Soil Moisture Shifts: Unlocking the Secrets Behind Intensifying Heat Waves

Regions in Eastern North America and Central Europe may experience their most severe heat waves becoming twice as intense as earlier estimates due to shifts in soil moisture, as indicated by a fresh study.

Directed by Professor Douglas Maraun from the University of Graz, with contributions from the University of Reading, the investigation revealed that an increase of 2°C in global temperatures could elevate the most intense heat occurrences in these areas by as much as 4°C.

Featured in Nature Communications, this research indicates that the most extreme and damaging heat waves—such as those that affected Canada in 2021, India in 2022, and the Mediterranean in 2023—might worsen more significantly than average heat events amidst climate change, largely due to fluctuations in soil moisture.

Reinhard Schiemann, a research scientist at the University of Reading and a co-author of the study, stated, “It has long been recognized that heat waves generally intensify as average temperatures rise, yet it was previously uncertain how the rarest and most severe heat waves would alter in comparison to more moderate ones. The critical variable is soil moisture during heat events. Significant changes in soil moisture during extreme heat can either heighten or diminish the temperature rise, impacting the most intense heat waves differently than moderate ones.”

Influence of soil on extreme heat

The research team discovered that soil moisture during extreme heat events behaves differently compared to normal conditions. They identified three distinct states: excessively wet soil (where moisture has minimal impact on temperature), extremely dry soil (which offers little cooling effect), and an intermediate condition where soil moisture exerts the strongest influence on temperature.

As climate conditions warm, regions may transition between these states. For instance, some currently moist areas are anticipated to become drier, positioning them into this crucial intermediate state where soil moisture substantially affects temperature. In such locations, like Central Europe and the Eastern US, extreme heat events could intensify far beyond prior predictions. Conversely, other far drier regions might experience reduced extreme temperature increases if they receive additional precipitation.

This understanding emerged from the team’s analysis of multiple climate models, specifically examining how soil moisture alters on the hottest days annually. The interplay between soil moisture and temperature varies by location and is largely influenced by anticipated shifts in rainfall patterns. While certain regions may receive increased rainfall and others may not, changes in soil moisture will significantly determine the intensity of extreme heat waves in the future.

The conclusions imply that existing climate risk evaluations may require updates, as they might be grossly underestimating the severity of future extreme heat events across various regions.