Do Emergent Constraints on Carbon Cycle Feedbacks hold in CMIP6?

Abstract

Emergent constraints help to better understand Earth system processes in a changing climate and to constrain future climate projections. Here, we analyze the robustness of two previously found emergent constraints on carbon cycle feedbacks using models from the Coupled Model Intercomparison Project (CMIP) of Phases 5 and 6. First, an emergent constraint on the carbon-climate feedback is evaluated, which is found to be robust regarding the choice of model ensemble. For the combined CMIP5 and CMIP6 ensembles, the sensitivity of tropical land carbon uptake to tropical warming is constrained to −37 ± 14 GtC/K. Second, we analyze an emergent constraint on the carbon-concentration feedback. This emergent constraint (originally derived from the CMIP5 models) is not evident in the CMIP6 ensemble. This is in part because the historical increase in the amplitude of the CO2 seasonal cycle is more accurately simulated in CMIP6, such that the models are all now close to the observational constraint.