To what extent can interannual CO2 variability constrain carbon cycle sensitivity to climate change in CMIP5 Earth System Models?

We analyze the carbon-climate feedback in eight Earth System Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). We focus on tropical land carbon change and find decreases (-31.02 to -169.32 GtCK-1) indicating tropical ecosystems will release carbon as temperature warms, thus contributing to a positive feedback identified in earlier studies. We further investigate the relationship between tropical land carbon change and sensitivity of historical atmospheric CO2 growth rate to tropical temperature variability and find a weak linear relationship. This sensitivity for most models is stronger than observed. We further use this emergent constraint to constrain uncertainties in model-projected future carbon-climate changes and find little effect in narrowing the model spread, but the mean sensitivity is slightly smaller. This contrasts with earlier Coupled Carbon Cycle Climate Model Intercomparison Project results, highlighting the challenge in constraining future projections by modern observations and the necessity for evaluating such relationships continuously.