Land Carbon Sink Research Articles

Identification and characterization of abrupt changes in the land uptake of carbon

A recent study of the net land carbon sink estimated using the Mauna Loa, Hawaii atmospheric CO2record, fossil fuel estimates, and a suite of ocean models suggests that the mean of the net land carbon uptake remained approximately constant for three decades and increased after 1988/1989. Due to the large variability in the net land uptake, it is not possible to determine the exact timing and nature of the increase robustly by visual inspection. Here, we develop a general methodology to objectively determine the nature and timing of the shift in the net land uptake based on the Schwarz Information Criterion. We confirm that it is likely that an abrupt shift in the mean net land carbon uptake occurred in 1988. After taking into account the variability in the net land uptake due to the influence of volcanic aerosols and the El Niño Southern Oscillation, we find that it is most likely that there is a remaining step increase at the same time (p‐values of 0.01 and 0.04 for Mauna Loa and South Pole, respectively) of about 1 Pg C/yr. Thus, we conclude that neither the effect of volcanic eruptions nor the El Niño Southern Oscillation are the causes of the sudden increase of the land carbon sink. By also applying our methodology to the atmospheric growth rate of CO2, we demonstrate that it is likely that the atmospheric growth rate of CO2 exhibits a step decrease between two fitted lines in 1988–1989, which is most likely due to the shift in the net land uptake of carbon.

Evolution of hydrological and carbon cycles under a changing climate

AbstractHydrological and carbon cycles are inherently coupled and play a pivotal role in the earth system. Water stress is one of the most limiting factors constraining vegetation productivity in both direct and indirect ways (mainly through nutrition cycling). In this article, we first reviewed the concepts of hydrological and carbon cycles and their interactions and then summarized recent advances in our understanding of how global climate change affects water stress, ecosystem carbon cycling, disturbances, and water use efficiency. A warming climate may continue to benefit plant growth in energy limited areas such as high latitudes and altitudes. However, over many water‐limited regions of the world, a warming climate has led to a drying trend, posing negative effects on land carbon sink capability, as shown by reductions in vegetation growth and increases in ecosystem disturbances (wildfires, insect outbreaks, and tree mortality). Carbon cycle and water use efficiency have responded to climate change in different ways in different areas. Further integrating information from climate records, flux measurements at eddy flux towers, and observations from satellites will greatly advance our understanding of the hydrological and carbon cycles and their evolution. Advancement in our understanding of the evolution of hydrological and carbon cycles under the changing climate will help us to adapt and mitigate the adverse effects of climate change. Copyright © 2011 John Wiley & Sons, Ltd.

The Joint UK Land Environment Simulator (JULES), model description – Part 2: Carbon fluxes and vegetation dynamics

Abstract. The Joint UK Land Environment Simulator (JULES) is a process-based model that simulates the fluxes of carbon, water, energy and momentum between the land surface and the atmosphere. Many studies have demonstrated the important role of the land surface in the functioning of the Earth System. Different versions of JULES have been employed to quantify the effects on the land carbon sink of climate change, increasing atmospheric carbon dioxide concentrations, changing atmospheric aerosols and tropospheric ozone, and the response of methane emissions from wetlands to climate change. This paper describes the consolidation of these advances in the modelling of carbon fluxes and stores, in both the vegetation and soil, in version 2.2 of JULES. Features include a multi-layer canopy scheme for light interception, including a sunfleck penetration scheme, a coupled scheme of leaf photosynthesis and stomatal conductance, representation of the effects of ozone on leaf physiology, and a description of methane emissions from wetlands. JULES represents the carbon allocation, growth and population dynamics of five plant functional types. The turnover of carbon from living plant tissues is fed into a 4-pool soil carbon model. The process-based descriptions of key ecological processes and trace gas fluxes in JULES mean that this community model is well-suited for use in carbon cycle, climate change and impacts studies, either in standalone mode or as the land component of a coupled Earth system model.

Forest biomass allometry in global land surface models

[1] A number of global land surface models simulate photosynthesis, respiration, and disturbance, important flows in the carbon cycle that are widely tested against flux towers and CO2 concentration gradients. The resulting forest biomass is examined in this paper for its resemblance to realistic stands, which are characterized using allometric theory. The simulated biomass pools largely do not conform to widely observed allometry, particularly for young stands. The best performing models had an explicit treatment of stand-thinning processes, which brought the slope of the allometry of these models closer to observations. Additionally, models that had relatively shorter wood turnover times performed were generally closer to observed allometries. The discrepancy between the pool distribution between models and data suggests estimates of NEE have biases when integrated over the long term, as compared to observed biomass data, and could therefore compromise long-term predictions of land carbon sources and sinks. We think that this presents a practical obstacle for improving models by informing them better with data. The approach taken in this paper, examining biomass pools allometrically, offers a simple approach to improving the characteristic behaviors of global models with the relatively sparse data that is available globally by forest inventory.

Pinning down the land carbon sink

Over half of all anthropogenic carbon dioxide emissions into the atmosphere are removed naturally by land and ocean carbon sinks. New analysis indicates that the land sink is increasing in some surprising places.

Regional trends in terrestrial carbon exchange and their seasonal signatures

The trends of terrestrial carbon exchange and their mechanistic drivers are key components in understanding how carbon reservoirs will respond to climate change. Here we show trends in seasonal non-fossil land-atmosphere carbon exchange from 1980 to 2008 using atmospheric CO2 inversion results. Four indices were analysed: growing-season net flux (GSNF), dormant-season net flux (DSNF), amplitude and annual net carbon flux (NCF). We find that the global land carbon sink is intensifying at -0.057 ± 0.01 PgC yr-2, resulting in -1.65 ± 0.29 PgC of additional uptake over the period examined. This increased total land uptake is driven by a decline in the DSNF (-0.04 ± 0.01 PgC yr-2) and intensification of the GSNF (-0.02 ± 0.008 PgC yr-2). Regional analysis shows the dominant role of the southern half of the African continent; intensification of the GSNF (-0.02 ± 0.005 PgC yr-2) and a decline in the DSNF (-0.013 ± 0.004 PgC yr-2) imply that Africa has shifted from a net carbon source in the 1980s to near-neutral emissions. By contrast, a weakening of the GSNF is found in temperate North America (0.015±0.007 PgC yr-2) and tropical America (0.01 ± 0.005 PgC yr-2).

Trends and regional distributions of land and ocean carbon sinks

Abstract. We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 Pg C yr−1 between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) Pg C yr−1 to a new relatively constant mean of −1.15 Pg C yr−1 between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO2, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

Estimated carbon residence times in three forest ecosystems of eastern China: Applications of probabilistic inversion

Carbon residence time is one critical parameter for predicting future land carbon sink dynamics but has not been well quantified for many plant and soil pools. This study applied a probabilistic inverse analysis of multiple observations to estimate mean residence times of carbon among three forest ecosystems in eastern China. Three assimilation experiments were conducted with either net ecosystem exchange data from eddy‐flux measurements or six biometric and soil data (i.e., foliage biomass, fine root biomass, woody biomass, litterfall, soil organic carbon, and soil respiration) or all data to evaluate their relative effectiveness on estimation of carbon residence times of different pools in a terrestrial ecosystem model. Estimated mean residence times of carbon ranged from 2 to 10 months for metabolic litter and microbial biomass pools, from 1 to 3 years for foliage, fine root biomass, and structural litter pools, and from 17 to 1361 years for woody biomass, slow and passive soil organic matter pools at three forest sites. The residence times of carbon were longer for leaf, litter and microbes pools but shorter for fine root and wood pools in the young evergreen coniferous plantation at Qianyanzhou site than the two mature mixed forests at Changbaishan and Dinghushan sites. Carbon residence times were well constrained for three plant pools and slow soil organic matter by biometric and soil data, whereas residence times for metabolic and structural litter, and microbial biomass pools were constrained by daily net ecosystem exchange data. Overall, our study demonstrated that biometric, soil and net ecosystem exchange data are complementary in constraining mean residence times of an ecosystem model.

Sustainable Foresting: Easier Said Than Done

It is commonly held that planting forests helps to mitigate climate change, because forests sequester carbon dioxide into long-lived biomass and soils (1). However, our personal experience shows that managed forests are unlikely to increase the land carbon sink unless foresters are paid a fair price for the ecosystem services they provide.

Implications of future climate and atmospheric CO2 content for regional biogeochemistry, biogeography and ecosystem services across East Africa

AbstractWe model future changes in land biogeochemistry and biogeography across East Africa. East Africa is one of few tropical regions where general circulation model (GCM) future climate projections exhibit a robust response of strong future warming and general annual‐mean rainfall increases. Eighteen future climate projections from nine GCMs participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment were used as input to the LPJ dynamic global vegetation model (DGVM), which predicted vegetation patterns and carbon storage in agreement with satellite observations and forest inventory data under the present‐day climate. All simulations showed future increases in tropical woody vegetation over the region at the expense of grasslands. Regional increases in net primary productivity (NPP) (18–36%) and total carbon storage (3–13%) by 2080–2099 compared with the present‐day were common to all simulations. Despite decreases in soil carbon after 2050, seven out of nine simulations continued to show an annual net land carbon sink in the final decades of the 21st century because vegetation biomass continued to increase. The seasonal cycles of rainfall and soil moisture show future increases in wet season rainfall across the GCMs with generally little change in dry season rainfall. Based on the simulated present‐day climate and its future trends, the GCMs can be grouped into four broad categories. Overall, our model results suggest that East Africa, a populous and economically poor region, is likely to experience some ecosystem service benefits through increased precipitation, river runoff and fresh water availability. Resulting enhancements in NPP may lead to improved crop yields in some areas. Our results stand in partial contradiction to other studies that suggest possible negative consequences for agriculture, biodiversity and other ecosystem services caused by temperature increases.

Spatiotemporal patterns of terrestrial carbon cycle during the 20th century

We evaluated how climate change, rising atmospheric CO2 concentration, and land use change influenced the terrestrial carbon (C) cycle for the last century using a process‐based ecosystem model. Over the last century, the modeled land use change emitted about 129 Pg of C to the atmosphere. About 76% (or 98 Pg C) of this emission, however, was offset by net C uptake on land driven by climate changes and rising atmospheric CO2 concentration. Thus, the modeled net release of C from the terrestrial ecosystems to the atmosphere from 1901 to 2002 is about 31 Pg C. Global net primary productivity (NPP) has significantly increased by 14% during the last century, especially since the 1970s. From 1980 to 2002, global NPP increased with an average increase rate of 0.4% yr−1. At global scale, such an increase seems to be primarily attributed to the increase in atmospheric CO2 concentration, and then to precipitation change. Over the last 2 decades, climate change and rising CO2 forced the land carbon sink (1.6 Pg C yr−1 for 1980s and 2.2 Pg C yr−1 for 1990s) to be larger than land use change driven carbon emissions (1.0 Pg C yr−1 for 1980s and 1.2 Pg C yr−1 for 1990s), resulting a net land sink of 0.5 Pg C yr−1 in the 1980s and of 1.0 Pg C yr−1 in the 1990s. The largest C emission from land use change appeared in tropical regions with an average emission of 0.6 Pg C yr−1 in 1980s and 0.7 Pg C yr−1 in 1990s, which is slightly larger than net carbon uptake due to CO2 fertilization and climate change. Thus, net carbon balance of tropical lands is close to neutral over the past 2 decades (about 0.13 Pg C yr−1 in 1980s and 0.03 Pg C yr−1 in 1990s). We also found that current global warming has already started accelerating C loss from terrestrial ecosystems, by enhanced decomposition of soil organic carbon. In response to warming trends only, the global net carbon uptake significantly decreased, offsetting about 70% of the increase in global net carbon uptake owing to CO2 fertilization during 1980–2002. The global terrestrial C cycle also shows large year‐to‐year variations, and different regions have quite distinct dominant drivers. Generally, interannual changes of carbon fluxes in tropical and temperate ecosystems are mainly explained by precipitation variability, while temperature variability plays a major role in boreal ecosystems.

Impact of changes in diffuse radiation on the global land carbon sink

Plant photosynthesis tends to increase with irradiance. However, recent theoretical and observational studies have demonstrated that photosynthesis is also more efficient under diffuse light conditions. Changes in cloud cover or atmospheric aerosol loadings, arising from either volcanic or anthropogenic emissions, alter both the total photosynthetically active radiation reaching the surface and the fraction of this radiation that is diffuse, with uncertain overall effects on global plant productivity and the land carbon sink. Here we estimate the impact of variations in diffuse fraction on the land carbon sink using a global model modified to account for the effects of variations in both direct and diffuse radiation on canopy photosynthesis. We estimate that variations in diffuse fraction, associated largely with the 'global dimming' period, enhanced the land carbon sink by approximately one-quarter between 1960 and 1999. However, under a climate mitigation scenario for the twenty-first century in which sulphate aerosols decline before atmospheric CO(2) is stabilized, this 'diffuse-radiation' fertilization effect declines rapidly to near zero by the end of the twenty-first century.

Global nitrogen deposition and carbon sinks

Land and ocean uptake of carbon dioxide plays a critical role in determining atmospheric carbon dioxide levels. Future increases in nitrogen deposition have been predicted to increase the size of these terrestrial and marine carbon sinks, but although higher rates of nitrogen deposition might enhance carbon uptake in northern and tropical forests, they will probably have less of an impact on ocean sink strength. Combined, the land and ocean sinks may sequester an additional 10% of anthropogenic cabon emissions by 2030 owing to increased nitrogen inputs, but a more conservative estimate of 1 to 2% is more likely. Thus nitrogen-induced increases in the strength of land and ocean sinks are unlikely to keep pace with future increases in carbon dioxide. Land and ocean carbon sinks play a critical role in determining atmospheric carbon dioxide levels. Nitrogen-induced increases in land and ocean sink strength are unlikely to keep pace with future increases in carbon dioxide.

Indirect radiative forcing of climate change through ozone effects on the land-carbon sink

The evolution of the Earth's climate over the twenty-first century depends on the rate at which anthropogenic carbon dioxide emissions are removed from the atmosphere by the ocean and land carbon cycles. Coupled climate-carbon cycle models suggest that global warming will act to limit the land-carbon sink, but these first generation models neglected the impacts of changing atmospheric chemistry. Emissions associated with fossil fuel and biomass burning have acted to approximately double the global mean tropospheric ozone concentration, and further increases are expected over the twenty-first century. Tropospheric ozone is known to damage plants, reducing plant primary productivity and crop yields, yet increasing atmospheric carbon dioxide concentrations are thought to stimulate plant primary productivity. Increased carbon dioxide and ozone levels can both lead to stomatal closure, which reduces the uptake of either gas, and in turn limits the damaging effect of ozone and the carbon dioxide fertilization of photosynthesis. Here we estimate the impact of projected changes in ozone levels on the land-carbon sink, using a global land carbon cycle model modified to include the effect of ozone deposition on photosynthesis and to account for interactions between ozone and carbon dioxide through stomatal closure. For a range of sensitivity parameters based on manipulative field experiments, we find a significant suppression of the global land-carbon sink as increases in ozone concentrations affect plant productivity. In consequence, more carbon dioxide accumulates in the atmosphere. We suggest that the resulting indirect radiative forcing by ozone effects on plants could contribute more to global warming than the direct radiative forcing due to tropospheric ozone increases.

Comparing atmospheric transport models for future regional inversions over Europe – Part 1: mapping the atmospheric CO<sub>2</sub> signals

Abstract. The CO2 source and sink distribution across Europe can be estimated in principle through inverse methods by combining CO2 observations and atmospheric transport models. Uncertainties of such estimates are mainly due to insufficient spatiotemporal coverage of CO2 observations and biases of the models. In order to assess the biases related to the use of different models the CO2 concentration field over Europe has been simulated with five different Eulerian atmospheric transport models as part of the EU-funded AEROCARB project, which has the main goal to estimate the carbon balance of Europe. In contrast to previous comparisons, here both global coarse-resolution and regional higher-resolution models are included. Continuous CO2 observations from continental, coastal and mountain sites as well as flasks sampled on aircrafts are used to evaluate the models' ability to capture the spatiotemporal variability and distribution of lower troposphere CO2 across Europe. 14CO2 is used in addition to evaluate separately fossil fuel signal predictions. The simulated concentrations show a large range of variation, with up to ~10 ppm higher surface concentrations over Western and Central Europe in the regional models with highest (mesoscale) spatial resolution. The simulation – data comparison reveals that generally high-resolution models are more successful than coarse models in capturing the amplitude and phasing of the observed short-term variability. At high-altitude stations the magnitude of the differences between observations and models and in between models is less pronounced, but the timing of the diurnal cycle is not well captured by the models. The data comparisons show also that the timing of the observed variability on hourly to daily time scales at low-altitude stations is generally well captured by all models. However, the amplitude of the variability tends to be underestimated. While daytime values are quite well predicted, nighttime values are generally underpredicted. This is a reflection of the different mixing regimes during day and night combined with different vertical resolution between models. In line with this finding, the agreement among models is increased when sampling in the afternoon hours only and when sampling the mixed portion of the PBL, which amounts to sampling at a few hundred meters above ground. The main recommendations resulting from the study for constraining land carbon sources and sinks using high-resolution concentration data and state-of-the art transport models through inverse methods are given in the following: 1) Low altitude stations are presently preferable in inverse studies. If high altitude stations are used then the model level that represents the specific sites should be applied, 2) at low altitude sites only the afternoon values of concentrations can be represented sufficiently well by current models and therefore afternoon values are more appropriate for constraining large-scale sources and sinks in combination with transport models, 3) even when using only afternoon values it is clear that data sampled several hundred meters above ground can be represented substantially more robustly in models than surface station records, which emphasize the use of tower data in inverse studies and finally 4) traditional large scale transport models seem not sufficient to resolve fine-scale features associated with fossil fuel emissions, as well as larger-scale features like the concentration distribution above the south-western Europe. It is therefore recommended to use higher resolution models for interpretation of continental data in future studies.

A climate-change risk analysis for world ecosystems

We quantify the risks of climate-induced changes in key ecosystem processes during the 21st century by forcing a dynamic global vegetation model with multiple scenarios from 16 climate models and mapping the proportions of model runs showing forest/nonforest shifts or exceedance of natural variability in wildfire frequency and freshwater supply. Our analysis does not assign probabilities to scenarios or weights to models. Instead, we consider distribution of outcomes within three sets of model runs grouped by the amount of global warming they simulate: <2 degrees C (including simulations in which atmospheric composition is held constant, i.e., in which the only climate change is due to greenhouse gases already emitted), 2-3 degrees C, and >3 degrees C. High risk of forest loss is shown for Eurasia, eastern China, Canada, Central America, and Amazonia, with forest extensions into the Arctic and semiarid savannas; more frequent wildfire in Amazonia, the far north, and many semiarid regions; more runoff north of 50 degrees N and in tropical Africa and northwestern South America; and less runoff in West Africa, Central America, southern Europe, and the eastern U.S. Substantially larger areas are affected for global warming >3 degrees C than for <2 degrees C; some features appear only at higher warming levels. A land carbon sink of approximately 1 Pg of C per yr is simulated for the late 20th century, but for >3 degrees C this sink converts to a carbon source during the 21st century (implying a positive climate feedback) in 44% of cases. The risks continue increasing over the following 200 years, even with atmospheric composition held constant.

Insights from simulations with high‐resolution transport and process models on sampling of the atmosphere for constraining midlatitude land carbon sinks

We analyze the requirements for detecting changes in midlatitude land carbon sources or sinks from sampling the atmospheric CO2 concentration. Programs to sample the continental lower troposphere have only recently started, and it is not yet clear which atmospheric sampling strategy is most adequate. To shed some light on this question, we use simulations of atmospheric CO2 over Eurasia with two regional‐scale atmospheric transport models. Our analysis focuses on the detection of the monthly mean CO2 signal caused by a perturbation of Eurasian summer biospheric fluxes by 20% (0.06 PgC/month). The main results are (1) that several measurements per day, preferably during the afternoon, are necessary to permit the detection of the additional land sink and (2) that the ratio between signal and background variation, corrected for autocorrelation in time, suggests no preferred level in the vertical for sampling. However, (3) the signals in the free troposphere are very small (0.2 ppm per 0.06 PgC/month) given the precision of atmospheric measurements. In contrast, signals in the planetary boundary layer (PBL) are on the order of 1 ppm per 0.06 PgC/month. This suggests that optimal sampling on continents should concentrate on the mixed portion of the PBL during afternoon. (4) Finally, the spatial correlation structure of the atmospheric CO2 concentrations suggests that a horizontal sampling density on the order of a few 100 km is needed.

Is carbon within the global terrestrial biosphere becoming more oxidized? Implications for trends in atmospheric O2

AbstractMeasurements 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.

Global terrestrial carbon storage and uncertainties in its temperature sensitivity examined with a simple model

AbstractThe future of the land carbon sink is a significant uncertainty in global change projections. Here, key controls on global terrestrial carbon storage are examined using a simple model of vegetation and soil. Equilibrium solutions are derived as a function of atmospheric CO2 and global temperature, these environmental variables are then linked in an idealized global change trajectory, and the lag between the dynamic and equilibrium solutions is derived for different linear rates of increase in atmospheric CO2. Terrestrial carbon storage is departing significantly from equilibrium because CO2 and temperature are increasing on a similar timescale to ecosystem change, and the lag is found to be proportional to the rate of forcing. Thus peak sizes of the land carbon sink, and any future land carbon source, are proportional to the rate of increase of CO2. A switch from a land carbon sink to a source occurs at a higher CO2 and temperature under more rapid forcing. The effects of parameter uncertainty in temperature sensitivities of photosynthesis, plant respiration and soil respiration, and structural uncertainty through the effect of fixing the ratio of plant respiration to photosynthesis are explored. In each case, the CO2 fertilization effect on photosynthesis is constrained to reproduce the 1990 atmospheric CO2 concentration within a closed global model. New literature compilations are presented for the temperature sensitivities of plant and soil respiration. A lower limit, Q10=1.29, for soil respiration significantly increases future land carbon storage. An upper limit, Q10=3.63, for soil respiration underpredicts the increase in carbon storage since the Last Glacial Maximum. Fixing the ratio of plant respiration to photosynthesis (R/P) at 0.5 generates the largest and most persistent land carbon sink, followed by the weakest land carbon source.

Strong carbon cycle feedbacks in a climate model with interactive CO2 and sulphate aerosols

Carbon cycle feedbacks are a significant source of uncertainty in climate change projections, with the potential for strong positive feedbacks to accelerate the rate of anthropogenic global warming during the 21st century. A climate change experiment is presented which uses a General Circulation Model (GCM) in which both interactive carbon and sulphur cycles have been included for the first time, along with the natural climate forcings due to solar changes and volcanic aerosol. These extra climate forcing factors have a significant impact on both 20th century climate change and the contemporary land and ocean carbon sinks. The additional forcings act to delay by more than a decade the conversion of the land carbon sink to a source, but ultimately result in a more abrupt rate of CO2 increase with the land carbon source (which reaches 7 GtC yr−1 by 2100) exceeding the ocean carbon sink (which saturates at 5 GtC yr−1 by 2100) beyond about 2080.