On the left is the global circulation pattern on a normal day. The picture on the right shows the pattern in extreme weather. In normal events moderate wind speeds prevail in the north and south direction. In the extreme situations, however, it comes to unusual currents. (Picture, Michael Mann, Penn State)
Read aloud Good weather is a matter of change: cool sunshine must be followed by cooler rainy weather, otherwise there will be droughts or, in the reverse case, floods. According to statistics, such extreme weather events are becoming increasingly common in recent decades. A study now clarifies the cause of this critical trend: global warming seems to create conditions that falter the weather.

"The weather has always been crazy" - stubbornly hold such views downplay the effect of climate change. From statistics, however, it is clear that the weather is increasingly "crazy" these days. "The California drought in 2016, the flood in Pakistan in 2010, and the 2003 heatwave in Europe are all a very disturbing series of extremes, " says Michael Mann of Pennsylvania State University, State College.

Fingerprint of human activity

This trend is not only generally striking, but also within the framework of climate change models. "These events occur more often than would be expected from the direct effects of global warming. So there has to be an additional effect of climate change here, "says Mann. As he and his colleagues have now shown, the climatic developments favor extreme meanders in the large air currents - the jet streams. This leads to the heaped weather extremes. "So we finally see a clear fingerprint of human activity, " says Mann.

The results of the international research team are based on the analysis of satellite data, climate models and historical weather records. "High-quality satellite data has only been available for a relatively short time - too short to draw reliable conclusions. Reliable temperature measurements, on the other hand, have been around since 1870. Therefore, we took these measurements to reconstruct the changes over time, "says co-author Kai Kornhuber of the Potsdam Institute for Climate Impact Research (PIK). "We looked at dozens of different climate models, computer simulations, and observational data to study effects on planetary waves, " says Kornhuber.

Climate change is stalling currents

These planetary waves are gigantic streams of air that orbit the earth and wave up and down between the tropics and the Arctic. They transport heat and moisture. These currents are driven mainly by the temperature differences between the equator and the poles. As the researchers explain, these temperature differences are now decreasing as the Arctic heats up faster than other regions. In addition, the land masses warm up faster than the oceans, especially in the summer. Both changes affect the global winds, the scientists explain. display

The weather extremes occur when the weather changes to a halt. "This occurs under special conditions that favor so-called quasi-resonant amplification. This makes the north-south windings of the jet stream very large, and it blocks the forward movement of the waves from west to east, explains co-author Stefan Rahmstorf from PIK. It turned out that the temperature distribution, which favors the stalling of the planetary waves, has increased in almost 70 percent of the simulations. The scientists also found in their data a particular distribution of temperatures just during the periods when the eastern forward motion of the planetary waves stopped.

As the researchers report, most of these changes occurred only in the last four decades. "The fact that the jet stream often winds heavily over a long period of time is a relatively new phenomenon - which makes it even more significant, " says co-author Dim Coumou of the Free University of Amsterdam. "We now have to investigate this further - in addition to the good evidence, we continue to have unanswered questions." Once again, however, is his conclusion: "We can limit the risks of increasing weather extremes, if we limit our emissions of greenhouse gases, " said the climatologist,

Original work of the researchers:

  • Scientific Reports, DOI: 10.1038 / srep45242
© science.de - Martin Vieweg
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