Extreme summer heat on three continents has the same causes | NOW

Although the Dutch heat record was not broken last Tuesday, records were set in France and the United Kingdom. In England it became more than 40 degrees for the first time. Meanwhile, the heat and drought in southern Europe led to major forest fires.

This week, large parts of the United States also had to deal with maximum temperatures of around 40 degrees. Heat warnings were in place for more than 100 million residents from coast to coast.

In Shanghai, the absolute record temperature of 40.9 degrees was equaled two weeks ago. Much of China has been hit by another heat wave this week. In the northwestern province of Xinjiang, there are even warnings of flooding from faster melting glaciers.

Climate change is everywhere

Those heat waves are thousands of miles apart, yet they are all related to the same two phenomena.

The first is, of course, climate change. Because the average temperature on the entire globe is increasing, the average summer temperature is rising. Heat waves are also getting hotter. “That link has been statistically demonstrated everywhere,” says climate researcher Karin van der Wiel of the KNMI. “We can say very clearly that all heat waves today have an imprint of climate change.”

Increasing drought also contributes to the heat. If the soil is very dry, as is currently the case in southern Europe, less solar energy is converted into evaporation of water. Instead, the air heats up extra, resulting in even higher temperatures.

Records ‘crushed’ more often

If a heat wave occurs, climate change also increases the chance that temperature records will be broken. In addition, the likelihood of records being ‘crushed’ increases. Meteorologists speak of this when the old record is exceeded by more than 1 degree. In the Netherlands, that happened in 2019, when the national record temperature of 40.7 degrees was recorded in Gilze-Rijen.

According to calculations that the KNMI performed with climate models, there is now about a 2 percent chance that the temperature record will be smashed in a year. At the beginning of the last century, that chance was more than twice as small. The models show that the chance will continue to increase for the time being to around 3 percent in 2060. Then, on average, temperature records will be crushed once every thirty years.

Jet stream is going to twist

But ‘general’ global warming isn’t the only reason heat waves are more common. Changes in the jet stream can also contribute to the development of heat waves.

The jet stream is an air current at about 10 kilometers altitude, which moves from west to east over the Northern Hemisphere. The jet stream is sometimes ‘tight’ and then blows it back from above the Atlantic Ocean to the Netherlands.

But if the jet stream is weaker, it can start to ‘squirm’. He can then ensure, for example, that warm air from North Africa, Spain and Portugal is brought to the north. That caused heat in Western Europe last week. The twisting jet stream causes high and low pressure areas to linger for a long time, allowing hot air to stay in one place for many days.

Europe gets more extra heat waves

The jet stream currently has seven twists and turns across the northern hemisphere, says Professor of Climate Extremes Dim Coumou (VU Amsterdam). This shape often causes heat in Europe and North America in the summer, and currently also leaves a warm area hanging over China.

Although the jet stream has an influence in various places, the effect is greatest here. “In Europe, we see that extreme temperatures are increasing faster,” says Coumou.

That also has to do with a ‘double jet stream’ that hangs more and more over the continent in the summer, Coumou and fellow scientists recently wrote in the scientific journal. Nature Communications. There is then a relatively strong westerly wind above the north of Scandinavia and above the Mediterranean Sea.

“We see that the double jet stream has increased over the last 40 years,” says Coumou. But it’s not yet clear whether the changes in the jet stream are caused by climate change, or by something else. “We want to look at that in follow-up studies.”

We have had resonance conditions for a planetary wave with wave number 7 during the past week, with large wave amplitudes. Think meanders in the jet stream. This Leads to simultaneous heat waves in different areas of the Northern hemisphere. Graph shows tropospheric winds.

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rahmstorf

Moment of places

17:33 – 20 July 2022