Emergent constraints on climate-carbon cycle feedbacks in the CMIP5 Earth system models

An emergent linear relationship between the long-term sensitivity of tropical land carbon storage to climate warming (γ_LT) and the short-term sensitivity of atmospheric carbon dioxide (CO₂) to interannual temperature variability (γ_IAV) has previously been identified by Cox et al. (2013) across an ensemble of Earth system models (ESMs) participating in the Coupled Climate-Carbon Cycle Model Intercomparison Project (C⁴MIP). Here we examine whether such a constraint also holds for a new set of eight ESMs participating in Phase 5 of the Coupled Model Intercomparison Project. A wide spread in tropical land carbon storage is found for the quadrupling of atmospheric CO₂, which is of the order of 252 ± 112 GtC when carbon-climate feedbacks are enabled. Correspondingly, the spread in γ_LT is wide (-49 ± 40 GtC/K) and thus remains one of the key uncertainties in climate projections. A tight correlation is found between the long-term sensitivity of tropical land carbon and the short-term sensitivity of atmospheric CO₂ (γ_LT versus γ_IAV), which enables the projections to be constrained with observations. The observed short-term sensitivity of CO₂ (-4.4 ± 0.9 GtC/yr/K) sharpens the range of γ_LT to -44 ± 14 GtC/K, which overlaps with the probability density function derived from the C⁴MIP models (-53 ± 17 GtC/K) by Cox et al. (2013), even though the lines relating γ_LT and γ_IAV differ in the two cases. Emergent constraints of this type provide a means to focus ESM evaluation against observations on the metrics most relevant to projections of future climate change. Key Points Tropical land carbon loss is a key uncertainty in climate change projections CO₂ interannual variability is linearly related to tropical carbon loss in CMIP5 Observed variability in CO₂ constrains projections of future carbon losses