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.

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.

Deepti Singh.

A recent study, coauthored by Deepti Singh, a climate scientist at Washington State University, 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 severe by the 2010s, with peak intensities 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, Dr. Singh said.

She 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.

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New York Times