Recent scientific studies show that up to 500,000 people die annually from extreme heat, surpassing the toll of natural disasters such as earthquakes, hurricanes and floods combined.

The new 2025 SONAR report from Swiss Re identifies this rise in dangerously high temperatures as one of the most pressing new threats. It is made far more likely by the climate crisis.

The report emphasises that this hazard extends beyond human mortality, affecting sectors such as energy and telecommunications.

"Extreme heat used to be considered the 'invisible peril' because the impacts are not as obvious as of other natural perils," said Jérôme Haegeli, Swiss Re's Group chief economist. "With a clear trend to longer, hotter heatwaves, it is important we shine a light on the true cost to human life, our economy, infrastructure, agriculture and healthcare system."

More intense, frequent, and prolonged heatwaves are becoming increasingly evident. July 2024 saw the planet’s warmest three-day stretch ever recorded.

In the United States, historical data reveals that, since the 1960s, these events have nearly tripled, with each now reaching higher peak temperatures and extending about one day longer on average.

The toll on public health is significant. Around 480,000 deaths annually are linked to extreme temperatures, says the report. Heat stress contributes to a range of issues – from fatigue and heatstroke to failure of vital organs – and worsens pre-existing conditions, especially among older adults and those who are pregnant.

High temperatures, when combined with strong winds, can heighten wildfire risks. Swiss Re Institute reports that between 2015 and 2024, global insured wildfire damages amounted to $78.5bn.

The SONAR report also notes how various industries face growing exposure to extreme heat. In telecommunications, for instance, rising temperatures can compromise cooling infrastructure in data centres or damage ground-based cables.

Legal liabilities are also increasing. Companies are already encountering lawsuits linked to failure in preventing heat-related damage. A notable 2021 case involved a $52bn claim against fossil fuel firms over losses attributed to weather extremes driven by climate change.

This suggests that heat-related legal action may become more common, influencing insurance claims.

Rising temperatures can also worsen other emerging threats. These include unfamiliar biological hazards such as heat-activated fungal infections, as well as long-standing risks like agricultural failure and workplace injuries from prolonged exposure to high heat.

This comes as the UK has been experiencing record high temperatures.

The UK experienced its first-ever 40°C temperature in 2022, said the Guardian. The government’s Met Office recently reported that there is a 50% chance of reaching that level again in the next dozen years. It also warned that 45°C events could occur as the climate continues to warm.

June 21, 2025 was hottest day of 2025 so far, it said, with parts of Surrey reaching 33.2C. The UK is usually temperate, and such record high temperatures are almost certainly a sign of the climate crisis.

According to the Guardian, scientists have determined that the likelihood of the heat in Southeast England had increased dramatically – by a factor of 100 – due to human-driven climate change.

Without climate change, a 32°C (89.6°F) day in June would have occurred roughly once every 2,500 years, researchers say, reported the newspaper. Current summer heat in England has risen to between 2°C and 4°C warmer than similar events in past decades.

This level of heat can lead to loss of life, especially among seniors and those with pre-existing health concerns. From 2020 to 2024, over 10,000 people in the UK died prematurely due to summer heatwaves, according to the UK Health Security Agency, said the Guardian.

Extended periods of high temperature pose particular danger, as they allow no time for the body to recover.

Forecasters had forecast daytime highs in the Southeast to remain above 28°C for three straight days. Researchers concluded that this pattern is now ten times more likely because of global warming.

The warming climate led to record-breaking global temperatures in 2024. Meanwhile, emissions from fossil fuel use are still climbing. Continued growth in carbon pollution could push global temperatures beyond 1.5°C above preindustrial levels within just two years – an internationally agreed limit. Surpassing it would worsen the types of extreme events already affecting the UK and other parts of the world.

Dr Friederike Otto, from Imperial College London, told the Guardian: “Heatwaves are called the silent killer, because we don’t see people dropping dead on the street, but killers they are. In Europe in 2022, more than 60,000 people died in the summer from extreme heat.”

Maja Vahlberg from the Red Cross Red Crescent Climate Centre also said: “Sadly most people die from heat indoors and alone, especially older people and those with underlying health conditions, such as lung or heart disease.”

Professor Mike Tipton, a human physiology expert at the University of Portsmouth, said in an interview with the newspaper: “The human body is not designed to tolerate prolonged exposure to this sort of extreme heat. It is undeniable that climate change is now costing British lives. Those politicians and commentators who pour scorn on climate action should reflect on this fact because, until we stop emitting greenhouse gases (GHGs), these episodes are only likely to become more extreme.”

According to Scientific American, which focussed on the extreme heatwave currently engulfing much of the US, overnight heat is especially dangerous. When temperatures stay high after sunset, mortality rates increase beyond normal expectations. More individuals end up in emergency hospital departments, hospitalisations rise, and risks such as early labour grow. Sleep disruption and declines in mental health are also reported.

 “If it doesn’t cool down at night, then your core body temperature can’t really get back to what is normal for you,” Kristie Ebi, an epidemiologist with the University of Washington, told Scientific American. “You’re starting the next morning with a higher baseline.” During extended heat events, mortality starts climbing after roughly a day. “It’s not the instantaneous exposure; it’s the buildup over the course of a day, not getting relief at night. That starts affecting the cells and organs,” Ebi notes.