Millions of Americans are once again gripped by dangerous heat. Hot air blanketed Europe over the weekend, leaving parts of France and Spain feeling normal in July or August. High temperatures scorched northern and central China even as torrential rains caused flooding in the south of the country. Some places in India started experiencing extraordinary heat in March, although the onset of monsoon rains has brought some relief.
It’s too early to say whether climate change is directly responsible for causing severe heatwaves in these four powerful economies — which also happen to be the largest emitters of heat-trapping gases — at about the same time, just a few days into the summer.
While global warming is making extreme heat more common worldwide, a deeper analysis is needed to tell scientists whether specific weather events were made more likely or more intense because of human-induced warming. (A team of researchers studying this spring’s devastating heat in India found that climate change was 30 times more likely.)
Still, concurrent heatwaves seem to increasingly affect certain groups of distant places of late, for reasons related to the jet stream and other air streams that affect weather systems worldwide.
Studies have shown that parts of North America, Europe and Asia are connected in this way. Scientists are still trying to determine how these patterns might change as the planet warms further, but for now that means concurrent extreme heat will likely continue to affect these places where so much of the world’s economic activity is concentrated.
“To have a heat wave, we need the heat, and we need the atmospheric circulation pattern that allows the heat to build up,” said Daniel E. Horton, a climate scientist at Northwestern University. With global warming, he said, “we’re definitely getting more heat.” But climate change could also affect the way this heat is distributed around the world by air currents orbiting the Earth, he said.
Simultaneous weather extremes in many locations are not just meteorological curiosities. Individual heat waves can lead to illness and death, forest fires and crop failures. Concurrent forms could pose a threat to global food supplies, which have been under great strain this year by Russia’s invasion of Ukraine.
While heatwaves are shaped by complex local factors such as urbanization and land use, scientists no longer question whether climate change is making them worse. Soon, the world’s most devastating heatwaves simply won’t have a historical analogy from the time shortly before humans started pumping greenhouse gases into the atmosphere, some scientists argue, negating the question of whether climate change is a major driver.
Warming over the past few decades has already made it difficult for scientists to know what to call a heat wave and what to treat as just another warm weather standard, said Andrew Dessler, a climate scientist at Texas A&M University.
For example, if the threshold for a heat wave is just the mercury crossing 100 degrees Fahrenheit for days, then it’s “not unexpected at all,” said Dr. Dessler, to see them appear more regularly in different regions at once. “As time goes by, more and more of the planet will experience those temperatures, until eventually, with enough global warming, every area of land in the northern hemisphere at mid-latitude would be above 100 degrees,” he said.
But even when scientists look at how often temperatures exceed a certain level relative to a moving average, they still find a large increase in the frequency of concurrent heat waves.
A recent study that did this found that the average number of days between May and September with at least one major heat wave in the Northern Hemisphere doubled between the 1980s and the 2010s, to about 152 from 73. But the number of days increased by two or more heat waves was seven times higher, from 20 to about 143. That’s almost every day from May to September.
The study also found that these simultaneous heatwaves hit larger areas and were more intense by the 2010s, with peak temperatures nearly one-fifth higher than in the 1980s. On days when there was at least one major heat wave somewhere in the Northern Hemisphere, there were an average of 3.6 per day, the study found.
These “dramatic” increases came as a surprise, said Deepti Singh, a climate scientist at Washington State University and author of the study.
dr. Singh and her co-authors also looked at where concurrent heat waves occurred most frequently over those four decades. One pattern stood out: Large concurrent heatwaves increasingly hit parts of eastern North America, Europe, and central and eastern Asia between 1979 and 2019 — “more than what we’d expect simply due to the effect of warming,” said Dr. . Singh.
The study did not try to predict whether heat waves following this pattern will become more frequent as global warming continues, she said.
Scientists are trying to determine how the meandering of the jet stream, which has long shaped weather patterns for billions of people, might change in this warming era. One factor is the rapid warming of the Arctic, which is narrowing the temperature difference between the northern and southern bands of the Northern Hemisphere. How exactly this might affect extreme weather is still a matter of debate.
But those temperature differences are the main forces driving the wind that keeps the weather systems moving across the planet. As temperature differences narrow, these airflows can slow down, said Kai Kornhuber, a climate scientist with the Lamont-Doherty Earth Observatory at Columbia University. That means extreme events such as heat waves and heavy rains are likely to last longer.
“The longer a heat wave lasts, the more you push natural and societal systems to the brink,” said Dr. Kornhuber.
Climate change already means the world will see more extreme weather events and more extremes happening at the same time, he said. “This circulation changes, they will work on top of it,” he said, “and would make extremes even more severe and even more frequent.”
