What’s behind heat waves in Europe?

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

Today, temperatures across Europe are soaring again, near triple digits, from Spain to the British Isles and spreading eastwards. Heat-fueled wildfires are burning in many countries and much of the continent is in the grip of a long drought.

And there are still two months of summer left.

Scientists say the persistent extreme heat already this year is consistent 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, as temperatures are on average about 2 degrees Fahrenheit (1.1 degrees Celsius) higher than they were at the late 19th century, before carbon dioxide emissions and other heat trapping. gases have become generalized. Thus, extreme heat starts from a higher starting point.

But beyond that there are other factors, some involving atmospheric and ocean circulation, that can make Europe a heatwave hotspot.

No two heat waves are exactly the same. The current scorching temperatures that hit England and Wales on Monday were caused in part by a region of upper-level low-pressure air that was locked in off the coast of Portugal for days. It’s called a “cutoff low” in the jargon of atmospheric scientists, because it was cut off from a river of westerly winds, the mid-latitude jet stream, which circles the planet at high altitude.

Areas of low pressure tend to pull air towards them. In this case, the low pressure area steadily drew air from North Africa towards it and towards Europe. “It’s pumping warm 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 have increased in frequency and intensity over the past four decades, and linked this increase at least in part to changes in the jet stream. Researchers found that many European heat waves occur when the jet stream has temporarily split in two, leaving an area of ​​weak winds and high-pressure air between the two branches, conducive to the build-up of heat. extreme heat.

Efi Rousi, a senior scientist at Germany’s Potsdam Institute for Climate Research and lead author of the study, said the current heat wave appeared to be related 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 cut-off low, Dr Rousi said, as well as an area of ​​weak winds over Europe which allowed the heat to persist.

“It seems like it’s really helping this heat wave build up,” she said.

There may be other reasons why Europe is experiencing 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 detect specific influences, Dr Rousi said.

Dr Kornhuber said warming in the Arctic, which is happening much faster than in other parts of the world, could play a role. As the Arctic warms at a faster rate, the temperature differential between it and the equator decreases. This leads to less summer winds, which has the effect of extending the duration of weather systems. “We’re seeing 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, could affect Europe’s climate. Dr Rousi published a paper last year which showed, using computer simulations, that a weakening of the current as the world warms 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 increase the likelihood of others occurring in the same region, as a period of extreme heat dries out the ground.

When there is some moisture in the ground, some of the solar energy is used to evaporate the water, which produces a slight cooling effect. But when a heat wave wipes out almost all the moisture from the ground, there isn’t much left to evaporate when the next wave of warm air arrives. Thus, more of the sun’s energy bakes the surface, adding to the heat.

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