The heatwave scorching western Europe is the most severe and widespread ever and is only possible due to the climate crisis driven by fossil fuel burning, scientists have said.

Almost half of Europe’s 850 largest cities are also enduring their worst ever heat stress, a combination of temperature and humidity, they found. Muggier conditions mean sweating is less effective at cooling the body, making heatwaves even more dangerous.

The analysis comes as the UK recorded its hottest ever June temperature on Thursday, 36.4C (97.5F) in Somerset, and much of western Europe recorded a sharp rise in medical emergencies, including some deaths.

In summer 2022, more than 60,000 people died due to heat in Europe. The statistical analysis needed to assess the impact of the current heatwave will take time to complete. Nonetheless, the heatwave is certain to exact a heavy toll and is also disrupting lives and livelihoods, with schools closed, hospitals struggling and rail and air journeys cancelled across the continent.

The new analysis by scientists from the World Weather Attribution (WWA) consortium shows how rapidly extreme heat is worsening as carbon pollution continues to pile up in the atmosphere. As recently as 2003, a heatwave like the current one in Europe would have been 2C cooler due to the lower level of global heating at the time. In 1976, another famous heatwave year, it would have been 3.5C cooler.

The sweltering night-time temperatures currently harming people’s sleep are about 100 times more likely today than in 2003. The scientists warned that without urgent climate action, future heat conditions would get even more extreme and the current summer could seem relatively cool in retrospect.

“This is the most severe and widespread heatwave to have ever affected this large a region of Europe,” said Dr Theodore Keeping, an extreme weather research associate at Imperial College London and part of the WWA team. “We found that in the last 50 years, during which time the planet has warmed by 1.1C, the chance of a heatwave like this has changed immensely. This event would not have been possible in June without climate change. But do we expect this to be a cool summer going forward? That’s absolutely the case.”

He said many capital cities were experiencing not only their hottest recorded three-day period in June but the hottest three-day period at any time of year. At least 100 million people in Europe were expected to face temperatures above 35C on Thursday.

The scientists used wet bulb globe temperatures to assess the additional impact of high humidity. “It accounts for the ability of the human body to cool itself down. With the worst conditions ever experienced in 45% of cities over 50,000 people, the health impacts of this heatwave are likely to be extremely high,” Keeping said. “The speed of change is startling.”

Commenting on the WWA analysis, Simon Stiell, the UN’s climate chief, said: “Climate change is running rampant, caused by the world’s addiction to burning coal, oil and gas. But the solutions are equally clear: a faster shift to clean energy – which is now much cheaper than fossil fuels – as well as protecting forests and building climate resilience.”

The WWA team used both observed and reliable forecast temperature data to analyse the hottest three-day period across a large area of western Europe, which is sitting under a “heat dome”. Using peer-reviewed methods, they found unequivocally that climate change was the driving force behind the severity of the heat.

They ruled out natural variability of the weather, in particular any influence from the El Niño event that has begun in the Pacific Ocean. The current weather pattern, a blocked high-pressure system trapping hot air over Europe and drawing warm air up from the Sahara, is not unusual in summer, the scientists said. Instead, the level of heat has been supercharged by global heating.

Carolina Pereira Marghidan, of the Red Cross Red Crescent Climate Centre, said: “After the devastating 2003 heatwave in Europe, many countries invested in early warning systems and action plans. Research shows that those have saved many lives, but it’s not enough.”

She said intensifying heat was increasingly affecting health, transport, energy systems and daily life. “We need greater investment in heat-resilient homes, cities and infrastructure to keep people safe.”

The UK government’s official adviser, the Climate Change Committee, said in May that the country’s infrastructure was “built for a climate that no longer exists” and needed urgent improvement to protect people from the climate crisis. The UK Health Security Agency (UKHSA) found that more than 10,000 people died in Britain owing to summer heatwaves between 2020 and 2024.

On Wednesday, the London ambulance service responded to its highest ever number of life-threatening emergencies in a single day – 641. Older people, children and those with vulnerabilities are most at risk, but the current red alerts from the UKHSA and Met Office warn that everyone is in danger. On Thursday the UKHSA extended its red heat-health alert by 24 hours, to 11pm on Friday.

A study of a smaller and less intense European heatwave in 2024 found that in 12 cities alone, more than 2,300 people lost their lives in three days because of the higher temperatures. “We found two-thirds of the 2,300 would not have died if it wasn’t for climate change,” said Prof Friederike Otto, a climate scientist at Imperial College London and a co-founder of WWA.

“Scientists like me are beginning to sound like a broken record, reacting year after year to heat extremes that climb ever higher,” she said. “Yes this is climate change, yes it’s us, yes we have the solutions, no we’re not implementing them fast enough. It’s really now a question of what kind of future we want for ourselves, and whether we’re willing to do what it takes to secure it.”

Last October, health experts said rising global heat was now killing one person a minute around the world.