Millions of Americans are once again in the grips of dangerous heat. Hot air blanketed Europe last weekend, causing parts of France and Spain to feel the way it usually does in July or August. High temperatures scorched northern and central China even as heavy rains caused flooding in the country’s south. Some places in India began experiencing extraordinary heat in March, though the start of the monsoon rains has brought some relief.

It’s too soon to say whether climate change is directly to blame for causing severe heat waves in these four powerhouse economies — which also happen to be the top emitters of heat-trapping gases — at roughly the same time, just days into summer.

While global warming is making extreme heat more common worldwide, deeper analysis is required to tell scientists whether specific weather events were made more likely or more intense because of human-induced warming. (A team of researchers who studied this spring’s devastating heat in India found that climate change had made it 30 times as likely to occur.)

Even so, concurrent heat waves seem to be hitting certain groups of far-flung places with growing frequency of late, for reasons related to the jet stream and other rivers of air that influence weather systems worldwide.

Studies have shown that parts of North America, Europe and Asia are linked this way. Scientists are still trying to determine how these patterns might change as the planet warms further, but for now it means simultaneous heat extremes will probably continue affecting 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 accumulate,” said Daniel E. Horton, a climate scientist at Northwestern University. With global warming, he said, “we’re definitely getting more heat.” But climate change may also be affecting the way this heat is distributed around the world by globe-circling air currents, he said.

Simultaneous weather extremes in numerous locations aren’t just meteorological curiosities. Individual heat waves can lead to illness and death, wildfires, and crop failures. Concurrent ones can threaten global food supplies, which have been under perilous strain this year because of Russia’s invasion of Ukraine.

While heat waves are shaped by complex local factors such as urbanization and land use, scientists no longer have much doubt about whether climate change is making them worse. Soon, the world’s most devastating heat waves may simply have no historical analogue from the time shortly before humans starting pumping greenhouse gases into the atmosphere, some scientists argue, rendering obsolete the question of whether climate change is a main driver.

The warming of recent decades has already made it hard for scientists to know what to call a heat wave and what to treat as simply a new normal for hot weather, said Andrew Dessler, a climate scientist at Texas A&M University.

If the threshold for a heat wave is just the mercury exceeding 100 degrees Fahrenheit for days in a row, for instance, then it’s “not at all unexpected,” Dr. Dessler said, to see them occurring more regularly in several regions at once. “As time goes on, more and more of the planet will be experiencing those temperatures, until eventually, with enough global warming, every land area in the mid-latitude Northern Hemisphere would be above 100 degrees,” he said.

Yet even when scientists look at how often temperatures exceed a certain level relative to a moving average, they still find a big increase in the frequency of simultaneous heat waves.

One recent study that did this found that the average number of days between May and September with at least one large heat wave in the Northern Hemisphere doubled between the 1980s and the 2010s, to around 152 from 73. But the number of days with two or more heat waves was seven times higher, growing to roughly 143 from 20. That’s nearly every single day from May to September.

The study also found that these concurrent heat waves affected larger areas and were more intense by the 2010s, with peak temperatures that were almost one-fifth higher than in the 1980s. On days when there was at least one large heat wave somewhere in the Northern Hemisphere, there were 3.6 of them happening per day on average, the study found.

These “dramatic” increases came as a surprise, said Deepti Singh, a climate scientist at Washington State University and an author of the study.

Dr. Singh and her co-authors also looked at where concurrent heat waves occurred most frequently during those four decades. One pattern stood out: Large simultaneous heat waves struck parts of eastern North America, Europe, and central and eastern Asia increasingly often between 1979 and 2019 — “more than what we would expect simply by the effect of warming,” Dr. Singh said.

The study did not try to predict whether heat waves along this pattern will become more frequent as global warming continues, she said.

Scientists are working to pin down how the meandering of the jet stream, which has long shaped weather patterns for billions of people, might be changing in this warming era. One factor is the rapid warming of the Arctic, which narrows the difference in temperatures between the northern and southern bands of the Northern Hemisphere. How exactly this might be affecting extreme weather is still a matter of debate.

But those temperature differences are key forces driving the winds that keep weather systems moving around the planet. As the temperature differences narrow, these air currents may be slowing down, said Kai Kornhuber, a climate scientist with the Lamont-Doherty Earth Observatory at Columbia University. That means extreme events like heat waves and heavy downpours are likely to last longer.

“The longer a heat wave lasts, the more you push natural and societal systems to the edge,” Dr. Kornhuber said.

Climate change already means the world will see more extreme weather events, and more extremes occurring simultaneously, he said. “These circulation changes, they will act on top of it,” he said, “and would make extremes even more severe and even more frequent.”



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