Heat waves are getting more dangerous with climate change — and we may still be underestimating them

CNN

23-06-2025

The intensifying and expansive heat wave affecting around 150 million people in the United States from Wisconsin to Washington, DC, bears the hallmarks of human-caused global warming.
Hundreds of daily temperature records are threatened during the next few days, particularly along the East Coast, and some all-time June high temperature records could be tied or broken as well.
Milestones for record warm overnight low temperatures are also being set — another sign of climate change. Nighttime temperatures have been warming faster than daytime, which exacerbates the health consequences from heat waves. This is especially the case in cities, where the urban heat island effect keeps temperatures high overnight.
The US heat wave comes nearly in tandem with searing high temperatures in Western Europe, which global warming made far more likely and intense.
Of all the forms of extreme weather — droughts, floods, hurricanes — heat waves are the ones that scientists can most reliably tie to climate change caused by fossil fuel pollution. As the world warms, the odds of extreme heat events increase dramatically, while the odds and severity of record cold extremes decrease.
'The physical process of how more greenhouse gases in the atmosphere lead to hotter and more frequent heatwaves is well-understood and straightforward,' said Fredi Otto, a climate scientist who leads the World Weather Attribution project, an international effort that examines the role climate change plays in individual weather events.
'Every heatwave that is occurring today is hotter than it would have been without human-induced climate change,' she said.
The heat waves we are experiencing now are occurring in 1.2 degrees Celsius (2.16 degrees Fahrenheit) of global warming, with even more severe heat likely in coming decades as global average temperatures continue to climb.
In recent years, researchers have found that some heat waves would have been impossible without the temperature boost from global warming. Others have been made tens to hundreds of times more likely, and hotter than they would have been without the effects of climate pollution.
This was the case with the Pacific Northwest heat wave of 2021, a Siberian heat wave in 2020 and a United Kingdom heat wave in 2022, among other more recent events.
In short, climate change is causing heat waves to become more common, intense and longer-lasting. They are also hitting both earlier and later in the warm season, and in many parts of the world are becoming more humid, too, which makes them more dangerous.
Some studies have shown that global weather patterns conducive to simultaneous heat waves on different continents, such as the US and European heat waves this month, are becoming more common during the Northern Hemisphere summer.
According to a rapid scientific analysis, the UK heat wave this month, which brought temperatures up to 92 degrees Fahrenheit in Surrey, England, was 100 times more likely to occur in today's climate than prior to the human-caused global warming era.
Three straight days of temperatures above 82 degrees in southeastern England are now about 100 times more likely to occur compared to the pre-global warming era, the analysis said, before humans began burning fossil fuels like coal, oil and gas for energy.
In the US, heat waves are the deadliest form of extreme weather. The ongoing one is particularly threatening because it is the first of the season, bringing conditions more common during the peak of summer, not in June.
Temperatures are set to soar near or into the triple digits along the Amtrak Acela corridor from Washington to New York City, with heat indices climbing as high as 110 degrees or even higher in some locations. Such conditions will make it dangerous to be outdoors for extended periods of time, and the lack of overnight heat relief will compound the public health threat.
In most parts of the world outside of the tropics, including the US and the UK, heat waves occurring now would have been up to 7 degrees cooler without the burning of fossil fuels, Otto said.
'Through its influence on extreme heat, human-induced climate change puts a massive burden on societies, leading to thousands of premature deaths, and a large strain on infrastructure and ecosystems,' Otto said. 'In addition, extreme heat is leading to agricultural losses and a large loss in productivity.'
Otto sounded an additional note of caution: Computer models 'severely underestimate' the extreme heat trends we are seeing so far, which means projections for future extreme heat are likely also underestimates.
This is thought to be because of changes in air pollutants known as aerosols as well as shifts in weather patterns that may also be caused by climate change.
Michael Mann, a climate researcher at the University of Pennsylvania, has also found models struggle with heat wave projections in the context of climate change. His research has found an increase in the occurrence of persistent heat domes in recent years, like the one occurring now, which have rarely been picked up by the climate models.
'Climate models are likely understating the relationship between climate change and persistent summer weather extremes today and under predicting the potential for future increases in such extremes,' he told CNN.