Our changing climate will make future European summer heatwaves like the one in 2003, blamed for killing 35,000, more likely but harder to forecast a season in advance. That’s what research done by Benjamin Quesada from the Laboratoire des Sciences du Climat et de l’Environnement (LSCE) in Gif-sur-Yvette, France, suggests. Together with fellow climatologists, he has found that water trapped in Southern European soil during wet winters and springs keeps the continent reliably cool in summer. “Under global warming, climate models almost all agree about drier soils in Southern Europe and more frequent and long lasting summer heatwaves,” Benjamin told Simple Climate. Losing that cooling influence currently makes the weather less predictable but with a higher chance of being hot – though Benjamin hopes his findings will eventually bring more accurate forecasts.
The dramatic heatwaves in 2003 and 2010 took Europe by surprise. That has motivated the continent’s scientists to try to understand them and therefore predict them better. The role that water absorbed in soil plays has been one area that they’ve looked at. Their research shows a vicious “feedback” cycle where drier soils mean that less water reaches the atmosphere to create clouds. In turn, more heat from the sun reaches the ground and dries it out yet more. “Soil moisture can be seen as a buffer,” Benjamin said. “On dry soil, solar energy will directly heat soil, and isn’t ‘wasted’ first in evaporation as in wet soil.” Usually an escape from this cycle can come thanks to factors like winds circling the planet and carrying clouds with them, he added.
The rain in Spain – and Italy, and Greece
Benjamin and his co-workers wanted to find out the ways that soil moisture influences European climate, in a way that might allow detailed prediction. To do this, they looked at the soil-temperature link using both measurements of the past and model predictions of the future. They considered records from 238 weather stations from 1948 to 2011. For each summer, the scientists looked for how often there were days in June-August ranking in the hottest 10 per cent for that 64 year period, plus how much rain there had been in the previous January-May.
Comparing these numbers in a paper published in scientific journal Nature Climate Change on Sunday, they first found that rainfall across all Europe predicted the number of hot summer days poorly. But when they just looked at rainfall in Southern Europe, they found that after wet springs there were few of the hottest days. Summers after dry springs generally either had a high or low number of hottest days, with few falling in between. That suggested that without the protective effect of the soil moisture, conditions were determined by sunny or cloudy and rainy weather coming in from the Atlantic.
To see how this might affect heatwave prediction as the world warms, the team turned to 14 computerised climate models. They first checked how well the models generated the rainfall and hot day data they had already analysed and ranked them according to how well they performed, Benjamin explained. “Then, we checked whether projections with the best models differ from the others,” he said. “The best ones predict significantly drier future winter/spring seasons in Southern Europe and rather warmer summers in Continental Europe.”
Benjamin said that the next step after this research would be combining local data and models to try and use this new perspective on European weather to make better predictions. Those predictions will be within a season, rather than a season ahead, but they will mean these results will help society adapt better to “extreme weather”, he said.