In Milan, it was 30.3°C — a five-day average that sent elderly residents to hospitals and pushed emergency services beyond their limit. In Madrid, it was slightly cooler but deadlier. And in London, where you’d expect rain and gloomy weather, ambulances lined the streets near overheated apartments.

This isn’t something we’re used to seeing but it’s something that’s becoming more and more common due to climate change.

According to a new rapid study from Imperial College London and the London School of Hygiene & Tropical Medicine, human-caused climate change tripled the number of deaths during the late June 2025 heatwave that scorched twelve major European cities. Over just ten days, more than 2,300 people died due to the extreme heat, and researchers estimate that 1,500 of those deaths wouldn’t have occurred without the global warming driven by our fossil fuels.

This shouldn’t be a common event

Heatwaves do happen naturally. Yet, due to climate change, extreme heatwaves are becoming both more common and more intense. To quantify the effects of climate change, scientists used a method known as “event attribution.” Essentially, they compared what happened in the present world to what would have happened in a version of the world that hadn’t warmed by 1.3°C. The difference is striking.

In Madrid, researchers estimate that over 90% of heat-related deaths could be linked directly to climate change. Milan had the highest toll in sheer numbers, with 317 deaths out of 499 tied to warming. Paris saw 235 climate-related deaths, and London, 171.

Most victims were over the age of 65, though some were much younger. Many died quietly in their homes or hospitals, often without official attribution to heat. This is why experts refer to extreme heat as a silent killer, a disaster that happens in plain sight but rarely grabs headlines like floods or fires.

“Heatwaves are silent killers. Unlike floods or storms, their impact can be invisible: people who die during extreme heat usually have pre-existing health conditions, and heat is rarely recorded as a contributing cause of death. This makes the true toll of heatwaves both underestimated by the public and difficult to measure directly,” says Garyfallos Konstantinoudis, Research Fellow at the Grantham Institute, who was not involved in the study.

How heat kills

Our bodies cool down by sweating. But when it’s both hot and humid (as it was across much of Europe) sweat evaporates slowly or not at all. Internal temperatures rise and, especially for vulnerable individuals or outdoor workers, this can lead to heatstroke, heart failure, or fatal respiratory stress.

““Heatwaves are deadly. They disproportionately affect vulnerable groups — older adults, young children, people with chronic illnesses, and those without secure housing. Prolonged heat can overwhelm the body’s ability to regulate temperature, leading to dehydration, heat exhaustion, and potentially fatal heatstroke,” adds Akshay Deoras, Research Scientist, National Centre for Atmospheric Science & Department of Meteorology, University of Reading, who was not involved in the study.

This heatwave was unusual in that it came so early in the summer. People weren’t expecting it, and they weren’t expecting it to linger. The urban heat island effect makes cities significantly hotter than surrounding rural areas because concrete, asphalt, and buildings trap and radiate heat, especially at night. Nighttime temperatures stayed high, a phenomenon known as “tropical nights,” which put even more pressure on the body.

Some countries tried to react. For instance, Italy restricted outdoor labor and France shut down over a thousand schools temporarily. Even with these measures, Europe paid a big toll during this heatwave.

Yet, this is not just a European problem. While Europe is currently warming faster than any other continent during summer, extreme heat is a growing global threat. Similar deadly heatwaves have struck South Asia, the Middle East, and North America in recent years, killing thousands and overwhelming healthcare systems.

“Every 2 to 5 years now”

Again, such events can also happen naturally. But in a natural world, they would happen exceedingly rarely. In a world without any man-made greenhouse gas emissions, this type of heatwave would happen once or twice a century in many cities. In our world, it’s happening every 2-5 years.

Plus, if our emissions trends continue, things could get much worse. By mid-century, 163 million Europeans could be facing record-breaking summers every year, and the toll could increase dramatically.

Some cities are trying to adapt. Urban planners are planting more trees and designing green spaces that reduce the “urban heat island” effect. But these responses are uneven, underfunded, and in many places, too late.

“Heatwaves are not just periods of hot weather, they are serious public health emergencies that can be deadly, specifically for vulnerable groups such as older adults, children, individuals with chronic illnesses, and those facing poverty, social isolation, or homelessness. Reducing harm requires strong preparedness: early warning systems, clear public advice, and resilient healthcare services,” notes Dr Raquel Nunes, Associate Professor in Health and Environment at the University of Warwick, not involved with the study.

The study calls for a ramp-up of early warning systems, better insulation for homes, and long-term urban planning that prioritizes human survivability in a warmer world. But it also notes that even if we adapt, the heat is coming faster than we can prepare for it.

Unlike storms, heatwaves often don’t have names. But maybe they should. Giving names makes disasters real. Makes the loss tangible.

So, if we had to name this one, maybe call it what it really was: The Heatwave That Shouldn’t Have Happened.

You can read the study in its entirety here.