Europe: climate extremes overtake forecasts

Extreme hot days have become hotter in Central Europe since 1950 by five degrees

Europe during the heatwave at the end of July 2019. Extremely hot days have become more frequent and hotter, especially in Central Europe. © European Space Agency
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A clear signal: in Europe, the number of summer hot days since 1950 has tripled. At the same time, these heat extremes have become more than two degrees hotter, as a study shows. This trend is particularly clear in Central Europe: Here, the heat days have even become hotter by five degrees, which is more than most climate models predict, as the researchers report.

Not only are the average temperatures rising worldwide due to climate change, but also the heat extremes are intensifying. Thus, in the summer of 2018, but also this year in Germany, numerous new temperature records were achieved. According to forecasts, in the future such "century summers" could become the rule in many regions. In extreme cases, even three quarters of the world's population would be regularly exposed to potentially life-threatening heat waves.

Almost 70 years and 4, 000 weather stations

But what about the heat extremes in Europe? It is clear that a higher average temperature leads to hotter summer days. The big question, though, is whether days with outlier temperatures may have increased more than the natural range of temperatures. This is exactly what Ruth Lorenz and her colleagues at ETH Zurich have investigated.

For their study, the researchers evaluated data from around 4, 000 weather stations in Europe from the period from 1950 to the end of 2018. On this basis, they analyzed how often the hottest one percent days occurred and the coldest days. In addition, they determined the maximum temperatures of these heat or cold days and their change in relation to the summer mean temperatures.

Three times as many extreme hot days

The result: Since 1950, the number of extreme heat days in Europe has tripled. There were only about two such extreme days a year on average, but today there are already six. The strongest increase in summer heat extremes has occurred in the period since 1996, the researchers report. They found this trend not only in the daytime temperatures, but even more pronounced for exceptionally hot nights. display

By contrast, extreme colds have fallen sharply. "They've lost about two to three times, " Lorenz and her team report. While there were about five extreme cold days in the winter of 1950, today there are only two on average. "In the last three decades, there has not been a single winter with an abnormal number of cold nights across Europe, " said the researchers.

Changes in daily maximum temperatures (TXx) and minimum temperatures (TNm) in Central Europe (CEU), Northern Europe (NEW) and the Mediterranean (MED) Lorenz et al., Geophysical Research Letters / AGU

Five degrees more in Central Europe

But the hot days have not just become more - they are getting hotter too: on average, the maximum temperature of such extreme days in Europe has increased by 0.33 degrees per decade. Since 1950, this adds up to a total of 2.3 degrees more, as the scientists report. In many regions of Europe, the peak temperatures of the hottest days have risen far more sharply than the summer mean temperatures. "These trends are thus much stronger than we would expect on the basis of natural climate variability", says Lorenz. "This is really a signal of climate change."

The hardest hit is Central Europe: Here, the extreme heat days have in places even heated by 0.8 degrees per decade. "That's more than five degrees over the entire time period, " the researchers said. Similarly, longer heat periods of three to seven days duration have intensified. Overall, the heat extremes in Central Europe have thus been about 50 percent stronger than the summer average. In northern Europe and the Mediterranean, on the other hand, little has changed.

Underestimated by climate models

This effect of climate change on heat extremes surpasses the predictions of many regional climate models, the researchers noted in their study. "The observed trend surpasses the simulated in nearly 75 percent of the models, " they report. "The majority of models tend to underestimate the intensification of extreme heat and overstate the warming of extreme colds."

The scientists are therefore clear that climate change in Europe is also manifested in an increase in the heat extremes. Therefore, summer heat extremes are not just randomly hotter than they used to be - it's a result of global warming. (Geophysical Research Letters, 2019; doi: 10.1029 / 2019GL082062)

Source: American Geophysical Union

- Nadja Podbregar