Study: May and June heatwaves killed 2,700 people, with climate change responsible for 42%

Two heatwaves swept across England and Wales in the spring and early summer of 2026, and the toll was staggering. Researchers from Imperial College London, the Met Office, and the London School of Hygiene and Tropical Medicine have now estimated that approximately 2,700 people died from heat-related causes during those nine days in May and eleven days in June—550 in the first wave, 2,200 in the second. These were not ordinary warm spells. Daytime temperatures in May ran 15 degrees Celsius above the monthly average. June brought not just extreme heat but oppressive humidity, paired with nights that refused to cool. Record-breaking temperatures of 35.1 degrees Celsius in May and 37 degrees in June made these events exceptional not just for Britain but for all of western Europe, and their early arrival in the year made them particularly alarming.

What makes these numbers carry a different weight is what the research reveals about their cause. The scientists compared actual mortality during the heatwaves against modeled death rates in a climate untouched by human activity. The difference is stark: climate change—driven primarily by fossil fuel burning—was directly responsible for 42 percent of all heat-related deaths across the two events. In May, that meant 327 of the 550 deaths. In June, 825 of the 2,200. More than 1,100 people died because the planet is warmer than it would otherwise be. The warming itself is measurable: maximum daytime temperatures now run 3 to 4 degrees Celsius hotter than they would in a pre-industrial climate, a shift that compounds the danger with each passing year.

The mechanisms of heat death are not mysterious but they are relentless. Heat exhaustion and heat stroke are the obvious killers, but the research documents a wider cascade: stress on the heart and kidneys, the worsening of chronic conditions like heart disease, lung disease, and diabetes. The heat also destabilizes mental health, increases violence, and raises the risk of drowning. For the vulnerable—the elderly, the isolated, those already managing illness—a sustained heatwave becomes a crisis without obvious escape. During the June event, London's ambulance service experienced its busiest week on record, fielding a surge of life-threatening emergencies that strained a system already at capacity.

Dr. Mark McCarthy of the Met Office called the two heatwaves "exceptional," noting that May heatwaves are rare enough that thresholds are seldom met. This year shattered that pattern. The June event was notable not only for its intensity but for the combination of extreme daytime heat, high humidity, and persistently hot nights—a trio that amplifies the stress on human bodies, infrastructure, transport, agriculture, and hospitals. Dr. Clair Barnes from Imperial College London framed heatwaves as "the most dangerous kind of extreme weather," yet public perception often reduces them to images of ice cream and sunbathing. "People need to be aware that we are now seeing dangerous climate-change fuelled heat that is claiming lives, disrupting schools and hospitals and shutting down transport and infrastructure," she said. The World Health Organisation recommends that European authorities plan for extreme heat with the same rigor they apply to winter flu. Britain has not yet done so.

The research arrives as a warning about what lies ahead. The independent Climate Change Committee has warned that the UK is fundamentally unprepared for extreme heat, projecting that 92 percent of homes could be overheating by mid-century without urgent intervention. The committee has called for maximum temperature rules in workplaces and for air conditioning in hospitals, care homes, and schools—basic infrastructure that does not yet exist at scale. Professor Lea Berrang Ford of the UK Health Security Agency acknowledged that while the mortality estimates are modeled rather than observed, they "help illustrate the scale of risk associated with extreme heat and the growing threat climate change poses to our wellbeing." The UKHSA is preparing its own official analysis based on actual mortality data, expected in the coming weeks.

The political response has been divided. Energy Secretary Ed Miliband framed climate action as a matter of intergenerational responsibility, emphasizing the government's commitment to net-zero emissions and energy security. Zack Polanski, leader of the Green Party, took a sharper tone, saying the deaths rest on the hands of politicians who have prioritized profit over public safety and failed to act over years. "Without action to protect people from extreme heat, we will see thousands more deaths in the years to come," he warned. The London Ambulance Service, meanwhile, offered practical guidance: stay hydrated, avoid strenuous activity during peak heat, limit alcohol. These are individual measures in response to a systemic crisis—necessary but insufficient.

What the research makes clear is that the heatwaves of May and June 2026 were not anomalies but harbingers. They killed more than 2,700 people in a matter of weeks. More than 1,100 of those deaths were directly attributable to human-caused climate change. The question now is whether Britain will treat extreme heat as the public health emergency it has become, or whether it will wait for the next wave, and the one after that, watching the toll climb.