± 13.4 (mesquite), 2.1 ± 4.4 (open), and 3.9 ± 5.2 µg N2 O m–2 h–1 (sacaton) during the 2002 monsoon. Limited monsoon 2003 rainfall reduced N2 O emissions by 47% in the mesquite, but N2 O production increased in the open (55%) and sacaton (5%) sites. Following a dry winter and spring 2002 (15 mm total rainfall), premonsoon CH4 consumption at all sites was close to zero, but following monsoon moisture input, the CH4 sink averaged 26.1 ± 6.3 µg CH4 m–2 h–1 through April 2003. Laboratory incubations showed potentials for CH4 oxidation from 0 to 45 cm, suggesting that as the soil surface dried, CH4 oxidation activity shifted downward in the sandy soils. Predicted climate change shifts in annual precipitation from one dominated by summer monsoon rainfall to one with higher winter precipitation may reduce soil CO2 and N2 O emissions while promoting CH4 oxidation rates in semiarid riparian soils of the Southwest, potentially acting as a negative feedback for future global warming.
ABSTRACT: For atmospheric CO2 concentration stabilization, we projected allowable carbon emission with an Earth system model based on a general circulation model. Our calculations on centennial timescale in various scenarios reveal how saturation with respect to CO2 and climate-carbon cycle feedback reduce natural carbon uptake, and hence allowable emission. In 450 ppm stabilization scenario, for example, climate-carbon cycle feedback reduces the accumulative allowable carbon emission until year 2300 from 1248 to 980 Pg C. The Emission at the year 2050 is about the half of the year 2000 level for the SP450 scenario. Terrestrial carbon cycle is especially susceptible to climate-carbon cycle feedback, and is a significant source of projection uncertainty. Our model responds nonlinearly to CO2 and climate, suggesting process-based models are indispensable tool for future climate-carbon cycle projections.
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.
ABSTRACT: This paper discusses the physical linkage between the surface and the atmosphere, and demonstrates how even slight changes in surface conditions can have a pronounced effect on weather and climate. Observational and modeling evidence are presented to demonstrate the influence of landscape type on the overlying atmospheric conditions. The albedo, and the fractional partitioning of atmospheric turbulent heat flux into sensible and latent fluxes is shown to be particularly important in directly affecting local and regional weather and climate. It is concluded that adequate assessment of global climate and climate change cannot be achieved unless mesoscale landscape characteristics and their changes over time can be accurately determined.
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.
W. H. Schlesinger, J. F. Reynolds, G. L. Cunningham, L. F. Huenneke, W. M. Jarrell, R. A. Virginia, W. G. Whitford (1990). Biological feedbacks in global desertification. Science 247 (4946): 1043-1048
ABSTRACT: Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.
Updegraff, Karen, Pastor, John, Bridgham, Scott D., Johnston, Carol A. (1995). Environmental and substrate controls over carbon and nitrogen mineralization in northern wetlands. Ecological Applications 5 (1): 151-163.
ABSTRACT: Northern wetlands may be a potential carbon source to the atmosphere upon global warming, particularly with regard to methane. However, recent conclusions have largely been based on short-term field measurements. We incubated three wetland soils representing a range of substrate quality for 80 wk in the laboratory under both aerobic and anaerobic conditions at 15° and 30°C. The soils were obtained from aScirpus -Carex -dominated meadow in an abandoned beaver pond and from the surface and at 1 m depth of a spruce (Picea )-Sphagnum bog in Voyageurs National Park, Minnesota. Substrate quality was assessed by fractionation of carbon compounds and summarized using principal components analysis. Nitrogen and carbon mineralization, the partitioning of carbon between carbon dioxide and methane, pH, and Eh were measured periodically over the course of the incubation. The responses of nitrogen mineralization, carbon mineralization, and trace gas partitioning to both temperature and aeration depended strongly on the substrate quality of the soils. Sedge meadow soil had the highest nitrogen and carbon mineralization rates and methane production under anaerobic conditions, and carbon mineralization under aerobic conditions, but the surface peats had the highest nitrogen mineralization rates under aerobic conditions. Methanogenesis was highest in the sedge soil but less sensitive to temperature than in the peats. A double exponential model showed that most of the variation in nitrogen and carbon mineralization among the soils and treatments was accounted for by differences in the size and kinetics of a relatively small labile pool. The kinetics of this pool were more sensitive to changes in temperature and aeration than that of the larger recalcitrant pool. Principal components analysis separated the soils on the basis of labile and recalcitrant carbon fractions. Total C and N mineralization correlated positively with the factor representing labile elements, while methanogenesis also showed a negative correlation with the factor representing recalcitrant elements. Estimates of atmospheric feedbacks from northern wetlands upon climatic change must account for extreme local variation in substrate quality and wetland type; global projections based on extrapolations from a few field measurements do not account for this local variation and may be in error.
Vourlitis,G. C., Boynton,B., Verfaillie J.,Jr, Zulueta,R., Hastings,S. J., Oechel,W. C., Hope,A., Stow,D. (2000). Physiological models for scaling plot measurements of CO2 flux across an arctic tundra landscape. Ecologcial Applications 10 (1): 60-72
ABSTRACT: Regional estimates of arctic ecosystem CO2 exchange are required because of the large soil carbon stocks located in arctic regions, the potentially large global-scale feedbacks associated with climate-change-induced alterations in arctic ecosystem C sequestration, and the substantial small-scale (1–10 m2 ) heterogeneity of arctic vegetation and hydrology. Because the majority of CO2 flux data for arctic ecosystems are derived from plot-scale studies, a scaling routine that can provide reliable estimates of regional CO2 flux is required. This study combined data collected from chamber measurements of CO2 exchange, meteorology, hydrology, and surface reflectance with simple physiological models to quantify the diurnal and seasonal dynamics of whole-ecosystem respiration (R), gross primary production (GPP), and net CO2 exchange (F) of wet- and moist-sedge tundra ecosystems of arctic Alaska. Diurnal fluctuations in R were expressed as exponential functions of air temperature, whereas diurnal fluctuations in GPP were described as hyperbolic functions of diurnal photosynthetic photon flux density (PPFD). Daily integrated rates of R were expressed as an exponential function of average daily water table depth and temperature, whereas daily fluctuations in GPP were described as a hyperbolic function of average daily PPFD and a sigmoidal function of the normalized difference vegetation index (NDVI) calculated from satellite imagery. These models described, on average, 75–97% of the variance in diurnal R and GPP, and 78–95% of the variance in total daily R and GPP. Model results suggest that diurnal F can be reliably predicted from meteorology (radiation and temperature), but over seasonal time scales, information on hydrology and phenology is required to constrain the response of GPP and R to variations in temperature and radiation.
Using these physiological relationships and information about the spatial variance in surface features across the landscape, measurements of CO2 exchange in 0.5-m2 plots were extrapolated to the hectare scale. Compared to direct measurements of hectare-scale F made using eddy covariance, the scaled estimate of seasonally integrated F was within 20% of the observed value. With a minimum of input data, these models allowed plot measurements of arctic ecosystem CO2 exchange to be confidently scaled in space and time.
ABSTRACT: We examined climate-carbon cycle feedback by performing a global warming experiment using MIROC-based coupled climate-carbon cycle model. The model showed that by the end of the 21st century, warming leads to a further increase in carbon dioxide (CO2 ) level of 123 ppm by volume (ppmv). This positive feedback can mostly be attributed to land-based soil-carbon dynamics. On a regional scale, Siberia experienced intense positive feedback, because the acceleration of microbial respiration due to warming causes a decrease in the soil carbon level. Amazonia also had positive feedback resulting from accelerated microbial respiration. On the other hand, some regions, such as western and central North America and South Australia, experienced negative feedback, because enhanced litterfall surpassed the increased respiration in soil carbon. The oceanic contribution to the feedback was much weaker than the land contribution on global scale, but the positive feedback in the northern North Atlantic was as strong as those in Amazonia and Siberia in our model. In the northern North Atlantic, the weakening of winter mixing caused a reduction of CO2 absorption at the surface. Moreover, weakening of the formation of North Atlantic Deep Water caused reduced CO2 subduction to the deep water. Understanding such regional-scale differences may help to explain disparities in coupled climate-carbon cycle model results.
ABSTRACT: Climate warming will thaw permafrost, releasing trapped carbon from this high-latitude reservoir and further exacerbating global warming.