The level of deaths from European heatwaves has the potential to reach levels seen during the COVID pandemic.

The warning comes as researchers revealed the weather patterns that produced some of Europe’s most extreme heat waves over the past three decades could prove far more lethal if they strike in today’s hotter climate.

“We showed that if these same weather systems were to occur after we’ve trapped a lot more heat in the atmosphere with greenhouse gases, the intensity of the heat waves gets stronger and the death toll rises,” said lead study author Christopher Callahan, who completed the research as a Stanford Doerr School of Sustainability postdoctoral scholar and recently joined the Indiana University faculty.

Global average temperatures in recent years have approached 1.5 degrees Celsius above pre-industrial levels and about 0.7 degrees above the 2003 average, when a heat wave killed more than 20,000 people across Europe. This year, 2025, researchers estimated thousands of people may have lost their lives because of extreme heat during the fourth-hottest summer in European history.

Using a combination of artificial intelligence and statistical techniques from economics, Callahan and colleagues estimate that 2003-like weather patterns could cause 17,800 excess deaths across the continent in a single week against the backdrop of today’s climate, compared to 9,000 with no global warming. At 3 degrees Celsius above pre-industrial levels, their modelling shows weekly excess deaths from a 2003-like weather system could reach 32,000.

Deadly heat waves in Europe have repeatedly followed the stalling of a high-pressure system, or “heat dome,” over land already parched from months of low rainfall.

In the summer of 2003, an extreme version of this combination held temperatures around 38 degrees Celsius (100 degrees Fahrenheit) for two weeks straight across much of Western Europe. In France, refrigerated trucks stored bodies as morgues reached capacity. The temperatures were so extreme that the event essentially broke conventional probability calculations, which suggested that without climate change it might be a one-in-a-million-year event.

“That event, which was devastating from a health point of view, was extremely statistically rare at the time that it happened, and yet we know it’s possible the weather conditions that produced it could happen again, but in what is now a much warmer climate,” said co-author Noah Diffenbaugh, the William Wrigley professor in the Stanford Doerr School of Sustainability.

Until now, however, researchers have not known the likely death toll if those same weather conditions arise in the current climate two decades later, or in the future after additional global warming.

Scientists said they  have known for decades that extreme heat waves can be expected to intensify as the planet continues to warm, and growing evidence shows that heat-related mortality risks can increase exponentially as it gets hotter. The new study showed how this could play out in Europe.

“These events could be as bad as some of the worst weeks of COVID by mid-century” said co-author Marshall Burke, a Stanford professor of environmental social sciences.

“A lot of the reason for excess deaths is because we are just so underprepared for these events. Similar to during COVID when the health system was just fully disrupted, people can’t get to the hospital, hospitals have to discharge people early,” he added. “So, even if you have something bad happen to you that’s not related to heat at all, your care is going to suffer and health outcomes will worsen.”

Global average temperatures in recent years have approached 1.5 degrees Celsius above pre-industrial levels and about 0.7 degrees above the 2003 average, when a heat wave killed more than 20,000 people across Europe.