Two months ago, France experienced its warmest May on record, with record highs in some cities. Last month, France was again ravaged by a spring heat wave that also hit Spain, Italy and other countries. Then, this month, Poland and other parts of Eastern Europe suffered during a period of extreme heat.
Now temperatures are rising again across Europe, at or near triple digits from Spain to the British Isles and spreading east. Heat-fuelled wildfires are burning in many countries, and much of the continent is ravaged by prolonged drought.
And there are still two months of summer to go.
According to scientists, the persistent extreme heat is already part of a trend 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 plays a role, as does heatwaves around the world, as temperatures average about 2 degrees Fahrenheit (1.1 degrees Celsius) higher than they were at the end of the 19th century, before the release of carbon dioxide and other heat trap gases became widespread. Extreme heat thus rises from a higher starting point.
But beyond that, there are other factors, including atmospheric and ocean circulation, that can make Europa a heat wave hotspot.
No two heat waves are exactly alike. The current scorching temperatures that reached England and Wales on Monday were partly caused by an area of highest-level low-pressure air that has come to a standstill for days off the coast of Portugal. It is 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, which orbits the planet at high altitudes.
Low pressure areas tend to pull air towards them. In this case, the low-pressure zone is steadily drawing air from North Africa toward it and into Europe. “It pumps hot air north,” said Kai Kornhuber, a researcher at Columbia University’s Lamont-Doherty Earth Observatory.
dr. Kornhuber contributed to a study published this month who found that heatwaves in Europe had increased in frequency and intensity over the past four decades, linking the increase at least in part to changes in the jet stream. The researchers found that many European heatwaves happened when the jet stream had temporarily split in two, leaving an area of weak winds and high-pressure air between the two branches, conducive to extreme heat buildup.
Efi Rousi, a senior scientist at the Potsdam Institute for Climate Research in Germany and the study’s lead author, said the current heat wave appeared to be related to such a “double jet,” which she says has been in Europe for the past two years. to soften. This could have led to the onset of the cutoff low, said Dr. Rousi, as well as an area of light wind over Europe that allowed the heat to persist.
“Looks like this really helps build up this heat wave,” she said.
There may be other reasons why Europe is seeing more and more persistent heatwaves, although some of these are currently the subject of debate among scientists. Natural climate variability can make it difficult to discover specific influences, said Dr. Rousi.
dr. Kornhuber said warming in the Arctic, which is happening much faster than in other parts of the world, may play a role. As the Arctic warms faster, the temperature difference between the Arctic and the equator decreases. This leads to a decrease in summer winds, causing weather systems to hang around 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, could affect Europe’s climate. dr. Rousi published a paper last year in which computer simulations showed that a weakening of the flow as the world warms 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 cause others to occur in the same area as a period of extreme heat dries out the soil.
When there is some moisture in the soil, some of the solar energy is used to evaporate the water, leading to a slightly cooling effect. But if one heat wave wipes out almost all soil moisture, there’s little left to evaporate in the next wave of hot air. So more of the solar energy bakes the surface, adding to the heat.
Raymond Zhong reporting contributed.