The extreme heat wave that has enveloped Spain and Portugal and is spreading north and east is just the latest such event in Western Europe, which now experiences periods of life-threatening heat almost every summer. . This year, parts of the region suffered from intense heat even before the start of summer.
Global warming has worsened heat waves in Europe and elsewhere, for the simple reason that they start from a higher reference temperature than ever before. Average global temperatures have risen 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 basic warming, other mechanisms lead to heat waves. In the present, low-pressure air from southern Europe pulls warm air from the Sahara to the north. This area of low pressure is expected to drift north and east, bringing warm air to France and Britain and parts of central Europe.
A recent study confirmed that Western Europe has become what researchers call a heatwave hotspot over the past four decades, with events increasing in frequency and cumulative intensity (defined as heat above a certain threshold ).
What’s more, according to the study, changes in frequency and intensity are happening faster in Europe than in many other parts of the world, including another hotspot, the western United States.
The study, published this month in Nature Communications, found that atmospheric circulation, particularly the state of the mid-latitude jet stream, is contributing to the accelerating heat wave trend in Western Europe.
The jet stream is a river of fast winds from west to east in the upper atmosphere. Sometimes it splits in two. Heat waves can develop in areas of weak winds and high air pressure, called blocking highs, between the north and south flanks of the jet stream.
The researchers found that these instances of “double jets” had increased in frequency and lasted longer, and that these changes explained the changes in the heat waves.
Efi Rousi, senior scientist at Germany’s Potsdam Institute for Climate Research and lead author of the study, said it was unclear what caused the jet stream to split. The blocking peaks could develop on their own and cause the jet stream to separate, she said, “or it could be the opposite, that the jet stream separates for other reasons, allowing the blockage from developing.
“We don’t know exactly what the trigger is,” added Dr. Rousi.