The release of carbon from tropical forests may exacerbate future climate change1, but the magnitude of the effect in climate models remains uncertain2. Coupled climate–carbon-cycle models generally agree that carbon storage on land will increase as a result of the simultaneous enhancement of plant photosynthesis and water use efficiency under higher atmospheric CO2 concentrations, but will decrease owing to higher soil and plant respiration rates associated with warming temperatures3. At present, the balance between these effects varies markedly among coupled climate–carbon-cycle models, leading to a range of 330 gigatonnes in the projected change in the amount of carbon stored on tropical land by 2100. Explanations for this large uncertainty include differences in the predicted change in rainfall in Amazonia4,5 and variations in the responses of alternative vegetation models to warming6. Here we identify an emergent linear relationship, across an ensemble of models7, between the sensitivity of tropical land carbon storage to warming and the sensitivity of the annual growth rate of atmospheric CO2 to tropical temperature anomalies8. Combined with contemporary observations of atmospheric CO2 concentration and tropical temperature, this relationship provides a tight constraint on the sensitivity of tropical land carbon to climate change. We estimate that over tropical land from latitude 30° north to 30° south, warming alone will release 53 ± 17 gigatonnes of carbon per kelvin. Compared with the unconstrained ensemble of climate–carbon-cycle projections, this indicates a much lower risk of Amazon forest dieback under CO2-induced climate change if CO2 fertilization effects are as large as suggested by current models9. Our study, however, also implies greater certainty that carbon will be lost from tropical land if warming arises from reductions in aerosols10 or increases in other greenhouse gases11.
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We acknowledge funding from the NERC NCEO programme (P.M.C. and C.M.L.); the EU Greencycles II project (P.M.C. and P.F.); the EU FP7 ‘CARBONES’ project (D.P. and C.D.J.); the Joint DECC/Defra Met Office Hadley Centre Climate Programme (GA01101) (D.P., B.B.B. and C.D.J.); the CEH Science Budget (C.H.) and the Newton Institute programme on ‘Mathematical and Statistical Approaches to Climate Modelling and Prediction’, during which this research was first formulated (P.M.C., B.B.B. and C.H.). We also acknowledge the modelling groups that provided results to C4MIP.
This file contains Supplementary Table 1 and Supplementary Figures 1-3.