“Giant airstreams encircle our globe in the upper troposphere – we call them planetary waves,” explains Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research (PIK) and co-author of the second paper. “Now evidence is mounting that humanity is messing with these enormous winds. Fueled by human-made greenhouse-gas emissions, global warming is probably distorting the natural patterns.” Usually the waves, conveying chains of high- and low-pressure domains, travel eastwards between the equator and the North Pole. “Yet when they get trapped due to a subtle resonance mechanism,” says Schellnhuber, “they slow down so the weather in a given region gets stuck. Rains can grow into floods, sunny days into heat waves, and tinder-dry conditions into wildfires.”
Investigating the Arctic Factor and connecting the dots
“While it might not sound so bad to have more prolonged sunny episodes in summer, this is in fact a major climate risk,” says Dim Coumou from the Potsdam Institute for Climate Impact Research (PIK) and Vrije Universiteit Amsterdam, lead-author of the review paper and co-author of the wildfire case study. “We have rising temperatures due to human-caused global warming which intensifies heat waves and heavy rainfall, and on top of that we could get dynamical changes that make weather extremes even stronger – this is quite worrying.” This summer is an impressive example of how stalling weather can impact societies: persistent hot and dry conditions in Western Europe, Russia and parts of the US threaten cereal yields in these breadbaskets.