Do Emergent Constraints on Carbon Cycle Feedbacks Hold in CMIP6?

Sabrina Zechlau , Manuel Schlund , Peter M. Cox , Pierre Friedlingstein (1) , Veronika Eyring

1 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)

Emergent constraints on carbon cycle feedbacks in response to warming and increasing atmospheric CO₂ concentration have previously been identified in Earth system models 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 multimodel 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.

Mots clés: carbon cycle; emergent constraint; CMIP5; CMIP6; fertilization effect; temperature warming

Domaines: Planète et Univers [physics]

Sabrina Zechlau, Manuel Schlund, Peter M. Cox, Pierre Friedlingstein, Veronika Eyring. Do Emergent Constraints on Carbon Cycle Feedbacks Hold in CMIP6?. Journal of Geophysical Research: Biogeosciences, 2022, 127, ⟨10.1029/2022JG006985⟩. ⟨insu-03993969⟩