What is behind Europe’s heatwaves?

Two months ago, France experienced its hottest May on record, with record highs in some cities. Last month, France was hit again by a spring heatwave, which also affected Spain, Italy and other countries. Then, this month, Poland and other parts of Eastern Europe suffered extreme heat.

Now temperatures are soaring again across Europe, from Spain to the British Isles and from Spain to the British Isles in triple digits or near triple digits, spreading eastward. Wildfires fueled by the heat are burning in many countries, and much of the continent is suffering from a prolonged drought.

And there are still two months of summer left.

Scientists say the ongoing extreme heat is trending as early as this year. Heat waves in Europe, they say, are increasing in frequency and intensity faster than in almost any other part of the planet, including the western United States.

Global warming is playing a role, as are heat waves around the world, as temperatures average about 1.1 degrees Celsius higher than they were in the late 19th century, before emissions of carbon dioxide and other heat-trapping gases became widespread. So extreme heat takes off from a higher starting point.

But beyond that, there are other factors, some affecting atmospheric and oceanic circulation, that could make Europe a heatwave hotspot.

No two heat waves are exactly alike. The current scorching temperatures that hit England and Wales on Monday were caused in part by a region of low upper-level air pressure that has stalled off the coast of Portugal for days. It is known as the “limiting low” in the parlance of atmospheric scientists because it was cut off by a flow of westerly winds, the mid-latitude jet stream, which orbits the planet at high altitudes.

Low pressure zones tend to attract air. In this case, the low-pressure zone steadily draws air from North Africa to it and to Europe. “It’s pumping hot air north,” said Kai Kornhuber, a researcher at the Lamont-Doherty Earth Observatory, part of Columbia University.

dr Kornhuber contributed to a study published this month that found that heat waves in Europe had increased in frequency and intensity over the past four decades, and linked the increase, at least in part, to changes in the jet stream. The researchers found that many European heatwaves occurred when the jet stream temporarily split in two, leaving an area of ​​low winds and high-pressure air between the two branches, conducive to the build-up of extreme heat.

Efi Rousi, senior scientist at the Potsdam Institute for Climate Research in Germany and lead author of the study, said the current heatwave appears to be linked to such a “double beam” that she says has been sweeping across Europe for the past two weeks. This could have led to the formation of the cutoff low, said Dr. Rousi, as well as an area of ​​light winds over Europe that kept the heat on.

“It seems that this is really helping this heatwave build up,” she said.

There may be other reasons why Europe is experiencing more and longer-lasting heatwaves, although some of these are currently the subject of debate among scientists. Natural climate variability can make it difficult to pinpoint specific influences, said Dr. rousi

dr Kornhuber said warming in the Arctic, which is occurring much faster than other parts of the world, could play a role. As the Arctic warms faster, the temperature difference between it and the equator decreases. This leads to a decrease in summer winds, causing weather systems to linger longer. “We’re seeing an increase in persistence,” he said.

There is also evidence that changes in one of the world’s largest ocean currents, the Atlantic Meridional Overturning Circulation, may be affecting climate in Europe. dr Rousi published a paper last year that used computer simulations to show that as the world warmed, weakening the flow would lead to changes in atmospheric circulation, leading to drier summers in Europe.

As in other parts of the world, a heat wave in Europe can cause others to occur in the same area as a period of extreme heat dries the ground.

If there is some moisture in the soil, part of the sun’s energy is used to evaporate the water, resulting in a slight cooling effect. But when a heat wave wipes out almost all soil moisture, there is little left to evaporate when the next wave of hot air arrives. So more solar energy burns the surface and increases the heat.

Raymond Zhong contributed reporting.

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