Extreme weather and climate events like storms, heavy precipitation and droughts and heat waves prevent the update of 3 giga-tonnes of carbon by the global vegetation. A team of scientists under the lead of Markus Reichstein, Director of the Max Planck Institute for Biogeochemistry in Jena, Germany, investigated the effect of extremes on the carbon cycle from the terrestrial ecosystem perspective for the first time.

In the current issue of Nature, they use Earth observation methods and numerical models to show that especially extreme droughts lead to a strong reduction in the carbon sequestration of forests, grass- and croplands.

This reduction in the regional and global carbon uptake has the potential to influence the global climate. Especially large scale events like the heat wave in western and southern Europe in the year 2003 provide the evidence that such extremes events have a much stronger and long lasting impact on the carbon cycle than expected so far.

One part of the question is the response of arable ecosystems: plants take up carbondioxide, soils are an important storage for the carbon produced by plants, which they release driven by increasing temperature. However, in the case of croplands we observe a complex interplay of these natural processes with the human management either increasing or reducing the impacts of an event.

"In general the timing of an event in the course of the development of crops clearly influences the magnitude of the impact on the carbon cycle. Extreme temperature in spring can foster growth, prevent pollination, or have no effect at all, depending on when they appear in the cropping cycle and the type of crop" says Martin Wattenbach from the GFZ German Research Centre for Geosciences, who contributed to this part of the study.

"Rice yields are reduced when temperatures rise above 37 C, but only in the short period of pollination in spring"

It is possible for farmers to mitigate extremes like droughts and heat waves by, for example, irrigation. However, they are limited by the amount of water available at the time of the event and their technical resources.

Since information on spatial and temporal patterns of management practise such as irrigation and annual crop distribution during an extreme event are largely unknown, the demand for further research remains very high. In addition to this part of the carbon cycle playing a relevant role in climate, the long term supply with agricultural produce may also be affected.