Do Emergent Constraints on Carbon Cycle Feedbacks hold in CMIP6?

Abstract

Emergent constraints on carbon cycle feedbacks in response to warming and increasing atmospheric CO2 concentration have previously been identified in Earth system models (ESMs) participating in the Coupled Model Intercomparison Project (CMIP) Phase 5. Here we examine whether two of these emergent constraints also hold for CMIP6. The spread of the sensitivity of tropical land carbon uptake to tropical warming in an idealized simulation with a 1% per year increase of atmospheric CO2 shows only a slight decrease in CMIP6 (-52 ± 35 GtC/K) compared to CMIP5 (-49 ± 40 GtC/K). For both model generations, the observed interannual variability in the growth rate of atmospheric CO2 yields a consistent emergent constraint on the sensitivity of tropical land carbon uptake with a constrained range of -37 ± 14 GtC/K for the combined ensemble (i.e., a reduction of ~30% in the best estimate and 60% in the uncertainty range relative to the multi-model mean of the combined ensemble). A further emergent constraint is based on a relationship between CO2 fertilization and the historical increase in the CO2 seasonal cycle amplitude in high latitudes. However, this emergent constraint is not evident in CMIP6. 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.