Eastern North America and Central Europe could see their most extreme heat waves become twice as intense as previously projected due to variations in soil moisture, according to a new study.

Led by Professor Douglas Maraun at the University of Graz, and involving contributions from the University of Reading, the research found that when global temperatures rise by 2°C, the most severe heat events in these regions could increase by up to 4°C.

Published in Nature Communications, the study reveals that the most extreme and devastating heat waves—like those that struck Canada in 2021, India in 2022, and the Mediterranean in 2023—may intensify much more dramatically than typical heat events during climate change because of variations in soil moisture.

Reinhard Schiemann, research scientist at the University of Reading and co-author of the study, said, “It was already known that heat waves generally become more intense as average temperature increases, but it was previously unclear how the rarest and most extreme heat waves will change compared to more moderate heat waves. The key factor is soil moisture during heat events. When soil moisture changes significantly during extreme heat, it can either amplify or reduce the temperature increase, and this can affect the most extreme heat waves more—or less—than moderate heat waves.”

How soil affects extreme heat

The research team found that soil moisture during extreme heat events works differently than during normal conditions. They identified three key states: very wet soil (where moisture doesn’t affect temperature much), very dry soil (where there’s little cooling effect), and a middle state where soil moisture has the strongest influence on temperature.

As the climate warms, regions can shift between these states. For example, some currently wet areas are expected to become drier, moving them into this critical middle state where soil moisture strongly affects temperature. In these regions, like Central Europe and Eastern US, extreme heat events could become much more intense than previously predicted. Other areas that are already dry might see less extreme temperature increases if they receive more rainfall.

The research team discovered this by analyzing multiple climate models, looking specifically at how soil moisture changes during the hottest days of the year. This relationship between soil moisture and temperature varies by region and depends heavily on expected changes in rainfall patterns. While some regions might see more rainfall and others less, these changes in soil moisture will play a crucial role in determining how extreme future heat waves become.

The findings suggest that current climate risk assessments may need to be updated, as they could be significantly underestimating the intensity of future extreme heat events in many regions.