Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability

A linear relationship between the sensitivity of tropical land carbon storage to warming and the sensitivity of the annual growth rate of atmospheric CO 2 to tropical temperature anomalies provides a tight constraint on the sensitivity of tropical land carbon to climate change. Carbon balance in tropical forests In response to climate change, tropical forests may release vast amounts of carbon, accelerating the pace of further climate change. Or they may not: research to date has been conflicting and controversial. Peter Cox and colleagues now use the response of the tropical land carbon cycle to interannual climate variability to constrain the likely future response. They find that the tropics will emit 53 ± 17 gigatonnes of carbon per degree of warming, a much more muted response that suggested in previous work. The release of carbon from tropical forests may exacerbate future climate change 1 , but the magnitude of the effect in climate models remains uncertain 2 . 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 CO 2 concentrations, but will decrease owing to higher soil and plant respiration rates associated with warming temperatures 3 . 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 Amazonia 4 , 5 and variations in the responses of alternative vegetation models to warming 6 . Here we identify an emergent linear relationship, across an ensemble of models 7 , between the sensitivity of tropical land carbon storage to warming and the sensitivity of the annual growth rate of atmospheric CO 2 to tropical temperature anomalies 8 . Combined with contemporary observations of atmospheric CO 2 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 CO 2 -induced