July 6, 2022, 11:00 ET
July 6, 2022, 23:00 ETS Smoke and fire in O Barco de Valdeorras, in the region of Galencia, in the northwest of Spain, Sunday. Extreme heat is fueling forest fires across the country. Credit … Brais Lorenzo / EPA, via Shutterstock
The extreme heat wave that has surrounded Spain and Portugal and is spreading north and east is just the latest event of its kind in Western Europe, which now experiences potentially lethal heat periods almost every summer. This year, some parts of the region suffered from intense heat even before summer began.
Global warming has worsened heat waves in Europe and elsewhere, for the basic reason that they start from a reference temperature higher than ever. Average global temperatures have risen by about 1.1 degrees Celsius (about 2 degrees Fahrenheit) since the late 19th century, when widespread emissions of heat-trapping carbon dioxide began.
But beyond this reference warming, other mechanisms cause heat waves. Currently, low-pressure air in southern Europe is dragging hot air from the Sahara to the north. This low pressure area is expected to drift north and east, bringing hot air to France and Britain and parts of central Europe.
A recent study confirmed that Western Europe has become what researchers call a heat wave hot spot over the past four decades, with events that have increased in frequency and cumulative intensity (defined as heat exceeding a certain threshold).
In addition, the study found, changes in frequency and intensity occur more rapidly in Europe than in many other parts of the world, including another hot spot, the western United States.
The study, published this month in Nature Communications, found that atmospheric circulation, specifically the state of the current in mid-latitude lightning, contributed to the acceleration of the heat wave trend in Western Europe.
The jet stream is a river of fast winds from west to east in the upper atmosphere. It is sometimes divided into two. Heat waves can develop in areas of weak winds and high pressure air, known as maximum blockages, between the north and south flanks of the jet stream.
The researchers found that these cases of “double jets” have increased in frequency and lasted longer, and that these changes explain the changes in heat waves.
Efi Rousi, a senior scientist at the Potsdam Institute for Climate Research in Germany and lead author of the study, said it was unclear what was causing the division of the current into lightning. Blocking maxima could develop on their own and cause the lightning current to split, he said, “or it could be the opposite, that the lightning current splits for other reasons, and this allows the blockage to develop. .
“We don’t know exactly what the trigger is,” Dr. Rousi added.
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