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 the 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 strain this year because of Russia’s invasion of Ukraine. While heat waves are shaped by complex local factors such as urbanisation 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 started pumping greenhouse gases into the atmosphere, some scientists argue, rendering obsolete the question of whether climate change is the 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 37 degrees Celsius 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 37 degrees,” he said.

New York Times News Service