Long-term climate commitments projected with climate - carbon cycle models

Eight earth system models of intermediate complexity (EMICs) are used to project climate change commitments for the recent IPCC Fourth Assessment Report (IPCC AR4). Simulations are run until year 3000 AD and extend substantially further into the future than conceptually similar simulations with Atmosphere-Ocean General Circulation Models (AOGCMs) coupled to carbon cycle models. We investigate (1) the climate change commitment in response to stabilized greenhouse gases and radiative forcing, (2) the climate change commitment in response to earlier CO2 emissions, and (3) emission trajectories for profiles leading to stabilization of atmospheric CO2 and their uncertainties due to carbon cycle processes. Results over the 21st century compare reasonably well with results from AOGCMs and the suite of EMICs proves well suited to complement more complex models. We identify 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 year 2100. The additional warming by year 3000 is 0.6 to 1.6 K for the low-CO2 SRES B1 scenario and 1.3 to 2.2 K for the high-CO2 SRES A2 scenario. Correspondingly, the post-2100 thermal expansion commitment is 0.3 to 1.1 m for SRES B1 and 0.5 to 2.2 m for SRES A2. Sea level continues to rise due to thermal expansion for several centuries after CO2 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 21st century continue to impact atmospheric CO2 andclimate even at year 3000. All models consistently find that most of the anthropogenic carbon emissions are eventually taken up by the ocean (49 to 62 %) in year 3000, and that a substantial fraction (15 to 28 %) is still airborne even after carbon emissions have ceased for 900 years. Future stabilization of atmospheric CO2 and climate change requires a substantial reduction of CO2 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 CO2 fertilization is not operating. We identify large differences among EMICs in both the response to increasing atmospheric CO2 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 the impact of climate sensitivity related uncertainty on projected allowable emissions is substantially smaller than the uncertainty related to different carbon cycle settings