by Makari Krause

With climate change proceeding full-bore, questions are continuously raised about how the carbon cycle will react and whether climate change will be a negative or positive feedback loop. As atmospheric CO­ 2 concentrations increase, most scientists agree that the rate of photosynthesis will increase leading to an increase in stored carbon. This same increase in atmospheric CO­ 2 , however, will increase soil respiration, releasing additional CO­ 2 into the atmosphere. The balance between these two effects is crucial in determining what the net effect of increasing atmospheric CO­ 2 will be on the carbon cycle. Cox et al. (2013) use a number of different models to identify 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. The study focuses on land between latitudes 30˚ north and 30˚ south; their established linear relationship estimates that in this area warming will release 53±17 gigatonnes of carbon per Kelvin. While this sounds like a staggering amount, it is much lower than current estimates and suggests that tropical forests will not experience as much warming-induced dieback as was previously thought.

In order to identify the effects of CO 2 on land and ocean carbon sinks, Cox et al. compare coupled climate models with uncoupled climate models. This uncoupling is achieved by making the land and ocean carbon cycles insensitive to climate change caused by the increase in atmospheric CO­ 2 . Comparing these two different types of models allowed the authors to separate the direct effects of CO­ 2 on land and ocean carbon sinks from other effects of climate change. The coupled simulations result in a much larger increase in atmospheric CO­ 2 than the uncoupled models; in fact the uncoupled models predict a significant increase in land carbon storage. This increase is due to the direct effects of increased atmospheric CO­ 2 on photosynthesis and water use efficiency. Cox et al. note that their model didn’t take nitrogen limitation into account, and while they agree that nitrogen could be a major factor in limiting the increase in photosynthesis at higher latitudes, they note that at the latitudes they are studying nitrogen limitation is not really a problem.

In order to separate the direct effects of CO­ 2 from other effects of climate change, Cox et al. develop a formula for identifying the change in tropical land carbon storage resulting from atmospheric CO­ 2 levels. Sensitivities of tropical land carbon storage to direct CO­ 2 effects are determined using the uncoupled simulations. Sensitivities to climate change are determined by subtracting the direct CO­ 2 effects from the total effects in the coupled simulations. Isolating the direct effects of CO­ 2 , Cox et al. estimate that tropical forests are at a low risk of dieback and will continue to store carbon even with increasing temperatures.

In order to find a constraint linking the sensitivity of tropical land carbon to interannual variability (IAV) in the growth rate of atmospheric CO 2 , Cox et al. compared IAV in the growth rate of atmospheric CO 2 and IAV in the annual mean tropical temperature. They found a linear correlation between these two observed variables and from this correlation develop a constraint to use on the climate models. When the constraint is applied to the coupled climate models it excludes many of the models that predict forest dieback, lowering the estimated probability of dieback from 21% to 0.24%.

While the negative impacts of increasing temperatures due to CO­ 2 may be offset by the benefits of carbon fertilization, if the warming arises from increases in other greenhouse gases or reductions in aerosols, this offsetting effect would be absent and the result could be a large forest dieback and increase in atmospheric CO­ 2 .

Cox, P. M., Pearson, D., Booth, B. B., Friedlingstein, P., Huntingford, C., Jones, C. D., & Luke, C. M., 2013. Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability. Nature 494, 341–344 http://www.nature.com/nature/journal/v494/n7437/abs/nature11882.html