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 magnitude and evolution of parameters that characterize feedbacks in the coupled carbon-climate system are compared across nine Earth system models (ESMs). The analysis is based on results from biogeochemically, radiatively, and fully coupled simulations in which CO 2 increases at a rate of 1% yr 21 . These simulations are part of phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CO 2 fluxes between the atmosphere and underlying land and ocean respond to changes in atmospheric CO 2 concentration and to changes in temperature and other climate variables. The carbon-concentration and carbonclimate feedback parameters characterize the response of the CO 2 flux between the atmosphere and the underlying surface to these changes. Feedback parameters are calculated using two different approaches. The two approaches are equivalent and either may be used to calculate the contribution of the feedback terms to diagnosed cumulative emissions. The contribution of carbon-concentration feedback to diagnosed cumulative emissions that are consistent with the 1% increasing CO 2 concentration scenario is about 4.5 times larger than the carbon-climate feedback. Differences in the modeled responses of the carbon budget to changes in CO 2 and temperature are seen to be 3-4 times larger for the land components compared to the ocean components of participating models. The feedback parameters depend on the state of the system as well the forcing scenario but nevertheless provide insight into the behavior of the coupled carbon-climate system and a useful common framework for comparing models.