European grasslands initially keep cool during heatwaves through enhanced evaporation, but heat up much more rapidly once soil moisture has been exhausted, reports a study published online this week in Nature Geoscience. By contrast, soil moisture is evaporated more slowly in forests, which leads to more initial heating but cooler temperatures in the long term.

Adriaan Teuling and colleagues analysed energy flux measurements from a network of observation towers across European grasslands and forests. They found that the land surface heats twice as fast over forests as over grasslands at the beginning of a heatwave. However, in the absence of rain, grasslands surpass forests as the main source of heating for the atmosphere once they have dried out.

This process may explain the extreme temperatures in France during the later stages of the summer 2003 heatwave.