The science behind climate deaths in heatwaves

The Hidden Toll of Heatwaves in Europe

A heatwave that scorched Europe in early July had barely ended when scientists released alarming estimates suggesting that as many as 2,300 people may have died across a dozen major cities during the extreme weather event. This figure was meant to raise awareness and serve as a timely warning to prevent further loss of life. Friederike Otto, one of the researchers involved in the study, emphasized the urgency of the message.

"We are still relatively early in the summer, so this will not have been the last heatwave," said Otto, a climate scientist at Imperial College London. "There is a lot that people and communities can do to save lives."

Heat has long been a silent killer in Europe, claiming tens of thousands of lives each summer. However, the toll often takes months or even years to be fully recognized. Otto and her team published their preliminary estimate just a week after temperatures peaked in western Europe. While the methods used were not new, this was the first study to directly link heatwave deaths to climate change so soon after the event.

From a public health perspective, early mortality estimates could be misinterpreted as official statistics. However, Raquel Nunes from the University of Warwick told DISCOVER TREND that the benefits of providing timely evidence outweigh these risks. She noted that this approach could have transformative potential for both public understanding and policy prioritization of heatwaves.

Climate Change and the Rising Danger of Heat

Science is increasingly showing that human-caused climate change is making heatwaves more intense and frequent. Unlike floods and fires, which are often dramatic and immediately visible, heat kills quietly. Prolonged exposure can lead to heat stroke, organ failure, and death. The elderly, the sick, and those working outdoors are particularly vulnerable, but younger individuals who exercise or work in high temperatures are also at risk.

Otto, a pioneer in the field of attribution science, has spent over a decade conducting studies on extreme weather events and heatwaves. Despite this, she feels society remains unprepared for the increasing frequency of such events. "People think it's 30 degrees Celsius instead of 27, what's the big deal? And we know it's a big deal."

When temperatures began to rise earlier this summer, scientists adjusted their approach. Collaborating with the London School of Hygiene & Tropical Medicine, they focused on the lethality of the heat rather than just its intensity. Between June 23 and July 2, they analyzed historic weather data and published mortality records to determine that climate change made the heatwave between 1°C and 4°C hotter across 12 cities. They estimated that 2,300 people likely died as a result.

In a notable first, the study found that 65% of these deaths—approximately 1,500 people in cities like London, Paris, and Athens—would not have occurred in a world without global warming. "That's a much stronger message," said Otto. "It brings it much closer to home what climate change actually means."

The Underestimated Threat of Heatwaves

The study was just a snapshot of the broader heatwave that hit western Europe during its hottest June on record. Temperatures soared to 46°C in Spain and Portugal, and the true toll was likely much higher. The authors noted that heat-related deaths are often undercounted, and since then, countries like Turkey, Greece, and Bulgaria have experienced fresh heatwaves and deadly wildfires.

Although groundbreaking, the study has not yet gone through peer review, a process that can take over a year. Otto argued that waiting until after the summer to publish would defeat the purpose, as the conversation around heatwaves tends to fade quickly. "I think it's especially important, in this context, to get the message out there very quickly."

Despite its limitations, the study relied on well-established scientific methods, according to several independent experts. Abhiyant Tiwari, a health and climate expert who worked on India’s first-ever heat action plan, suggested that tailoring this approach to local conditions could help cities better prepare for future heatwaves.

"I definitely see more such studies coming out in the future," said Tiwari from NRDC India. He added that India, which experiences extremely hot summers, is a prime candidate for similar research. With a template in place, more studies are likely to follow.