Western Europeans have had more than their fair share of heat waves in recent years, with frequent periods of stifling heat and potentially lethal conditions almost every summer. This year, parts of the region experienced intense heat even before summer began.
A new study confirms that over the past four decades Western Europe has become what scientists describe as a hotspot, with events of increasing frequency and cumulative intensity (defined as temperature above a certain threshold).
In addition, the study found that changes in the frequency and intensity of heat waves are occurring at a faster rate in Europe than in many other parts of the world — including another hot spot, the western United States.
Global warming is helping to exacerbate heatwaves around the world, for the basic reason that they start from a reference temperature that is higher than ever. Since the late 19th century, when widespread emissions of heat-trapping carbon dioxide began, global average temperatures have risen by about 1.1 degrees Celsius.
But the study, published last week by Nature Communications, found that there is another mechanism, this one involving atmospheric circulation, that has been contributing to the acceleration of the heatwave trend in western Europe.
Specifically, the researchers found a link between heat waves and the state of the jet stream, a stream of fast west to east winds in the upper atmosphere at mid-latitudes. Sometimes the jet stream splits in two. Heat waves can develop in areas of light winds and high air pressure, known as high blockage areas, between the northern and southern flanks of the jet stream.
The researchers found that these instances of split jet stream have been increasing in frequency and duration, 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 the study’s lead author, said it was unclear what was causing the jet stream to split. Areas of high blockage can develop on their own and cause the jet stream to split, she said, “or it could be the other way around — it could be that the jet stream splits for other reasons and that allows blockage to form.” .
“We don’t know exactly what the trigger is.”
