Extremely hot summers, the kind that were virtually unheard of decades ago, have become increasingly common.
The graphic above, based on a newly-updated analysis from researchers at Columbia University, shows how, in recent decades, local summer temperatures across the Northern Hemisphere have shifted conspicuously toward higher heat.
Not every summer is hotter today; some areas still see average or colder than average seasons from June through August. But the distribution of summer temperatures has shifted so that many more places endure hot summers more often than they did in the past. And the most drastic change has occurred at the hottest extreme.
Less than 1 percent of summers in the middle of the 20th century were extremely hot for their location, according to the analysis, compared with more than a quarter of summers over the last decade.
Mapping the temperature categories for individual years offers another, more detailed view of how high summer heat has spread around the world and where it has been felt most:
Northern Hemisphere Summers
Note: Data for June-July-August, Northern Hemisphere land.
This past summer, the hottest on record, shows the toll extreme heat can take. Blistering temperatures helped fuel destructive, deadly wildfires across the Mediterranean. Record highs forced Chinese cities to suspend outdoor work and open emergency cooling centers. Weeks of triple-digit heat stressed hospitals in the American southwest and led to an increase in heat-related deaths. Hotter summer temperatures affected cold places too, hastening ice melt and threatening permafrost.
“Extreme heat is one of the most direct ways in which we are experiencing the impacts of global warming,” said Deepti Singh, who leads the Climate Extremes Lab at Washington State University.
It’s also one of the clearest signals of how human emissions of greenhouse gasses are changing the planet’s climate, she added.
To understand how summers have changed, James Hansen, a retired NASA climate scientist and current professor at Columbia University, with colleagues Makiko Sato and Reto Ruedy, compared local summer temperatures over land for each decade since the 1950s to the average summer temperature for the location between 1951 and 1980, their baseline period.
During that period, about a third of summers across the Northern Hemisphere were in what the scientists defined as a “near average” or normal range for the period; a third were considered cold; a third were hot. Only a few summers, in a few places, were extremely cold or extremely hot.
By 2023, the scientists found, the distribution had shifted so that the vast majority of summers over the past decade were either hot or extremely hot. The updated analysis is based on a paper originally published in the journal Proceedings of the National Academy of Sciences in 2012.
The increase is consistent with what climate scientists have warned for decades: A seemingly small change in global average temperature (so far, about 1.2 degrees Celsius of warming since pre-industrial times) can lead to big changes in extreme heat.
Hotter summers also make the effects of global warming harder for the public to ignore, Dr. Hansen said via email. “People notice the extremes,” he wrote.
Dr. Hansen’s temperature curves also flatten out over time, as they move rightward, toward higher heat. Zeke Hausfather, a climate scientist at Berkeley Earth, said this largely reflects that some parts of the world are warming faster than others.
Overall, Dr. Hausfather said, the data provides “a good visualization of how when we focus just on the global average, we’re missing a lot of what’s happening.”
