What’s Behind Europe’s Heat Waves?

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

Now temperatures across Europe are soaring yet again, at or near triple digits from Spain to the British Isles and spreading east. Wildfires stoked by the heat are burning in many countries, and much of the continent is in the throes of a lengthy drought.

And there are still two months of summer left.

Scientists say the persistent extreme heat already this year is in keeping with a trend. Heat waves in Europe, they say, are increasing in frequency and intensity at a faster rate than almost any other part of the planet, including the Western United States.

Global warming plays a role, as it does in heat waves around the world, because temperatures are on average about 2 degrees Fahrenheit (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 involving the circulation of the atmosphere and the ocean, that may make Europe a heat wave hot spot.

No two heat waves are precisely the same. The current scorching temperatures that reached into England and Wales on Monday were caused in part by a region of upper level low-pressure air that has been stalled off the coast of Portugal for days. It’s known as a “cutoff low” in the parlance of atmospheric scientists, because it was cut off from a river of westerly winds, the mid-latitude jet stream, that circles the planet at high altitudes.

Low-pressure zones tend to draw air toward them. In this case, the low-pressure zone has been steadily drawing air from North Africa toward it and into Europe. “It’s pumping hot air northward,” said Kai Kornhuber, a researcher at 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 heat waves occurred when the jet stream had temporarily split in two, leaving an area of weak winds and high pressure air between the two branches that is conducive to the buildup of extreme heat.

Efi Rousi, a senior scientist at Potsdam Institute for Climate Research in Germany and the lead author of the study, said the current heat wave appeared to be linked to such a “double jet,” which she said has been in place over Europe for the past two weeks. This could have led to the creation of the cutoff low, Dr. Rousi said, as well as to an area of weak winds over Europe that allowed the heat to persist.

“It seems this is really favoring the buildup of this heat wave,” she said.

There may be other reasons Europe is seeing more, and more persistent, heat waves, although some of these are currently the subject of debate among scientists. Natural climate variability can make it difficult to tease out specific influences, Dr. Rousi said.

Dr. Kornhuber said warming in the Arctic, which is occurring much faster than other parts of the world, may play a role. As the Arctic warms at a faster rate, the temperature differential between it and the Equator decreases. This leads to a decrease in summertime winds, which has the effect of making weather systems linger for longer. “We do see an increase in persistence,” he said.

There are also indications that changes in one of the world’s major ocean currents, the Atlantic Meridional Overturning Circulation, may affect Europe’s climate. Dr. Rousi published a paper last year that showed, using computer simulations, that a weakening of the current as the world warmed would cause changes in atmospheric circulation leading to drier summers in Europe.

As in other parts of the world, a heat wave in Europe can make it more likely for others to occur in the same area, because a period of extreme heat dries out the soil.

When there is some moisture in the soil, some of the sun’s energy is used in evaporating the water, leading to a slight cooling effect. But when one heat wave wipes out almost all the soil moisture, there is little left to evaporate when the next wave of hot air arrives. So more of the sun’s energy bakes the surface, adding to the heat.

Raymond Zhong contributed reporting.

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