Eleven coupled climate-carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO 2 for the 1850-2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO 2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO 2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO 2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO 2 levels led to an additional climate warming ranging between 0.1°and 1.5°C.All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models.

The responses of carbon dioxide (CO₂) and other climate variables to an emission pulse of CO₂ into the atmosphere are often used to compute the Global Warming Potential (GWP) and Global Temperature change Potential (GTP), to characterize the response timescales of Earth System models, and to build reduced-form models. In this carbon cycle-climate model intercomparison project, which spans the full model hierarchy, we quantify responses to emission pulses of different magnitudes injected under different conditions. The CO₂ response shows the known rapid decline in the first few decades followed by a millennium-scale tail. For a 100 Gt-C emission pulse added to a constant CO₂ concentration of 389 ppm, 25 ± 9% is still found in the atmosphere after 1000 yr; the ocean has absorbed 59 ± 12% and the land the remainder (16 ± 14%). The response in global mean surface air temperature is an increase by 0.20 ± 0.12 °C within the first twenty years; thereafter and until year 1000, temperature decreases only slightly, whereas ocean heat content and sea level continue to rise. Our best estimate for the Absolute Global Warming Potential, given by the time-integrated response in CO₂ at year 100 multiplied by its radiative efficiency, is 92.5 × 10⁻¹⁵ yr W m⁻² per kg-CO₂. This value very likely (5 to 95% confidence) lies within the range of (68 to 117) × 10⁻¹⁵ yr W m⁻² per kg-CO₂. Estimates for time-integrated response in CO₂ published in the IPCC First, Second, and Fourth Assessment and our multi-model best estimate all agree within 15% during the first 100 yr. The integrated CO₂ response, normalized by the pulse size, is lower for pre-industrial conditions, compared to present day, and lower for smaller pulses than larger pulses. In contrast, the response in temperature, sea level and ocean heat content is less sensitive to these choices. Although, choices in pulse size, background concentration, and model lead to uncertainties, the most important and subjective choice to determine AGWP of CO₂ and GWP is the time horizon

[1] This paper presents a quantitative estimation of the impact of reservoirs on discharge and irrigation water supply during the 20th century at global, continental, and river basin scale. Compared to a natural situation the combined effect of reservoir operation and irrigation extractions decreased mean annual discharge to oceans and significantly changed the timing of this discharge. For example, in Europe, May discharge decreased by 10%, while in February it increased by 8%. At the end of the 20th century, reservoir operations and irrigation extractions decreased annual global discharge by about 2.1% (930 km 3 yr À1 ). Simulation results show that reservoirs contribute significantly to irrigation water supply in many regions. Basins that rely heavily on reservoir water are the Colorado and Columbia River basins in the United States and several large basins in India, China, and central Asia (e.g., in the Krishna and Huang He basins, reservoirs more than doubled surface water supply). Continents gaining the most are North America, Africa, and Asia, where reservoirs supplied 57, 22, and 360 km 3 yr À1 respectively between 1981 -2000, which is in all cases 40% more than the availability in the situation without reservoirs. Globally, the irrigation water supply from reservoirs increased from around 18 km 3 yr À1 (adding 5% to surface water supply) at the beginning of the 20th century to 460 km 3 yr À1 (adding almost 40% to surface water supply) at the end of the 20th century. The analysis is performed using a newly developed and validated reservoir operation scheme within a global-scale hydrology and vegetation model (LPJmL).

The responses of carbon dioxide (CO₂) and other climate variables to an emission pulse of CO₂ into the atmosphere are often used to compute the Global Warming Potential (GWP) and Global Temperature change Potential (GTP), to characterize the response time scales of Earth System models, and to build reduced-form models. In this carbon cycle-climate model intercomparison project, which spans the full model hierarchy, we quantify responses to emission pulses of different magnitudes injected under different conditions. The CO₂ response shows the known rapid decline in the first few decades followed by a millennium-scale tail. For a 100 Gt C emission pulse, 24 ± 10% is still found in the atmosphere after 1000 yr; the ocean has absorbed 60 ± 18% and the land the remainder. The response in global mean surface air temperature is an increase by 0.19 ± 0.10 °C within the first twenty years; thereafter and until year 1000, temperature decreases only slightly, whereas ocean heat content and sea level continue to rise. Our best estimate for the Absolute Global Warming Potential, given by the time-integrated response in CO₂ at year 100 times its radiative efficiency, is 92.7 × 10^−15 yr W m⁻² per kg CO₂. This value very likely (5 to 95% confidence) lies within the range of (70 to 115) × 10^−15 yr W m⁻² per kg CO₂. Estimates for time-integrated response in CO₂ published in the IPCC First, Second, and Fourth Assessment and our multi-model best estimate all agree within 15%. The integrated CO₂ response is lower for pre-industrial conditions, compared to present day, and lower for smaller pulses than larger pulses. In contrast, the response in temperature, sea level and ocean heat content is less sensitive to these choices. Although, choices in pulse size, background concentration, and model lead to uncertainties, the most important and subjective choice to determine AGWP of CO₂ and GWP is the time horizon

Simulations are run until the year 3000 A.D. and extend substantially farther into the future than conceptually similar simulations with atmosphere-ocean general circulation models (AOGCMs) coupled to carbon cycle models. In this paper the following are investigated: 1) the climate change commitment in response to stabilized greenhouse gases and stabilized total radiative forcing, 2) the climate change commitment in response to earlier CO 2 emissions, and 3) emission trajectories for profiles leading to the stabilization of atmospheric CO 2 and their uncertainties due to carbon cycle processes. Results over the twenty-first century compare reasonably well with results from AOGCMs, and the suite of EMICs proves well suited to complement more complex models. Substantial climate change commitments for sea level rise and global mean surface temperature increase after a stabilization of atmospheric greenhouse gases and radiative forcing in the year 2100 are identified. The additional warming by the year 3000 is 0.6-1.6 K for the low-CO 2 IPCC Special Report on Emissions Scenarios (SRES) B1 scenario and 1.3-2.2 K for the high-CO 2 SRES A2 scenario. Correspondingly, the post-2100 thermal expansion commitment is 0.3-1.1 m for SRES B1 and 0.5-2.2 m for SRES A2. Sea level continues to rise due to thermal expansion for several centuries after CO 2 stabilization. In contrast, surface temperature changes slow down after a century. The meridional overturning circulation is weakened in all EMICs, but recovers to nearly initial values in all but one of the models after centuries for the scenarios considered. Emissions during the twenty-first century continue to impact atmospheric CO 2 and climate even at year 3000. All models find that most of the anthropogenic carbon emissions are eventually taken up by the ocean (49%-62%) in year 3000, and that a substantial fraction (15%-28%) is still airborne even 900 yr after carbon emissions have ceased. Future stabilization of atmospheric CO 2 and climate change requires a substantial reduction of CO 2 emissions below present levels in all EMICs. This reduction needs to be substantially larger if carbon cycle-climate feedbacks are accounted for or if terrestrial CO 2 fertilization is not operating. Large differences among EMICs are identified in both the response to increasing atmospheric CO 2 and the response to climate change. This highlights the need for improved representations of carbon cycle processes in these models apart from the sensitivity to climate change. Sensitivity simulations with one single EMIC indicate that both carbon cycle and climate sensitivity related uncertainties on projected allowable emissions are substantial.n Current affiliation: IBP, ETH Zürich, Zürich, Switzerland.