Humphrey, V., Zscheischler, J., Ciais, P., Gudmundsson, L., Sitch, S., & Seneviratne, S. I. (2018). Sensitivity of atmospheric CO 2 growth rate to observed changes in terrestrial water storage. Nature, 560(7720), 628.

We have found that during global droughts the concentration of carbon dioxide in the atmosphere rises faster because stressed ecosystems absorb less carbon. This global effect is stronger than previously thought and appears to be underestimated in current climate models.

For the first time, we were able to quantify a yearly sensitivity of -1.33 Gigaton of carbon per Teraton of water (95% confidence interval −1.85 to −1.07 GtC per Tt H₂O). In other words, on average, for each kilogram of water that is missing, there is 1.33 additional grams of carbon in the atmopshere.

Why is this important? As human CO₂ emissions continue to increase, current models disagree on whether more droughts will limit the ability of ecosystems to take up CO₂, which would accelerate global warming. We have to make sure models get this right!

Our results suggest that current carbon cycle models tend to underestimate the global effect of droughts on terrestrial carbon uptake. Some known limitations of current models could explain why this happens. For instance, the representation of hydrological processes in those models might not be entirely adequate. Current parameterizations of the sensitivity of plants to water stress might need improvement. Finally, some important carbon processes that are strongly linked to the water cycle (i.e. carbon transport and mineralization in inland waters) are only just starting to be implemented in the next generation of models.