Coupled Climate Carbon Cycle Model Research Articles

Estimating the near-surface permafrost-carbon feedback on global warming

Abstract. Thawing of permafrost and the associated release of carbon constitutes a positive feedback in the climate system, elevating the effect of anthropogenic GHG emissions on global-mean temperatures. Multiple factors have hindered the quantification of this feedback, which was not included in climate carbon-cycle models which participated in recent model intercomparisons (such as the Coupled Carbon Cycle Climate Model Intercomparison Project – C4MIP) . There are considerable uncertainties in the rate and extent of permafrost thaw, the hydrological and vegetation response to permafrost thaw, the decomposition timescales of freshly thawed organic material, the proportion of soil carbon that might be emitted as carbon dioxide via aerobic decomposition or as methane via anaerobic decomposition, and in the magnitude of the high latitude amplification of global warming that will drive permafrost degradation. Additionally, there are extensive and poorly characterized regional heterogeneities in soil properties, carbon content, and hydrology. Here, we couple a new permafrost module to a reduced complexity carbon-cycle climate model, which allows us to perform a large ensemble of simulations. The ensemble is designed to span the uncertainties listed above and thereby the results provide an estimate of the potential strength of the feedback from newly thawed permafrost carbon. For the high CO2 concentration scenario (RCP8.5), 33–114 GtC (giga tons of Carbon) are released by 2100 (68 % uncertainty range). This leads to an additional warming of 0.04–0.23 °C. Though projected 21st century permafrost carbon emissions are relatively modest, ongoing permafrost thaw and slow but steady soil carbon decomposition means that, by 2300, about half of the potentially vulnerable permafrost carbon stock in the upper 3 m of soil layer (600–1000 GtC) could be released as CO2, with an extra 1–4 % being released as methane. Our results also suggest that mitigation action in line with the lower scenario RCP3-PD could contain Arctic temperature increase sufficiently that thawing of the permafrost area is limited to 9–23 % and the permafrost-carbon induced temperature increase does not exceed 0.04–0.16 °C by 2300.

CORRIGENDUM

The increase in atmospheric CO2 over this century depends on the evolution of the oceanic air–sea CO2 uptake, which will be driven by the combined response to rising atmospheric CO2 itself and climate change. Here, the future oceanic CO2 uptake is simulated using an ensemble of coupled climate–carbon cycle models. The models are driven by CO2 emissions from historical data and the Special Report on Emissions Scenarios (SRES) A2 high-emission scenario. A linear feedback analysis successfully separates the regional future (2010–2100) oceanic CO2 uptake into a CO2-induced component, due to rising atmospheric CO2 concentrations, and a climate-induced component, due to global warming. The models capture the observation-based magnitude and distribution of anthropogenic CO2 uptake. The distributions of the climate-induced component are broadly consistent between the models, with reduced CO2 uptake in the subpolar Southern Ocean and the equatorial regions, owing to decreased CO2 solubility; and reduced CO2 uptake in the midlatitudes, owing to decreased CO2 solubility and increased vertical stratification. The magnitude of the climate-induced component is sensitive to local warming in the southern extratropics, to large freshwater fluxes in the extratropical North Atlantic Ocean, and to small changes in the CO2 solubility in the equatorial regions. In key anthropogenic CO2 uptake regions, the climate-induced component offsets the CO2-induced component at a constant proportion up until the end of this century. This amounts to approximately 50% in the northern extratropics and 25% in the southern extratropics and equatorial regions. Consequently, the detection of climate change impacts on anthropogenic CO2 uptake may be difficult without monitoring additional tracers, such as oxygen.

Sensitivity of atmospheric CO<sub>2</sub> and climate to explosive volcanic eruptions

Abstract. Impacts of low-latitude, explosive volcanic eruptions on climate and the carbon cycle are quantified by forcing a comprehensive, fully coupled carbon cycle-climate model with pulse-like stratospheric aerosol optical depth changes. The model represents the radiative and dynamical response of the climate system to volcanic eruptions and simulates a decrease of global and regional atmospheric surface temperature, regionally distinct changes in precipitation, a positive phase of the North Atlantic Oscillation, and a decrease in atmospheric CO2 after volcanic eruptions. The volcanic-induced cooling reduces overturning rates in tropical soils, which dominates over reduced litter input due to soil moisture decrease, resulting in higher land carbon inventories for several decades. The perturbation in the ocean carbon inventory changes sign from an initial weak carbon sink to a carbon source. Positive carbon and negative temperature anomalies in subsurface waters last up to several decades. The multi-decadal decrease in atmospheric CO2 yields a small additional radiative forcing that amplifies the cooling and perturbs the Earth System on longer time scales than the atmospheric residence time of volcanic aerosols. In addition, century-scale global warming simulations with and without volcanic eruptions over the historical period show that the ocean integrates volcanic radiative cooling and responds for different physical and biogeochemical parameters such as steric sea level or dissolved oxygen. Results from a suite of sensitivity simulations with different magnitudes of stratospheric aerosol optical depth changes and from global warming simulations show that the carbon cycle-climate sensitivity γ, expressed as change in atmospheric CO2 per unit change in global mean surface temperature, depends on the magnitude and temporal evolution of the perturbation, and time scale of interest. On decadal time scales, modeled γ is several times larger for a Pinatubo-like eruption than for the industrial period and for a high emission, 21st century scenario.

Uncertainty of climate response to natural and anthropogenic forcings due to different land use scenarios

The A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS) climate model (CM) of intermediate complexity is extended by a spatially explicit terrestrial carbon cycle module. Numerical experiments with the IAP RAS CM are performed forced by the reconstructions of anthropogenic and natural forcings for the 16th to the 20th centuries and by combined SRES (Special Report on Emission Scenarios) A2-LUH (Land Use Harmonization) anthropogenic scenarios for the 21st century. Hereby, the impact of uncertainty in land-use scenarios on results of simulations with a coupled climate-carbon cycle model is tested. The simulations of the model realistically reproduced historical changes in carbon cycle characteristics. In the IAP RAS CM, climate warming reproduced in the 20th and 21st centuries enhanced terrestrial net primary production but terrestrial carbon uptake was suppressed due to an overcompensating increase in soil respiration. Around year 2100, the simulations the model forced by different land use scenarios diverged markedly, by about 70 Pg (C) in terms of biomass and soil carbon stock but they differed only by about 10 in terms of atmospheric carbon dioxide content.

Regional Impacts of Climate Change and Atmospheric CO2 on Future Ocean Carbon Uptake: A Multimodel Linear Feedback Analysis

Abstract The increase in atmospheric CO2 over this century depends on the evolution of the oceanic air–sea CO2 uptake, which will be driven by the combined response to rising atmospheric CO2 itself and climate change. Here, the future oceanic CO2 uptake is simulated using an ensemble of coupled climate–carbon cycle models. The models are driven by CO2 emissions from historical data and the Special Report on Emissions Scenarios (SRES) A2 high-emission scenario. A linear feedback analysis successfully separates the regional future (2010–2100) oceanic CO2 uptake into a CO2-induced component, due to rising atmospheric CO2 concentrations, and a climate-induced component, due to global warming. The models capture the observation-based magnitude and distribution of anthropogenic CO2 uptake. The distributions of the climate-induced component are broadly consistent between the models, with reduced CO2 uptake in the subpolar Southern Ocean and the equatorial regions, owing to decreased CO2 solubility; and reduced CO2 uptake in the midlatitudes, owing to decreased CO2 solubility and increased vertical stratification. The magnitude of the climate-induced component is sensitive to local warming in the southern extratropics, to large freshwater fluxes in the extratropical North Atlantic Ocean, and to small changes in the CO2 solubility in the equatorial regions. In key anthropogenic CO2 uptake regions, the climate-induced component offsets the CO2-induced component at a constant proportion up until the end of this century. This amounts to approximately 50% in the northern extratropics and 25% in the southern extratropics and equatorial regions. Consequently, the detection of climate change impacts on anthropogenic CO2 uptake may be difficult without monitoring additional tracers, such as oxygen.

Comparison of modeled and reconstructed changes in forest cover through the past 8000 years

Reproducing the tree cover changes throughout the Holocene is a challenge for land surface–atmosphere models. Here, results of a transient Holocene simulation of the coupled climate–carbon cycle model, CLIMBER2-LPJ, driven by changes in orbital forcing, are compared with pollen data and pollen-based reconstructions for several regions of Eurasia in terms of changes in tree fraction. The decline in tree fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the Holocene. The cooler and drier trend at the eastern side of the Eurasian continent, in Mongolia and China, also led to a decrease in tree cover in both model and data. In contrast, the Holocene trend towards a cooler climate in the continental interior (Kazakhstan) is accompanied by an increase in woody cover. There a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon-affected regions. In general the model-data comparison demonstrates that climate-driven changes during the Holocene result in a non-homogeneous pattern of tree cover change across the Eurasian continent. For the Eifel region in Germany, the model suggests a relatively moist and cool climate and dense tree cover. The Holzmaar pollen record agrees with the model for the intervals 8–3 ka and 1.7–1.3 ka BP, but suggests great reduction of the tree cover 3–2 ka and after 1.3 ka BP, when highly developed settlements and agriculture spread in the region.

Coupled climate–carbon simulations indicate minor global effects of wars and epidemics on atmospheric CO2 between ad 800 and 1850

Historic events such as wars and epidemics have been suggested as explanation for decreases in atmospheric CO2 reconstructed from ice cores because of their potential to take up carbon in forests regrowing on abandoned agricultural land. Here, we use a coupled climate–carbon cycle model to assess the carbon and climate effects of the Mongol invasion (~1200 to ~1380), the Black Death (~1347 to ~1400), the conquest of the Americas (~1519 to ~1700), and the fall of the Ming Dynasty (~1600 to ~1650). We calculate their impact on atmospheric CO2 including the response of the global land and ocean carbon pools. It has been hypothesized that these events have contributed to significant increases in land carbon stocks. However, we find that slow regrowth and delayed emissions from past land cover change allow for small increases of the land biosphere carbon storage only during long-lasting events. The effect of these small increases in land biosphere storage on global CO2 is reduced by the response of the global carbon pools and largely offset by concurrent emissions from the rest of the world. None of these events would therefore have affected the atmospheric CO2 concentration by more than 1 ppm. Only the Mongol invasion could have lowered global CO2, but by an amount too small to be resolved by ice cores.

Sensitivity of a coupled climate-carbon cycle model to large volcanic eruptions during the last millennium

The sensitivity of the climate–biogeochemistry system to volcanic eruptions is investigated using the comprehensive Earth System Model developed at the Max Planck Institute for Meteorology. The model includes an interactive carbon cycle with modules for terrestrial biosphere as well as ocean biogeochemistry. The volcanic forcing is based on a recent reconstruction for the last 1200 yr. An ensemble of five simulations is performed and the averaged response of the system is analysed in particular for the largest eruption of the last millennium in the year 1258. After this eruption, the global annual mean temperature drops by 1 K and recovers slowly during 10 yr. Atmospheric CO 2 concentration declines during 4 yr after the eruption by ca. 2 ppmv to its minimum value and then starts to increase towards the pre-eruption level. This CO 2 decrease is explained mainly by reduced heterotrophic respiration on land in response to the surface cooling, which leads to increased carbon storage in soils, mostly in tropical and subtropical regions. The ocean acts as a weak carbon sink, which is primarily due to temperature-induced solubility. This sink saturates 2 yr after the eruption, earlier than the land uptake. DOI: 10.1111/j.1600-0889.2010.00471.x

CO2 enhancement of forest productivity constrained by limited nitrogen availability.

Stimulation of terrestrial plant production by rising CO(2) concentration is projected to reduce the airborne fraction of anthropogenic CO(2) emissions. Coupled climate-carbon cycle models are sensitive to this negative feedback on atmospheric CO(2), but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO(2) fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO(2) concentration over 11 y in a free-air CO(2) enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO(2) compared with NPP in plots in current ambient CO(2), and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO(2) declined from 24% in 2001-2003 to 9% in 2008. Global analyses that assume a sustained CO(2) fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time--an expected consequence of stand development, which was exacerbated by elevated CO(2). Leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO(2); these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.

Historical and future anthropogenic emission pathways derived from coupled climate–carbon cycle simulations

Using a coupled climate–carbon cycle model, fossil fuel carbon dioxide (CO2) emissions are derived through a reverse approach of prescribing atmospheric CO2 concentrations according to observations and future projections, respectively. In the second half of the twentieth century, the implied fossil fuel emissions, and also the carbon uptake by land and ocean, are within the range of observational estimates. Larger discrepancies exist in the earlier period (1860–1960), with small fossil fuel emissions and uncertain emissions from anthropogenic land cover change. In the IPCC SRES A1B scenario, the simulated fossil fuel emissions more than double until 2050 (17 GtC/year) and then decrease to 12 GtC/year by 2100. In addition to A1B, an aggressive mitigation scenario was employed, developed within the European ENSEMBLES project, that peaks at 530 ppm CO2(equiv) around 2050 and then decreases to approach 450 ppm during the twenty-second century. Consistent with the prescribed pathway of atmospheric CO2 in E1, the implied fossil fuel emissions increase from currently 8 GtC/year to about 10 by 2015 and decrease thereafter. In the 2050s (2090s) the emissions decrease to 3.4 (0.5) GtC/year, respectively. As in previous studies, our model simulates a positive climate–carbon cycle feedback which tends to reduce the implied emissions by roughly 1 GtC/year per degree global warming. Further, our results suggest that the 450 ppm stabilization scenario may not be sufficient to fulfill the European Union climate policy goal of limiting the global temperature increase to a maximum of 2°C compared to pre-industrial levels.

Anthropogenic carbon dynamics in the changing ocean

Abstract. The long-term response of CO2 fluxes to climate change at the ocean surface and within the ocean interior is investigated using a coupled climate-carbon cycle model. This study also presents the first attempt to quantify the evolution of lateral transport of anthropogenic carbon under future climate change. Additionally, its impact on regional carbon storage and uptake is also evaluated. For the 1850–2099 period, our climate change simulation predicts oceanic uptake of anthropogenic carbon of about 538±23 Pg C. Another simulation indicates that changes in physical climate and its associated biogeochemical feedbacks result in a release of natural carbon of about 22±30 Pg C. The natural carbon outgassing is attributed to the reduction in solubility and change in wind pattern in the Southern Hemisphere. After the anthropogenic carbon passes through the air-sea interface, it is predominantly transported along the large scale overturning circulation below the surface layer. The spatial variations in the transport patterns in turn influence the evolution of future regional carbon uptake. In the North Atlantic, a slow down in the Atlantic Meridional Overturning Circulation weakens the penetration strength of anthropogenic carbon into the deeper ocean, which leads to a reduced uptake rate in this region. In contrast, more than half of the anthropogenic carbon taken up in the high latitude Southern Ocean region (south of 58° S) is efficiently and continuously exported northward, predominantly into intermediate waters. This transport mechanism allows continuous increase in future carbon uptake in the high latitude Southern Ocean, where the annual uptake strength could reach 39.3±0.9 g C m−2 yr−1, more than twice the global mean of 16.0±0.3 g C m−2 yr−1 by the end of the 21st century. Our study further underlines the key role of the Southern Ocean in controlling long-term future carbon uptake.

The African contribution to the global climate-carbon cycle feedback of the 21st century

Abstract. Future climate change will have impact on global and regional terrestrial carbon balances. The fate of African tropical forests over the 21st century has been investigated through global coupled climate carbon cycle model simulations. Under the SRES-A2 socio-economic CO2 emission scenario of the IPCC, and using the Institut Pierre Simon Laplace coupled ocean-terrestrial carbon cycle and climate model, IPSL-CM4-LOOP, we found that the warming over African ecosystems induces a reduction of net ecosystem productivity, making a 38% contribution to the global climate-carbon cycle positive feedback. Most of this contribution comes from African grasslands, followed by African savannahs, African tropical forest contributing little to the global climate-carbon feedback. However, the vulnerability of the African rainforest ecosystem is quite large. In contrast, the Amazon forest, despite its lower vulnerability, has a much larger overall contribution due to its 6 times larger extent.

Reversible and irreversible impacts of greenhouse gas emissions in multi-century projections with the NCAR global coupled carbon cycle-climate model

The legacy of historical and the long-term impacts of 21st century greenhouse gas emissions on cli- mate, ocean acidification, and carbon-climate feedbacks are investigated with a coupled carbon cycle-climate model. Emission commitment scenarios with zero emis- sions after year 2100 and 21st century emissions of 1,800, 900, and 0 gigatons of carbon are run up to year 2500. The reversibility and irreversibility of impacts is quantified by comparing anthropogenically-forced regional changes with internal, unforced climate variability. We show that the influence of historical emissions and of non-CO2 agents is largely reversible on the regional scale. Forced changes in surface temperature and precipitation become smaller than internal variability for most land and ocean grid cells in the absence of future carbon emissions. In contrast, continued carbon emissions over the 21st century cause irreversible climate change on centennial to millennial timescales in most regions and impacts related to ocean acidification and sea level rise continue to aggravate for centuries even if emissions are stopped in year 2100. Undersaturation of the Arctic surface ocean with respect to aragonite, a mineral form of calcium carbonate secreted by marine organisms, is imminent and remains widespread. The volume of super- saturated water providing habitat to calcifying organisms is reduced from preindustrial 40 to 25% in 2100 and to 10% in 2300 for the high emission case. We conclude that emission trading schemes, related to the Kyoto Process, should not permit trading between emissions of relatively short-lived agents and CO2 given the irreversible impacts of anthropogenic carbon emissions.

Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections

AbstractThe ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 °N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO2 concentration stimulate vegetation productivity, taking up CO2. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 ± 0.3 Pg C yr−1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 ± 20 Pg C over 1901 levels, of which 17 ± 8 Pg C comes from vegetation (43%) and 21 ± 16 Pg C from the soil (8%). Both CO2 fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO2 fertilization effect, permafrost, and the role of soil moisture. Unlike the CO2 fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid‐latitudes. These effects are further enhanced as a result of positive carbon cycle–climate feedbacks due to additional CO2 and warming.

Sensitivity of a coupled climate-carbon cycle model to large volcanic eruptions during the last millennium

The sensitivity of the climate–biogeochemistry system to volcanic eruptions is investigated using the comprehensive Earth System Model developed at the Max Planck Institute for Meteorology. The model includes an interactive carbon cycle with modules for terrestrial biosphere as well as ocean biogeochemistry. The volcanic forcing is based on a recent reconstruction for the last 1200 yr. An ensemble of five simulations is performed and the averaged response of the system is analysed in particular for the largest eruption of the last millennium in the year 1258. After this eruption, the global annual mean temperature drops by 1 K and recovers slowly during 10 yr. Atmospheric CO2 concentration declines during 4 yr after the eruption by ca. 2 ppmv to its minimum value and then starts to increase towards the pre-eruption level. This CO2 decrease is explained mainly by reduced heterotrophic respiration on land in response to the surface cooling, which leads to increased carbon storage in soils, mostly in tropical and subtropical regions. The ocean acts as a weak carbon sink, which is primarily due to temperature-induced solubility. This sink saturates 2 yr after the eruption, earlier than the land uptake.

Quantifying Carbon Cycle Feedbacks

Abstract Perturbations to the carbon cycle could constitute large feedbacks on future changes in atmospheric CO2 concentration and climate. This paper demonstrates how carbon cycle feedback can be expressed in formally similar ways to climate feedback, and thus compares their magnitudes. The carbon cycle gives rise to two climate feedback terms: the concentration–carbon feedback, resulting from the uptake of carbon by land and ocean as a biogeochemical response to the atmospheric CO2 concentration, and the climate–carbon feedback, resulting from the effect of climate change on carbon fluxes. In the earth system models of the Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP), climate–carbon feedback on warming is positive and of a similar size to the cloud feedback. The concentration–carbon feedback is negative; it has generally received less attention in the literature, but in magnitude it is 4 times larger than the climate–carbon feedback and more uncertain. The concentration–carbon feedback is the dominant uncertainty in the allowable CO2 emissions that are consistent with a given CO2 concentration scenario. In modeling the climate response to a scenario of CO2 emissions, the net carbon cycle feedback is of comparable size and uncertainty to the noncarbon–climate response. To quantify simulated carbon cycle feedbacks satisfactorily, a radiatively coupled experiment is needed, in addition to the fully coupled and biogeochemically coupled experiments, which are referred to as coupled and uncoupled in C4MIP. The concentration–carbon and climate–carbon feedbacks do not combine linearly, and the concentration–carbon feedback is dependent on scenario and time.

Challenges to Reproduce Vegetation Structure and Dynamics in Amazonia Using a Coupled Climate–Biosphere Model

Abstract The Amazon rain forest constitutes one of the major global stocks of carbon. Recent studies, including the last Intergovernmental Panel on Climate Change report and the Coupled Climate Carbon Cycle Model Intercomparison Project, have suggested that it may reduce in size and lose biomass during the twenty-first century through a savannization process. A better understanding of how this ecosystem structure, dynamics, and carbon balance may respond to future climate changes is needed. This article investigates how well a fully coupled atmosphere–biosphere model can reproduce vegetation structure and dynamics in Amazonia to the extent permitted by available data. The accurate representation of the coupled climate–biosphere dynamics requires the accurate representation of climate, net primary production (NPP), and its partition among the several carbon pool components. The simulated climate is validated against precipitation (within 5% of four datasets) and incident solar radiation (within 7% of observations). The authors also validate (i) simulated land cover, which reproduces well the observed patterns; (ii) NPP, within 5% of observations; and (iii) respiration rates, within 15% of observations. The performance of simulated variables that depend on carbon allocation, like NPP partitioning, leaf area index, and aboveground live biomass, although good on a regional mean, is significantly low when spatial patterns are considered. These errors may be attributed to fixed carbon allocation and residence time parameters assumed by the model. Carbon allocation apparently varies spatially, and to simulate this spatial variability is quite a challenge.

Characterizing uncertainty in modeling primary terrestrial ecosystem processes

The simulation results from models participating in the Coupled Climate Carbon Cycle Model Intercomparison Project (C4MIP) highlight the role of positive carbon‐climate feedback in accelerating growth of atmospheric CO2. The large range among models in the strength of this feedback indicates the uncertainty in our understanding of the response of the land and the oceans to continued climate warming and increasing CO2. Most of this uncertainty is associated with the response of terrestrial ecosystems to changes in climate and atmospheric CO2. The reasons for differences between models' responses to transient climate and CO2 forcing are not easily identified because of their complex parameterizations and spatially distributed processes. In this paper, we show that a simple box model can reasonably reproduce the globally averaged primary land‐atmosphere CO2 fluxes and carbon pools of two complex terrestrial ecosystem models (TEMs) over a range of emission scenarios. The parameters of the box model are calculated by fitting the box model to each TEM's response to transient climate and CO2 forcing. This approach is also applied to terrestrial carbon cycle components of carbon‐climate models participating in the C4MIP study. The resulting set of parameter values based on a common box model structure yields a wide range of parameter values, which suggests an absence of clear consensus in modeling primary terrestrial ecosystem processes and provides some insight into the reasons for divergent responses of terrestrial carbon cycle components.

Lifetime of Anthropogenic Climate Change: Millennial Time Scales of Potential CO2 and Surface Temperature Perturbations

AbstractMultimillennial simulations with a fully coupled climate–carbon cycle model are examined to assess the persistence of the climatic impacts of anthropogenic CO2 emissions. It is found that the time required to absorb anthropogenic CO2 strongly depends on the total amount of emissions; for emissions similar to known fossil fuel reserves, the time to absorb 50% of the CO2 is more than 2000 yr. The long-term climate response appears to be independent of the rate at which CO2 is emitted over the next few centuries. Results further suggest that the lifetime of the surface air temperature anomaly might be as much as 60% longer than the lifetime of anthropogenic CO2 and that two-thirds of the maximum temperature anomaly will persist for longer than 10 000 yr. This suggests that the consequences of anthropogenic CO2 emissions will persist for many millennia.

Constraining Carbon Cycle Feedback Using Paleodata: Palaeocarbon Modelling Intercomparison Project Kickoff Workshop; Totnes, United Kingdom, 26–28 January 2009

Climate‐induced changes in the terrestrial biosphere and the ocean modulate the release and uptake of carbon dioxide (CO2), and this, in turn, alters atmospheric composition and climate in a process called climate/carbon cycle feedback. The Coupled Carbon Cycle Climate Model Intercomparison Project (C4MIP), using models of the terrestrial and ocean carbon cycles in ocean‐atmosphere general circulation models, has shown that although the feedback is positive, its magnitude is highly uncertain (P. Friedlingstein et al., J. Clim., 19, 3337, 2006). It is important to quantify this feedback adequately because it affects the CO2 emissions consistent with any atmospheric CO2 stabilization.