The extreme heat wave that has enveloped Spain and Portugal and is spreading to the north and east is just the latest such event in Western Europe, which now experiences periods of potentially lethal hot weather almost every summer. This year, parts of the region suffered through 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 higher-than-ever baseline temperature. Average global temperatures have increased 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 baseline warming, other mechanisms lead to heat waves. In the current one, low pressure air in Southern Europe is drawing hot air from the Sahara northward. That low-pressure zone is expected to drift north and east, bringing the hot air to France and Britain and into parts of Central Europe.
A recent study confirmed that Western Europe has become what the researchers call a heat wave hot spot over the last four decades, with events increasing in frequency and cumulative intensity (defined as heat in excess of a certain threshold).
What’s more, the study found, the changes in frequency and intensity are happening faster 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 mid-latitude jet stream, contributed to the accelerating heat wave trend in Western Europe.
The jet stream is a river of fast west-to-east winds in the upper atmosphere. Sometimes it splits in two. Heat waves can develop in areas of weak winds and high-pressure air, known as blocking highs, between the northern and southern flanks of the jet stream.
The researchers found that these instances of “double jets” have been increasing in frequency and lasting longer, and that these changes account for the changes in heat waves.
Efi Rousi, a senior scientist at the Potsdam Institute for Climate Research in Germany and the study’s lead author, said it was unclear what was causing the jet stream to divide. The blocking highs could develop on their own and cause the jet stream to split, she said, “or it could be the opposite, that the jet stream splits for other reasons, and this allows the blocking to develop.
“We don’t know exactly what the trigger is,” Dr. Rousey added.