The extreme heat wave engulfing Spain and Portugal is the latest such event to spread north and east across Western Europe, which now experiences dangerously hot weather every summer. This year, some parts of the region were affected by extreme heat even before the onset of summer.
Global warming has worsened heat waves in Europe and elsewhere, for the basic reason that they start from near-zero baseline temperatures. Since the late 19th century, when widespread emissions of heat-trapping carbon dioxide began, average global temperatures have increased by about 1.1 degrees Celsius (about 2 degrees Fahrenheit).
But beyond this basic warming, other mechanisms can lead to heat waves. Currently, a low-pressure system in southern Europe is pulling warm air northward from the Sahara. That low pressure area is expected to move north and east, bringing warm air to France and Britain and parts of central Europe.
A recent study confirmed that the frequency and overall intensity (defined as heat above a certain threshold) of what researchers call heat wave hot spots have increased over the past four decades.
And, the study found, changes in frequency and intensity are occurring 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, atmospheric circulation, particularly the position of the mid-latitude jet stream, contributed to the acceleration of the heat wave over western Europe.
A jet stream is a river of fast west-east wind in the upper atmosphere. Sometimes split into two. Between the northern and southern sides of the jet stream, heat waves can form in areas of weak air and high pressure air.
These events of “twin jets” are increasing in frequency and lasting longer, and researchers attribute these changes to changes in heat waves.
It’s not clear what causes the jet stream to split, said Effie Rousi, a senior scientist at the Potsdam Institute for Climate Research in Germany and lead author of the study. Blocking highs may form themselves and cause the jet stream to split,” or vice versa, the jet stream splits for other reasons, allowing blocking to form.
“We don’t know exactly what the trigger is,” Dr. Rousey added.