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 (LT) and the short-term sensitivity of atmospheric carbon dioxide (CO2) to interannual temperature variability (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 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 (LT versus 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 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 LT and 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.