A new study has found a self-reinforcing interaction between temperature, soil moisture, and atmospheric pressure is being triggered at high temperatures, making extreme heatwaves 10 percent hotter than they would otherwise be. The study was published today in Science Advances and led by the Potsdam Institute for Climate Impact Research (PIK) and the International Institute for Applied Systems Analysis (IIASA).
Local temperature records have been shattered in heatwave hotspots where "mega heatwaves" have been increasing in frequency faster than more moderate extreme heat events in recent years. Notable examples include the 2022 heatwave in Europe, where temperatures in the UK topped 40°C for the first time, and the 2021 summer heatwave in the US Pacific Northwest, where recorded temperatures reached 4.6°C above previous records.
The new study identifies an emerging physical process underlying the most extreme heatwaves by using machine learning methods and statistical analysis. It found that most extreme heat events in the mid-latitudes (for example, Europe, southern Russia and Mongolia, eastern and north-western North America, north-western India, the western coast of South America, central and southern Africa, and northern interior Australia), are being topped up by soil moisture, atmospheric pressure and temperature interacting to drive higher temperatures.
High-pressure systems allow more solar radiation to reach the Earth's surface, heating up the land. Likewise, as temperatures rise, soils gradually dry out, reducing their ability to cool the surface through evaporation, which causes the land to heat.
"The two-way relationship between rising temperatures and drying soils has been understood for a while and is already factored into what we expect from a typical extreme heat event," explained PIK scientist and lead author Yinglin Tian.
"But when we look at these mega heatwaves that have been significantly outstripping previous temperature records, what we're seeing is that once you reach a certain temperature, all three factors: drying soils, high pressure and extreme heat, begin to interact and drive each other further in a way that we haven't understood before," Tian added.
The analysis shows that as the world has warmed from climate change, it has become more likely that the temperature threshold at which this dynamic is triggered is exceeded, and the reinforcing feedbacks begin to drive temperatures during a heatwave.
In Europe, the feedback interaction is triggered at around four Kelvin above average summer temperatures, or roughly 29°C. Between 1979-1983 the likelihood of temperatures in Central Europe exceeding this threshold was 6 percent. In 2018-2022, it was 23 percent.
This self-reinforcing dynamic is currently not well represented in the climate models that forecast extreme heat events.
"If we're right and this interaction is making the most extreme heat events even hotter, it means we could be in store for more severe future heatwave risks than what is currently expected," said Kai Kornhuber, a co-author of the paper from IIASA and the Max Planck Institute of Geoanthropology.
