Western Europeans have experienced more than their share of heat waves in recent years, with frequent periods of blisteringly hot and potentially lethal weather almost every summer. This year, parts of the region suffered through intense heat even before summer began.
A new study confirms 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.
Global warming has helped to make heat waves worse everywhere, for the basic reason that they start from a higher-than-ever baseline temperature. Average global temperatures have increased by about 1.1 degree Celsius (about 2 degrees Fahrenheit) since the late 19th century, when widespread emissions of heat-trapping carbon dioxide began.
But the study, published this week in Nature Communications, found another mechanism, one involving atmospheric circulation, that contributed to the accelerating heat wave trend in Western Europe.
Specifically, the researchers found a link between heat waves and the state of the jet stream, the river of fast west-to-east winds in the upper atmosphere at middle latitudes. Sometimes the jet stream 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 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. Rousi added.