Here’s how much climate change increases the odds of brutally hot summers

Simulations show warming is making the deadly heat gripping the Northern Hemisphere more likely

A photo of a man facing away from the camera and pouring water out of a 2-liter jug onto his head.

Millions of people across the Northern Hemisphere are sweltering in extreme heat waves, including this man trying to stay cool in Phoenix earlier this month.

Brandon Bell/Getty Images

The recent, record-breaking heat waves that have scorched the southwestern United States and northern Mexico, China and southern Europe were made dramatically more likely due to human-caused climate change, researchers report July 25 in a study from the World Weather Attribution network.

“This is absolutely not a surprise,” climate scientist Friederike Otto of Imperial College London said at a July 24 news briefing. But “while the weather is changing as expected, how much it hurts us is larger than expected.”

These intense and at times deadly heat waves are occurring as high-pressure systems stall across the Northern Hemisphere, creating barely budging heat domes (SN: 7/19/23).  Phoenix, for example, has reached at least 43.3° C (110° Fahrenheit) every day for more than three weeks. 

Otto and her colleagues used computers to simulate Earth’s climate, with and without human-caused climate change, to assess how likely the recent heat waves would have been under different climate conditions.

In a world without climate change, they found, the recent extreme heat in China would be expected roughly once every 250 years. Now, it’s a once-in-five-years phenomenon, or 50 times more likely to occur. Meanwhile the extreme heat waves in southern Europe and North America, which would have been virtually impossible without climate change, are now likely to occur once every 10 years and 15 years, respectively.

Should climate warming reach 2 degrees Celsius above preindustrial levels, these events could occur every two to five years, the simulations showed. We’ve already warmed by at least 1.1 degrees C since then (SN: 12/22/22).

Though it’s still too early to pin down the human toll of these extreme events, hundreds of deaths have already been reported from regions around the world, and power shortage concerns grow as the demand for cooling surges.

“The risks are rising faster than we are adapting,” Otto said. “We are much more vulnerable than we might have liked to believe in the past.”

Nikk Ogasa is a staff writer who focuses on the physical sciences for Science News. He has a master's degree in geology from McGill University, and a master's degree in science communication from the University of California, Santa Cruz.

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