The heatwave and drought that hit Europe in 2003 led to thousands of extra deaths and had widespread and long-lasting impacts on ecosystems. Some researchers have suggested that these events could become more common in the future, but we still have an incomplete understanding on what causes these extreme events.

This new study, helps to fit another important piece into the jigsaw. Researchers have shown that the initial surface conditions are an important control on the intensification of the heatwave. The land surface loses heat in two main ways, via conduction and radiation (aka sensible heat) and via evapotranspiration (when surface energy is used to evaporate water - aka latent heat). The latent heat flux is important as it is very effective at transferring heat away from he surface - the energy used to evaporate liquid water is not released again until the water condenses high in the atmosphere as clouds. The new study shows that when the land surface is already dry at the onset of the heatwave, there is little water available to be evaporated (the latent heat flux), so more energy is trapped at the surface, increasing temperatures further and increasing he severity of the heatwave.