An unexpected feedback loop may be driving mega heatwaves

Mid-latitude regions such as Europe are set to suffer more intense heatwaves than previously thought, according to new research that has uncovered a three-way feedback loop that amplifies extreme heat conditions.

Heatwaves across the world have shocked scientists with their scale and severity in recent years, with heat extremes dramatically outstripping both historical records and the predictions of climate models.

In the UK in 2022, temperatures exceeded 40°C (104°F) for the first time in history, despite predictions that such a milestone was probably still years away. Likewise, the 2021 heatwave in the Pacific Northwest saw temperatures hit extremes that far outstripped climate model projections.

Researchers have been scrambling to understand why very extreme heat is occurring more frequently, and with greater intensity, than expected at this level of warming.

One team thinks the answer lies partly in the soil. Scientists already knew that high-pressure systems allow more solar radiation to reach Earth’s surface, heating up the land. They also knew that higher temperatures dry soils more quickly, removing the cooling effect provided by evaporation, thereby further amplifying heatwaves.

Now it seems that when temperatures climb high enough, dry soil, high pressure and extreme heat can amplify each other further, with dry soil helping to sustain high-pressure systems in the atmosphere.

“Basically, the soils that are affected by circulation pattern also start to affect back on the circulation pattern itself,” says Kai Kornhuber at the International Institute for Applied Systems Analysis in Austria.

This three-way feedback effect makes extreme heatwaves 10 per cent hotter than they would otherwise be, according to the research by Kornhuber and his colleagues – a factor not fully accounted for in climate model projections.

This will have serious consequences for our ability to cope with future extreme heat. “Ten per cent hotter or colder can really make a difference between a heatwave and a record-shattering heatwave,” says Kornhuber. “We’re facing so many critical thresholds in social and biophysical systems that an extra 10 per cent can really lead to temperatures that are then quite damaging.”

This may include serious public health consequences, a breakdown of vital infrastructure such as transport links and energy grids, crop failures and the collapse of ecosystems.

As the world warms due to climate change, regions including Europe, Russia, North America, parts of South America and Australia will hit thresholds where this feedback effect is triggered increasingly often. Kornhuber says work is now needed to ensure climate models are properly including this feedback effect when projecting future extremes.

Dim Coumou at the Free University of Amsterdam says the work helps to solve “part of the puzzle” of why heatwaves are outstripping model projections in their severity. “It shows that climate models are not doing a good job in reproducing this feedback mechanism, and that is a concern,” he says.

Governments should urgently start to prepare for more extreme heat events, he adds: “A region like Europe has to prepare for more intense heatwaves in the coming decades.”