Climate Change and...

Annotated Bibliography

Carbon Dynamics

The Greenhouse Effect and Atmospheric CO2, CH4 & NOx

Aber, J. D., Ollinger, S. V., Federer, C. A., Reich, P. B., Goulden, M. L., Kicklighter, D. W., Melillo, J. M., Lathrop, R. G., Jr. (1995). Predicting the effects of climate change on water yield and forest production in the northeastern United States. Climate Research 5 (3): 207-222

ABSTRACT: Rapid and simultaneous changes in temperature, precipitation and the atmospheric concentration of CO2 are predicted to occur over the next century. Simple, well-validated models of ecosystem function are required to predict the effects of these changes This paper describes an improved version of a forest carbon and water balance model (PnET-II) and the application of the model to predict stand- and regional-level effects of changes in temperature, precipitation and atmospheric CO2 (gross and net) driven by nitrogen availability as expressed through foliar N concentration. Improvements was parameterized and run for 4 forest/site combinations and validated against available data for water yield, gross and net carbon exchange and biomass production. The validation exercise suggests that the determination of actual water availability to stands and the occurrence or non-occurrence of soil-based water stress are critical to accurate modeling of forest net primary production (NPP) and net ecosystem production (NEP). The model was then run for the entire New England/New York (USA) region using a 1 km resolution geographic information system. Predicted long-term NEP ranged from -85 to +275 g C m-2 yr-1 for the 4 forest/site combinations, and from -150 to 350 g cm-2 yr-1 for the region, with a regional average of 76 g C m-2 yr-1 . A combination of increased temperature (+6 degree C), decreased precipitation (-15%) and increased Water use efficiency (2x, due to doubling of CO2 ) resulted generally in increases in NPP and decreases in water yield over the region

Ball, T., Smith, K.A., Moncrieff, J.B. (2007). Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil. Global Change Biology 13 (10): 2128-2142

ABSTRACT: The influence of forest stand age in aPicea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO2 , CH4 and N2 O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2-year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30- and 20-year-old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30-year-old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO2 efflux was the dominant influence on the GHG budget, contributing 93–94% of the total GHG flux across the chronosequence compared with 6–7% from CH4 and N2 O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO2 were lower at the 20-year-old (20y) site than at the 30-year-old (30y) and CF sites, with soil temperature the dominant control. CH4 efflux was highest at the CF site, with peak flux 491±54.5μg m−2 h−1 and maximum annual flux 18.0±1.1 kg CH4 ha−1 yr−1 . No consistent uptake of CH4 was noted at any site. A linear relationship was found between log CH4 flux and the closeness of the water table to the soil surface across all sites. N2 O efflux was highest in the 30y site, reaching 108±38.3μgN2 O-N m−2 h−1 (171μgN2 Om−2 h−1 ) in midsummer and a maximum annual flux of 4.7±1.2 kg N2 O ha−1 yr−1 in 2001. Automatic chamber data showed a positive exponential relationship between N2 O flux and soil temperature at this site. The relationship between N2 O emission and soil volumetric moisture indicated an optimum moisture content for N2 O flux of 40–50% by volume. The relationship between C:N ratio data and integrated N2 O flux was consistent with a pattern previously noted across temperate and boreal forest soils.

C. R. Barnes (1999). Paleoceanography and paleoclimatology: an Earth system perspective. Chemical Geology 161 (1-3): 17-35

ABSTRACT: The purpose of this paper is to provide an overview of paleoceanography and paleoclimatology as a framework for other papers dealing with The Earth System: Geochemical Perspectives. An introduction to both paleoceanography and paleoclimatology is followed by examples of the temporal changes through the Phanerozoic. The important and interactive role of the biosphere is emphasized. Many important changes in the Earth system have affected the coupled ocean–atmosphere system and many, in turn, have been reflected in biotic events. Many such changes can be tracked through time using geochemical signatures as proxy indicators. Whereas the scale of past paleoceanographic and paleoclimatic changes have been generally appreciated for some time, the recognition of periodic rapid change in the ocean/climate state and the ability to study and measure these precisely is only a recent accomplishment. The potential for such rapid change in the ocean/climate/biosphere of the Earth system raises concerns for events in the near future that may be forced by anthropogenic activities that enhance natural variability.

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.

Bowden, R. D., Castro, M. S., Melillo, J. M., Steudler, P. A., Aber, J. D. (1993). Fluxes of greenhouse gases between soils and the atmosphere in a temperate forest following a simulated hurricane blowdown. Biogeochemistry 21 (2): 67-71

ABSTRACT: Fluxes of nitrous oxide (N2 O), carbon dioxide (CO2 ), and methane (CH4 ) between soils and the atmosphere were measured monthly for one year in a 77-year-old temperate hardwood forest following a simulated hurricane blowdown. Emissions of CO2 and uptake of CH4 for the control plot were 4.92 MT C ha−1 y−1 and 3.87 kg C ha−1 y−1 , respectively, and were not significantly different from the blowdown plot. Annual N2 O emissions in the control plot (0.23 kg N ha−1 y−1 ) were low and were reduced 78% by the blowdown. Net N mineralization was not affected by the blowdown. Net nitrification was greater in the blowdown than in the control, however, the absolute rate of net nitrification, as well as the proportion of mineralized N that was nitrified, remained low. Fluxes of CO2 and CH4 were correlated positively to soil temperature, and CH, uptake showed a negative relationship to soil moisture. Substantial resprouting and leafing out of downed or damaged trees, and increased growth of understory vegetation following the blowdown, were probably responsible for the relatively small differences in soil temperature, moisture, N availability, and net N mineralization and net nitrification between the control and blowdown plots, thus resulting in no change in CO2 or CH4 fluxes, and no increase in N2 O emissions.

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.

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.

Chen, J.M., Ju, W., Cihlar, J., Price, D., Liu, J., Chen, W., Pan, J., Black, A., Barr, A. (2003). Spatial distribution of carbon sources and sinks in Canada's forests.. Tellus: Series B 55 (2): 622-641

ABSTRACT: Annual spatial distributions of carbon sources and sinks in Canada's forests at 1 km resolution are computed for the period from 1901 to 1998 using ecosystem models that integrate remote sensing images, gridded climate, soils and forest inventory data. GIS-based fire scar maps for most regions of Canada are used to develop a remote sensing algorithm for mapping and dating forest burned areas in the 25 yr prior to 1998. These mapped and dated burned areas are used in combination with inventory data to produce a complete image of forest stand age in 1998. Empirical NPP age relationships were used to simulate the annual variations of forest growth and carbon balance in 1 km pixels, each treated as a homogeneous forest stand. Annual CO2 flux data from four sites were used for model validation. Averaged over the period 1990–1998, the carbon source and sink map for Canada's forests show the following features: (i) large spatial variations corresponding to the patchiness of recent fire scars and productive forests and (ii) a general south-to-north gradient of decreasing carbon sink strength and increasing source strength. This gradient results mostly from differential effects of temperature increase on growing season length, nutrient mineralization and heterotrophic respiration at different latitudes as well as from uneven nitrogen deposition. The results from the present study are compared with those of two previous studies. The comparison suggests that the overall positive effects of non-disturbance factors (climate, CO2 and nitrogen) outweighed the effects of increased disturbances in the last two decades, making Canada's forests a carbon sink in the 1980s and 1990s. Comparisons of the modeled results with tower-based eddy covariance measurements of net ecosystem exchange at four forest stands indicate that the sink values from the present study may be underestimated.

T. J. Crowley (1993). Climate change on tectonic time scales. Tectonophysics 222 (3-4): 277-294

ABSTRACT: Variations of the atmospheric CO2 level and the global mean surface temperature during the last 150 Ma are reconstructed by using a carbon cycle model with high-resolution input data. In this model, the organic carbon budget and the CO2 degassing from the mantle, both of which would characterize the carbon cycle during the Cretaceous, are considered, and the silicate weathering process is formulated consistently with an abrupt increase in the marine strontium isotope record for the last 40 Ma. The second-order variations of the atmospheric CO2 level and the global mean surface temperature in addition to the first-order cooling trend are obtained by using high-resolution data of carbon isotopic composition of marine limestone, seafloor spreading rate, and production rate of oceanic plateau basalt. The results obtained from this model are in good agreement with the previous estimates of palaeo-CO2 level and palaeoclimate inferred from geological, biogeochemical, and palaeontological models and records. The system analyses of the carbon cycle model to understand the cause of the climate change show that the dominant controlling factors for the first-order cooling trend of climate change during the last 150 Ma are tectonic forcing such as decrease in volcanic activity and the formation and uplift of the Himalayas and the Tibetan Plateau, and, to a lesser extent, biological forcing such as the increase in the soil biological activity. The mid-Cretaceous was very warm because of the high CO2 level (4–5 PAL) maintained by the enhanced CO2 degassing rate due to the increased mantle plume activities and seafloor spreading rates at that time, although the enhanced organic carbon burial would have a tendency to decrease the CO2 level effectively at that period. The variation of organic carbon burial rate may have been responsible for the second-order climate change during the last 150 Ma.

Dalal, R.C., Allen, D.E., Livesley, S.J., Richards, G. (2007). Magnitude and biophysical regulators of methane emission and consumption in the Australian agricultural, forest, and submerged landscapes: a review. Plant And SoilPlant Soil 309 (1-2): 43-76

ABSTRACT: Increases in the concentrations of atmospheric greenhouse gases, carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2 O) due to human activities are associated with global climate change. CO2 concentration in the atmosphere has increased by 33% (to 380 ppm) since 1750 ad, whilst CH4 concentration has increased by 75% (to 1,750 ppb), and as the global warming potential (GWP) of CH4 is 25 fold greater than CO2 it represents about 20% of the global warming effect. The purpose of this review is to: (a) address recent findings regarding biophysical factors governing production and consumption of CH4 , (b) identify the current level of knowledge regarding the main sources and sinks of CH4 in Australia, and (c) identify CH4 mitigation options and their potential application in Australian ecosystems. Almost one-third of CH4 emissions are from natural sources such as wetlands and lake sediments, which is poorly documented in Australia. For Australia, the major anthropogenic sources of CH4 emissions include energy production from fossil fuels (~24%), enteric fermentation in the guts of ruminant animals (~59%), landfills, animal wastes and domestic sewage (~15%), and biomass burning (~5%), with minor contributions from manure management (1.7%), land use, land-use change and forestry (1.6%), and rice cultivation (0.2%). A significant sink exists for CH4 (~6%) in aerobic soils, including agricultural and forestry soils, and potentially large areas of arid soils, however, due to limited information available in Australia, it is not accounted for in the Australian National Greenhouse Gas Inventory. CH4 emission rates from submerged soils vary greatly, but mean values ≤10 mgm−2 h−1 are common. Landfill sites may emit CH4 at one to three orders of magnitude greater than submerged soils. CH4 consumption rates in non-flooded, aerobic agricultural, pastoral and forest soils also vary greatly, but mean values are restricted to ≤100μg m−2 h−1 , and generally greatest in forest soils and least in agricultural soils, and decrease from temperate to tropical regions. Mitigation options for soil CH4 production primarily relate to enhancing soil oxygen diffusion through water management, land use change, minimised compaction and soil fertility management. Improved management of animal manure could include biogas capture for energy production or arable composting as opposed to open stockpiling or pond storage. Balanced fertiliser use may increase soil CH4 uptake, reduce soil N2 O emissions whilst improving nutrient and water use efficiency, with a positive net greenhouse gas (CO2 -e) effect. Similarly, the conversion of agricultural land to pasture, and pastoral land to forestry should increase soil CH4 sink. Conservation of native forests and afforestation of degraded agricultural land would effectively mitigate CH4 emissions by maintaining and enhancing CH4 consumption in these soils, but also by reducing N2 O emissions and increasing C sequestration. The overall impact of climate change on methanogenesis and methanotrophy is poorly understood in Australia, with a lack of data highlighting the need for long-term research and process understanding in this area. For policy addressing land-based greenhouse gas mitigation, all three major greenhouse gases (CO2 , CH4 and N2 O) should be monitored simultaneously, combined with improved understanding at process-level.

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.

P. G. Falkowski (1997). Evolution of the nitrogen cycle and its influence on the biological sequestration of CO2 in the ocean. Nature 387 (15 May): 272-275

ABSTRACT: Over geological time, photosynthetic carbon fixation in the oceans has exceeded respiratory oxidation of organic carbon. The imbalance between the two processes has resulted in the simultaneous accumulation of oxygen in, and drawdown of carbon dioxide from, the Earth's atmosphere, and the burial of organic carbon in marine sediments1–3 . It is generally assumed that these processes are limited by the availability of phosphorus4,5 , which is supplied by continental weathering and fluvial discharge5–7 . Over the past two million years, decreases in atmospheric carbon dioxide concentrations during glacial periods correlate with increases in the export of organic carbon from surface waters to the marine sediments8–11 , but variations in phosphorus fluxes appear to have been too small to account for these changes12,13 . Consequently, it has been assumed that total oceanic primary productivity remained relatively constant during glacial-to-interglacial transitions, although the fraction of this productivity exported to the sediments somehow increased during glacial periods12,14 . Here I present an analysis of the evolution of biogeochemical cycles which suggests that fixed nitrogen, not phosphorus, limits primary productivity on geological timescales. Small variations in the ratio of nitrogen fixation to denitrification can significantly change atmospheric carbon dioxide concentrations on glacial-to-interglacial timescales. The ratio of these two processes appears to be determined by the oxidation state of the ocean and the supply of trace elements, especially iron.

Fletcher, B. J., Brentnall, S. J., Anderson, C. W., Berner, R. A., Beerling, D. J. (2008). Atmospheric carbon dioxide linked with Mesozoic and early Cenozoic climate change. Nature Geoscience 1 (1): 43-48

ABSTRACT: The relationship between atmospheric carbon dioxide (CO2 ) and climate in the Quaternary period has been extensively investigated, but the role of CO2 in temperature changes during the rest of Earth's history is less clear. The range of geological evidence for cool periods during the high CO2 Mesozoic 'greenhouse world' of high atmospheric CO2 concentrations, indicated by models and fossil soils, has been particularly difficult to interpret. Here, we present high-resolution records of Mesozoic and early Cenozoic atmospheric CO2 concentrations from a combination of carbon-isotope analyses of non-vascular plant (bryophyte) fossils and theoretical modelling. These records indicate that atmospheric CO2 rose from ~420 p.p.m.v. in the Triassic period (about 200 million years ago) to a peak of ~1,130 p.p.m.v. in the Middle Cretaceous (about 100 million years ago). Atmospheric CO2 levels then declined to ~680 p.p.m.v. by 60 million years ago. Time-series comparisons show that these variations coincide with large Mesozoic climate shifts, in contrast to earlier suggestions of climate–CO2 decoupling during this interval. These reconstructed atmospheric CO2 concentrations drop below the simulated threshold for the initiation of glaciations on several occasions and therefore help explain the occurrence of cold intervals in a 'greenhouse world'.

J. B. Gaudinski, S. E. Trumbore, E. A. Davidson, S. Zheng (2000). Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes. Biogeochemistry 51 (1): 33-69

ABSTRACT: Temperate forests of North America are thought to be significant sinks of atmospheric CO2 . We developed a below-ground carbon (C) budget for well-drained soils in Harvard Forest Massachusetts, an ecosystem that is storing C. Measurements of carbon and radio carbon (14 C) inventory were used to determine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM):recognizable litter fragments (L), humified low density material (H), and high density or mineral-associated organic matter (M). Turnover times in all fractions increased with soil depth and were 2–5 years for recognizable leaf litter, 5–10 years for root litter, 40–100+ years for low density humified material and >100 years for carbon associated with minerals. These turnover times represent the time carbon resides in the plant + soil system, and may underestimate actual decomposition rates if carbon resides for several years in living root, plant or woody material.

Soil respiration was partitioned into two components using14 C: recent photosynthate which is metabolized by roots and microorganisms within a year of initial fixation (Recent-C), and C that is respired during microbial decomposition of SOM that resides in the soil for several years or longer (Reservoir-C).For the whole soil, we calculate that decomposition of Reservoir-C contributes approximately 41% of the total annual soil respiration. Of this 41%,recognizable leaf or root detritus accounts for 80% of the flux, and 20% is from the more humified fractions that dominate the soil carbon stocks. Measurements of CO2 and14 CO2 in the soil atmosphere and in total soil respiration were combined with surface CO2 fluxes and a soil gas diffusion model to determine the flux and isotopic signature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cm of the soil (O + A + Ap horizons). The average residence time of Reservoir-C in the plant + soil system is 8±1 years and the average age of carbon in total soil respiration (Recent-C + Reservoir-C) is 4±1 years.

The O and A horizons have accumulated 4.4 kg C m–2 above the plow layer since abandonment by settlers in the late-1800's. C pools contributing the most to soil respiration have short enough turnover times that they are likely in steady state. However, most C is stored as humified organic matter within both the O and A horizons and has turnover times from 40 to 100+ years respectively. These reservoirs continue to accumulate carbon at a combined rate of 10–30 g C mminus 2 yr–1 . This rate of accumulation is only 5–15% of the total ecosystem C sink measured in this stand using eddy covariance methods.

Heath, J., Black, H. I. J., Grant, H., Ineson, P., Kerstiens, G., Ayres, E., Possell, M., Bardgett, R. D. (2005). Atmospheric science: 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.

Hirsch, A. I., Michalak, A. M., Bruhwiler, L. M., Peters, W., Dlugokencky, E. J., Tans, P. P. (2006). Inverse modeling estimates of the global nitrous oxide surface flux from 1998–2001. Global Biogeochemical Cycles 20 (GB1008): doi:10.1029/2004GB002443

ABSTRACT: Measurements of nitrous oxide in air samples from 48 sites in the Cooperative Global Air Sampling Network made by NOAA/ESRL GMD CCGG (the Carbon Cycle Greenhouse Gases group in the Global Monitoring Division at the NOAA Earth System Research Laboratory in Boulder, Colorado) and the three-dimensional chemical transport model TM3 were used to infer global nitrous oxide fluxes and their uncertainties from 1998–2001. Results are presented for four semihemispherical regions (90°S–30°S, 30°S to equator, equator to 30°N, 30°N–90°N) and six broad “super regions” (Southern Land, Southern Oceans, Tropical Land, Tropical Oceans, Northern Land, and Northern Oceans). We found that compared to our a priori estimate (from the International Geosphere-Biosphere Programme's Global Emissions Inventory Activity), the a posteriori flux was much lower from 90°S–30°S and substantially higher from equator to 30°N. Consistent with these results, the a posteriori flux from the Southern Oceans region was lower than the a priori estimate, while Tropical Land and Tropical Ocean estimates were higher. The ratio of Northern Hemisphere to Southern Hemisphere fluxes was found to range from 1.9 to 5.2 (depending on the model setup), which is higher than the a priori ratio (1.5) and at the high end of previous estimates. Globally, ocean emissions contributed 26–36% of the total flux (again depending on the model setup), consistent with the a priori estimate (29%), though somewhat higher than some other previous estimates.

Karberg, N. J., Pregitzer, K. S., King, J. S., Friend, A. L., Wood, J. R. (2005). Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone. Oecologia 142 (2): 296-306

ABSTRACT: Global emissions of atmospheric CO2 and tropospheric O3 are rising and expected to impact large areas of the Earths forests. While CO2 stimulates net primary production, O3 reduces photosynthesis, altering plant C allocation and reducing ecosystem C storage. The effects of multiple air pollutants can alter belowground C allocation, leading to changes in the partial pressure of CO2 (p CO2 ) in the soil , chemistry of dissolved inorganic carbonate (DIC) and the rate of mineral weathering. As this system represents a linkage between the long- and short-term C cycles and sequestration of atmospheric CO2 , changes in atmospheric chemistry that affect net primary production may alter the fate of C in these ecosystems. To date, little is known about the combined effects of elevated CO2 and O3 on the inorganic C cycle in forest systems. Free air CO2 and O3 enrichment (FACE) technology was used at the Aspen FACE project in Rhinelander, Wisconsin to understand how elevated atmospheric CO2 and O3 interact to alter pCO2 and DIC concentrations in the soil. Ambient and elevated CO2 levels were 360±16 and 542±81 l l–1 , respectively; ambient and elevated O3 levels were 33±14 and 49±24 nl l–1 , respectively. Measured concentrations of soil CO2 and calculated concentrations of DIC increased over the growing season by 14 and 22%, respectively, under elevated atmospheric CO2 and were unaffected by elevated tropospheric O3 . The increased concentration of DIC altered inorganic carbonate chemistry by increasing system total alkalinity by 210%, likely due to enhanced chemical weathering. The study also demonstrated the close coupling between the seasonal13 C of soilp CO2 and DIC, as a mixing model showed that new atmospheric CO2 accounted for approximately 90% of the C leaving the system as DIC. This study illustrates the potential of using stable isotopic techniques and FACE technology to examine long- and short-term ecosystem C sequestration.

M. A. K. Khalil (1999). Non-CO2 greenhouse gases in the atmosphere. Annual Review of Energy and the Environment 24: 645-661

ABSTRACT: Although global warming is generally linked to increasing levels of carbon dioxide, there are many other gases produced from industrial, agricultural, and energy-generating sources that can also cause the Earth's temperature to rise. Individually these gases are not likely to make a significant contribution, but, taken together, it is believed that they can rival the effects of carbon dioxide. This paper reviews the current trends of the most abundant or the most effective of these non-CO2 greenhouse gases. Methane, nitrous oxide, and the major chlorofluorocarbons (F-11 and F-12) have been the most notable greenhouse gases other than CO2 . Although these gases will continue to play a role in global warming, new compounds are likely to become increasingly important. These include the fluorocarbon replacement compounds in the hydrofluorocarbon and the hydrochlorofluorocarbon groups and gases that are nearly inert in the atmosphere, persisting for thousands of years, such as the perfluorocarbons and sulfur hexafluoride.

L. R. Kump (1989). Chemical stability of the atmosphere and ocean. Palaeogeography, Palaeoclimatology, Palaeoecology 75 (1-2): 123-136

ABSTRACT: The Earth system processes most of the chemical components of its atmosphere and oceans in geologically short periods of time. It does this in a regulated way, one that maintains a remarkably constant surface environment. This we know primarily from the fossil record of uninterrupted, complex life on Earth that extends over the last billion years.

We are only beginning to understand the feedbacks that control the chemistry of the oceans and atmosphere. Numerical models of biogeochemical cycles are most stable when there is feedback between the amount of a chemical component in the ocean or atmosphere and its transfer to or from that reservoir. Coupling of subsystem, especially of those operating on different time scales, enhances stability.

An example of the role of feedback in stabilizing Earth's chemical environment is the mechanism of control of atmospheric oxygen. There appears to be no strong relationship between oxygen level and oxygen consumption. However, oxygen production may be a function of oxygen level; the burial rate of organic carbon (oxygen production) in marine sediments may be sensitive to bottom water oxygenation levels. Also, combustion may be an effective mechanism of transferring nutrients (namely phosphorus) from efficient, terrestrial ecosystems to less efficient, marine ecosystems. When O2 rises, fires become more frequent and P is transferred to the ocean, stimulating marine organic carbon burial but depressing global burial rates. Global O2 production rates decline, as does the O2 level: a negative feedback.

Models of ocean chemical composition are presently incapable of reproducing the temporal constancy indicated by geological observations. These models do not incorporate ion-exchange equilibria as important processes in marine geochemical cycles. When included, these equilibria significantly damp the fluctuations in ion ratios calculated by the extant models.

T. S. Ledley, E. T. Sundquist, S. E. Schwartz, D. K. Hall, J. D. Fellows, T. L. Killeen (1999). Climate change and greenhouse gases. EOS, Transactions of the American Geophysical Union 80 (39): 453

ABSTRACT: Infrared (IR) active gases, principally water vapor (H2 O), carbon dioxide (CO2 ), and ozone (O3 ), naturally present in the Earth’s atmosphere, absorb thermal IR radiation emitted by the Earth’s surface and atmosphere. The atmosphere is warmed by this mechanism and, in turn, emits IR radiation, with a significant portion of this energy acting to warm the surface and the lower atmosphere. As a consequence the average surface air temperature of the Earth is about 30° C higher than it would be without atmospheric absorption and reradiation of IR energy [Henderson-Sellers and Robinson, 1986; Kellogg, 1996; Peixoto and Oort, 1992].

This phenomenon is popularly known as the “greenhouse effect,” and the IR active gases responsible for the effect are likewise referred to as “greenhouse gases.” The rapid increase in concentrations of greenhouse gases since the industrial period began has given rise to concern over potential resultant climate changes.

The AGU Council approved a position statement on Climate Change and Greenhouse Gases in December 1998. The statement and a short summary of the procedures that were followed in its preparation, review, and adoption were published in the February 2, 1999, issue of Eos (p. 49) [AGU, 1999, also at AGU's Web site: http://www.agu.org/sci_soc/policy/climate_change.html]. The present article reviews scientific understanding of this issue, as presented in peer-reviewed publications. This understanding serves as the underlying basis of the
position statement.

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.

Manabe, S., Wetherald, R.T. (1985). Reduction in summer soil wetness induced by an increase in atmospheric carbon dioxide. Science 232 (4750): 626-628

ABSTRACT: The geographical distribution of the change in soil wetness in response to an increase in atmospheric carbon dioxide was investigated by using a mathematical model of climate. Responding to the increase in carbon dioxide, soil moisture in the model would be reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, western Europe, northern Canada, and Siberia. These results were obtained from the model with predicted cloud cover and are qualitatively similar to the results from several numerical experiments conducted earlier with prescribed cloud cover.

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.

Mitchell, J. F. B. (1989). The "greenhouse" effect and climate change. Reviews of Geophysics 27 (1): 115-139

ABSTRACT: The presence of radiatively active gases in the Earth’s atmosphere (water vapor, carbon dioxide, and ozone) raises its global mean surface temperature by 30 K, making our planet habitable by life as we know it. There has been an increase in carbon dioxide and other trace gases since the Industrial Revolution, largely as a result of man’s activities, increasing the radiative heating of the troposphere and surface by about 2 W m-2 . This heating is likely to be enhanced by resulting changes in water vapor, snow and sea ice, and cloud. The associated equilibrium temperature rise is estimated to be between 1 and 2 K, there being uncertainties in the strength of climate feedbacks, particularly those due to cloud. The large thermal inertia of the oceans will slow the rate of warming, so that the expected temperature rise will be smaller than the equilibrium rise. This increases the uncertainty in the expected warming to date, with estimates ranging from less than 0.5 K to over 1 K. The observed increase of 0.5 K since 1900 is consistent with the lower range of these estimates, but the variability in the observed record is such that one cannot necessarily conclude that the observed temperature change is due to increases in trace gases. The prediction of changes in temperature over the next 50 years depends on assumptions concerning future changes in trace gas concentrations, the sensitivity of climate, and the effective thermal inertia of the oceans. On the basis of our current understanding a further warming of at least 1 K seems likely. Numerical models of climate indicate that the changes will not be uniform, nor will they be confined to temperature. The simulated warming is largest in high latitudes in winter and smallest over sea ice in summer, with little seasonal variation in the tropics. Annual mean precipitation and runoff increase in high latitudes, and most simulations indicate a drier land surface in northern mid-latitudes in summer. The agreement between different models is much better for temperature than for changes in the hydrological cycle. Priorities for future research include developing an improved representation of cloud in numerical models, obtaining a better understanding of vertical mixing in the deep ocean, and determining the inherent variability of the ocean-atmosphere system. Progress in these areas should enable detection of a man-made "greenhouse" warming within the next two decades.

J. F. B. Mitchell, T. C. Johns, J. M. Gregory, S. F. B. Tett (2002). Climate response to increasing levels of greenhouse gases and sulphate aerosols. Nature 376 (10 August 2002): 501-504

ABSTRACT: Climate models suggest that increases in greenhouse-gas concentrations in the atmosphere should have produced a larger global mean warming than has been observed in recent decades, unless the climate is less sensitive than is predicted by the present generation of coupled general circulation models1,2 . After greenhouse gases, sulphate aerosols probably exert the next largest anthropogenic radiative forcing of the atmosphere3 , but their influence on global mean warming has not been assessed using such models. Here we use a coupled oceaná¤-atmosphere general circulation model to simulate past and future climate since the beginning of the near-global instrumental surface-temperature record4 , and include the effects of the scattering of radiation by sulphate aerosols. The inclusion of sulphate aerosols significantly improves the agreement with observed global mean and large-scale patterns of temperature in recent decades, although the improvement in simulations of specific regions is equivocal. We predict a future global mean warming of 0.3 K per decade for greenhouse gases alone, or 0.2 K per decade with sulphate aerosol forcing included. By 2050, all land areas have warmed in our simulations, despite strong negative radiative forcing in some regions. These model results suggest that global warming could accelerate as greenhouse-gas forcing begins to dominate over sulphate aerosol forcing.

Mosier,A. R., Halvorson,A. D., Peterson,G. A., Robertson,G. P., Sherrod,L. (2005). Measurement of net global warming potential in three agroecosystems. Nutrient Cycling in Agroecosystems 72 (1): 67-76

ABSTRACT: When appraising the impact of food and fiber production systems on the composition of the Earth's atmosphere and the ‘greenhouse’ effect, the entire suite of biogenic greenhouse gases – carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2 O) – needs to be considered. Storage of atmospheric CO2 into stable organic carbon pools in the soil can sequester CO2 while common crop production practices can produce CO2 , generate N2 O, and decrease the soil sink for atmospheric CH4 . The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40–44% of the GWP from both rain-fed sites and contributed 16–33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether a system is a sink or source of CO2 , i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO2 equivalents ha−1 y−1 . Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N2 O emission rates decline or increase with time after conversion to NT.

Mu, Q., M. Zhao, S. W. Running, M. Liu, H. Tian (2008). Contribution of increasing CO2 and climate change to the carbon cycle in China's ecosystems. Journal of Geophysical Research - Biogeosciences 113 (G01018)

ABSTRACT: Atmospheric CO2 and China's climate have changed greatly during 1961–2000. The influence of increased CO2 and changing climate on the carbon cycle of the terrestrial ecosystems in China is still unclear. In this article we used a process-based ecosystem model, Biome-BGC, to assess the effects of changing climate and elevated atmospheric CO2 on terrestrial China's carbon cycle during two time periods: (1) the present (1961–2000) and (2) a future with projected climate change under doubled CO2 (2071–2110). The effects of climate change alone were estimated by driving Biome-BGC with a fixed CO2 concentration and changing climate, while the CO2 fertilization effects were calculated as the difference between the results driven by both increasing CO2 and changing climate and those of variable climate alone. Model simulations indicate that during 1961–2000 at the national scale, changes in climate reduced carbon storage in China's ecosystems, but increasing CO2 compensated for these adverse effects of climate change, resulting in an overall increase in the carbon storage of China's ecosystems despite decreases in soil carbon. The interannual variability of the carbon cycle was associated with climate variations. Regional differences in climate change produced differing regional carbon uptake responses. Spatially, reductions in carbon in vegetation and soils and increases in litter carbon were primarily caused by climate change in most parts of east China, while carbon in vegetation, soils, and litter increased for much of west China. Under the future scenario (2071–2110), with a doubling CO2 , China will experience higher precipitation and temperature as predicted by the Hadley Centre HadCM3 for the Intergovernmental Panel on Climate Change Fourth Assessment. The concomitant doubling of CO2 will continue to counteract the negative effects of climate change on carbon uptake in the future, leading to an increase in carbon storage relative to current levels. This study highlights the role of CO2 fertilization in the carbon budget of China's ecosystems, although future studies should include other important processes such as land use change, human management (e.g., fertilization and irrigation), environmental pollution, etc.

Niyogi, D., Xue, Y. K. (2006). Soil moisture regulates the biological response of elevated atmospheric CO2 concentrations in a coupled atmosphere biosphere model. Global and Planetary Change 54 (1-2): 94-108

ABSTRACT: Terrestrial biosphere models/land surface models are routinely used to study the effects of CO2 doubling and climate change. The objective of this study is to show that the biological response associated with CO2 doubling is important, and that the effects intrinsically depend on the soil moisture state. Therefore, using a coupled biosphere–atmosphere model, we tested the hypothesis that the biological effects of CO2 changes in biosphere models are significantly coupled to the hydrological feedback via soil moisture availability in a terrestrial biosphere/land surface model. The results from a 15-day simulation of a photosynthesis-based land surface model, dynamically coupled to an atmospheric boundary layer and surface energy balance scheme, were analyzed to test the hypothesis. The objective was to analyze the biological effects of CO2 doubling under high as well as limiting soil moisture conditions for prescribed changes to the vegetation/land use type. The approach was to analyze the results from a coupled land surface-atmosphere model obtained by changing the biome type for each run. Sensitivity for all of the nine global vegetation type changes, as defined through the Simple Biosphere Model ver. 2 (SiB2) land cover classification, were analyzed for evapotranspiration and net carbon assimilation. The results indicated that: (i) the soil moisture (and its interaction with CO2 ) has a direct (first-order) effect on the biological effects of CO2 changes and the terrestrial ecosystem response; (ii) the biological impacts associated with CO2 changes in a biospheric model should be interpreted in consideration of the soil moisture status; and droughts or high soil moisture availability can enhance or completely balance or even reverse the effects associated with CO2 changes; (iii) for each vegetation type, the model results indicated a different response to soil moisture and CO2 changes; and resolving the direct and indirect effects explicitly, both C3 and C4 vegetation, appeared to be significantly affected by the biological effects of CO2 changes, and (iv) the explicit coupling between soil moisture/hydrological state and the CO2 changes need to be explicitly considered in projecting climate change impacts. The study results also indicated that feedback pathways can be efficiently determined by dissociating the direct and the interactive effects of CO2 impacts.

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.

Randerson, J.T., Masiello, C.A., Still C.J., Rahn, T., Poorter, H., Field, C.B. (2006). Is carbon within the global terrestrial biosphere becoming more oxidized? Implications for trends in atmospheric O2 . Global Change Biology 12 (2): 260-271

ABSTRACT: Measurements of atmospheric O2 and CO2 concentrations serve as a widely used means to partition global land and ocean carbon sinks. Interpretation of these measurements has assumed that the terrestrial biosphere contributes to changing O2 levels by either expanding or contracting in size, and thus serving as either a carbon sink or source (and conversely as either an oxygen source or sink). Here, we show how changes in atmospheric O2 can also occur if carbon within the terrestrial biosphere becomes more reduced or more oxidized, even with a constant carbon pool. At a global scale, we hypothesize that increasing levels of disturbance within many biomes has favored plant functional types with lower oxidative ratios and that this has caused carbon within the terrestrial biosphere to become increasingly more oxidized over a period of decades. Accounting for this mechanism in the global atmospheric O2 budget may require a small increase in the size of the land carbon sink. In a scenario based on the Carnegie–Ames–Stanford Approach model, a cumulative decrease in the oxidative ratio of net primary production (NPP) (moles of O2 produced per mole of CO2 fixed in NPP) by 0.01 over a period of 100 years would create an O2 disequilibrium of 0.0017 and require an increased land carbon sink of 0.1 Pg C yr−1 to balance global atmospheric O2 and CO2 budgets. At present, however, it is challenging to directly measure the oxidative ratio of terrestrial ecosystem exchange and even more difficult to detect a disequilibrium caused by a changing oxidative ratio of NPP. Information on plant and soil chemical composition complement gas exchange approaches for measuring the oxidative ratio, particularly for understanding how this quantity may respond to various global change processes over annual to decadal timescales.

Rothman, D.H. (2002). Atmospheric carbon dioxide levels for the last 500 million years. Proceedings of the National Academy of Sciences 99 (7): 4167-4171

ABSTRACT: The last 500 million years of the strontium-isotope record are shown to correlate significantly with the concurrent record of isotopic fractionation between inorganic and organic carbon after the effects of recycled sediment are removed from the strontium signal. The correlation is shown to result from the common dependence of both signals on weathering and magmatic processes. Because the long-term evolution of carbon dioxide levels depends similarly on weathering and magmatism, the relative fluctuations of CO2 levels are inferred from the shared fluctuations of the isotopic records. The resulting CO2 signal exhibits no systematic correspondence with the geologic record of climatic variations at tectonic time scales.

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.

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.

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