Slate schrieb vor ein paar Tagen völlig zurecht, dass sich die derzeitige Hitzewelle multiple Faktoren zur Ursache hat, was viele Klimawandel-Skeptiker zum Anlass nahmen, noch weiter herumzuzweifeln. Eine der Ursachen für die Hitzewelle ist ein langsam gewordener Jetstream in der Atmosphäre der nördlichen Welthalbkugel und dieses Phänomen lässt sich mittlerweile sehr wohl auf die erwärmende Arktis und damit auf den Klimawandel zurückführen, was Hitzewellen wie diese sehr viel wahrscheinlicher macht. Auch die europäische Hitzewelle aus dem Jahr 2003 lässt sich wohl auf dieses Phänomen zurückführen.
One reason is that the jet stream—a fast-flowing river of air snaking continually round the northern hemisphere at altitudes of around 6 kilometres—has stalled over Europe since May, and could continue to do so, trapping regions of high pressure that are cloudless, windless and extremely hot. “It’s been a key player in the astounding heatwaves across the UK and Scandinavia this summer,” says Jennifer Francis of Rutgers University in New Jersey.
She says evidence is mounting that accelerated warming of the Arctic is a major reason why the jet stream keeps getting stalled. The stream is driven by collisions between cold air descending southward from the Arctic and warm air pushing northward from the equator.
The greater the temperature difference between the colliding air streams, the more powerful the jet stream. But the temperature gap—and therefore the power of the jet stream—is being weakened because the Arctic is warming twice as fast as the rest of the planet, supplying the stream with increasingly warmer air. “Heatwaves over northern hemisphere continents in recent years fit the hypothesis that rapid Arctic warming is playing a role,” says Francis.
He says this summer’s extreme weather in the Northern Hemisphere is related to a near-stationary perturbation in the jet stream. Such patterns have been implicated in “many of the most extreme, persistent summer weather events in recent years, including the 2003 European heat wave, 2010 Moscow wildfires, 2011 Texas and Oklahoma drought [and the] 2016 Alberta wildfires.”
In a study published last year, Mann and his colleagues showed that such patterns are becoming more common as a result of human-caused climate change (Scientific Reports, doi.org/f9vwxh). Mann says the “amplified warming in the Arctic” seems to be a major contributor.