A heatwave in May set monthly temperature records across Europe; a heatwave in June became the hottest ever observed in western Europe. Now, in July, yet another heatwave is developing. Just 50 years ago, the June heatwave would have been virtually impossible. But global warming is making heatwaves more frequent, longer and more intense.

Worldwide, heat is the deadliest type of weather, killing more than each year. The number will increase, since even if we reached net zero tomorrow, the carbon dioxide we have already emitted will keep raising temperatures.

“This is just the start,” says at University College London. “Things are unfolding in a very, very major way now, because this isn’t just about it [being] too hot in London, and the long-term effects are going to be savage.”

Outside the tropics, the time of the year in which temperatures above 32°C occur has lengthened by 12 days in the past half-century. In Europe, the fastest-warming continent, the season of strong heat stress now starts on average in June and continues until almost September. Sometimes, like this year, it starts in May.

That increases people’s exposure to hot days and heatwaves. Parts of North America, Europe, South America and Africa now experience up to 50 more days of strong heat stress compared with the 1970s.

“If you’ve got heatwaves that last longer, and then you’ve got more heatwaves, people are going to be in that raised physiological state for longer,” says Neil Maxwell at the University of Brighton, UK. “That can lead to greater inflammatory marker responses, and that ultimately puts a greater stress upon individuals.”

Strong heat stress almost never occurred at night before 1998. But now, nighttime temperatures in western Europe and other places are increasing at of global warming as a whole.

A drop in body temperature triggers sleep. If the environment is too hot, it is harder to fall asleep, as well as to enter a state of deep sleep. And loss of sleep over several nights in a row can hinder reaction time and boost anxiety and stress.

“If you don’t get cooling periods at night, which we define in this country as less than 20°C at night, sustained temperatures without cooling have worse impacts,” says Montgomery.

The hottest summer ever seen led to apocalyptic scenes in Europe in 2022. Wildfires broke out in France, Portugal and Spain. Italy’s longest river, the Po, ran dry in places, and wrecks of Nazi ships full of explosives were discovered as the Danube fell to record lows. In the UK, temperatures exceeded 40°C (104°F) for the first time.

More than 60,000 people died because of these baking temperatures. The highest mortality rates were in Mediterranean countries, which had some of the biggest temperature anomalies, with temperatures reaching higher than 40°C in Italy, Greece and Spain. These countries also have some of the , whose bodies aren’t as resilient to heat and who are more likely to have chronic illnesses.

“You also get inflammatory responses from heat, so heat exposure in itself triggers all sorts of bad biology in your body, basically, that is directly harmful… and in particular in people with diseases,” says Montgomery.

The frequency of a heat stress day followed by a tropical night of at least 20°C has increased 73 per cent in Europe since the 1970s. These are called “compound events” because the body isn’t able to cool down and recover at night, compounding the heat stress.

Europe has also seen prolonged periods of heat stress become more common. And Africa is now almost three times more likely to suffer hot spells lasting three-quarters of the year or more.

Leaders like US President Donald Trump have made pledges to plant millions of trees while increasing CO2 emissions. But in the case of urban heat, trees can make a big difference. They create areas of shade, and they also draw moisture from the soil, which then evaporates from their leaves, cooling the environment. Neighbourhoods with tree canopies can be as much as than similar places.

But although many cities have started planting trees to deal with heat, a recent found that many still have swathes of territory below the 30 per cent canopy cover that can reduce dangerous heat island effects. More than 90 per cent of the buildings in Paris and London fall below this threshold.

This week’s heatwave is the hottest ever recorded in Europe, as well as the most humid, and it is likely to cause thousands of deaths.

Although a potential “super El Niño” is forming in the Pacific Ocean, this didn’t play a role in the heatwave, a study by the World Weather Attribution network of scientists has found. Instead, global warming is clearly to blame.

The study analysed how likely the average daily maximum temperature projected for 26 to 28 June in western and central Europe would have been in the cooler climates of 1976 and of 2003.

While the weather pattern – a low-pressure heat dome that is trapping hot air from the south – isn’t unusual, the temperatures are. Fifty years ago, a typical June heatwave would have been about 3.5°C cooler, and the temperatures seen over the next three days would have been a less-than-one-in-10,000-year occurrence.

Daytime temperatures have 44°C  (111°F) in one French town, and nighttime temperatures have remained above 30°C (86°F) in parts of Spain.

“This event would not have been possible in June without climate change,” at Imperial College London said at a media briefing on 25 June. “The three-day nighttime temperatures would not have been possible at any time of year without climate change.”

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The humidity has also been unprecedented, reaching more than 50 per cent in many British cities. Dew-point temperatures have been in the low 20s, as compared to the single digits during the July 2022 heatwave that set the UK’s temperature record.

The wet-bulb globe temperature, which measures not just air temperature but also humidity, heat radiation and air movement, has broken or is expected to break records in almost half of European cities, the study found.

Humidity amplifies health risks because it slows evaporation, making sweating less effective. While older people or those who have a chronic illness are in particular danger, so are migrants and people experiencing homelessness.

“What we see very clearly… is how unequal the effects of this heatwave are and how that really demonstrates the inequality that widens due to climate change,” said , also at Imperial College London. “Because it’s of course people who are particularly vulnerable who are most likely to lose their lives.”

While it is too soon to look at excess mortality, a previous study found a smaller heatwave in June and July 2025 killed 2300 people in London and 11 other European cities.

“The health impacts of this heatwave are likely to be extremely high across large parts of northern and central Europe,” said Keeping.

Heatwaves will become even more intense and frequent unless we rapidly cut fossil fuel emissions, the researchers stressed. And Europe, the fastest warming continent, is not ready, as it has an ageing, urban population living in cities built for a cooler era. In the UK, only 5 per cent of homes have air conditioning.

Besides AC, Europe should invest in passive cooling like building insulation, ventilation, green roofs and walls and trees along streets, they said. It should also expand its heat response to include oft-forgotten groups like people with mental health conditions and those who are pregnant, said Carolina Pereira Marghidan at the Red Cross Red Crescent Climate Centre.

“Europe has heat action plans, but research has also shown that sometimes they do not cover all the groups that may be vulnerable,” she said.