Mounting evidence shows that the increase of devastating weather extremes in summer is linked to human-made climate change, said the latest findings on climate change relayed to Database by the prestigious Potsdam Institute for Climate Impact Research in Germany, known for short as PIK.
Giant airstreams, also known as planetary waves, are circling the Earth, waving up and down between the Arctic and the tropics, transporting heat and moisture and causing droughts or floods in many areas of the globe, including the Philippines, PIK scientists said.
“The unprecedented 2016 California drought, the 2011 US heat wave and 2010 Pakistan flood, as well as the 2003 European hot spell, all belong to a most worrying series of extremes,” says Michael Mann from the Pennsylvania State University in the US, lead author of the PIK study soon to be published in Scientific Reports.
Mann said the increased incidence of these events exceeds what we would expect from the direct effects of global warming alone, so there must be an additional climate-change effect. In data from computer simulations, as well as observations, we identify changes that favor unusually persistent, extreme meanders of the jet stream that support such extreme weather events.
“Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity,” he said.
Coauthor Stefan Rahmstorf from the PIK in Germany explained that if the same weather persists for weeks on end in one region, then sunny days can turn into a serious heat wave and drought, or lasting rains can lead to flooding. “This occurs under specific conditions that favor what we call a quasi-resonant amplification that makes the north-south undulations of the jet stream grow very large. It also makes theses waves grind to a halt, rather than moving from west to east. Identifying the human fingerprint on this process is advanced forensics.”
Air movements are largely driven by temperature differences between the equator and the poles. Since the Arctic is more rapidly warming than other regions, this temperature difference is decreasing. Also, landmasses are warming more rapidly than the oceans, especially in summer. Both changes have an impact on those global air movements, PIK scientists said.
They said this includes the giant airstreams that are called planetary waves because they circle Earth’s Northern Hemisphere in huge turns between the tropics and the Arctic. The scientists detected a specific surface temperature distribution apparent during the episodes when the planetary waves eastward movement has been stalling, as seen in satellite data.
Using temperature measurements since 1870 to confirm findings in satellite data
“Good satellite data exists only for a relatively short time—too short to robustly conclude how the stalling events have been changing over time. In contrast, high-quality
temperature measurements are available since the 1870s, so we use this to reconstruct the changes over time,” says another coauthor Kai Kornhuber, also from PIK.
“We looked into dozens of different climate models—computer simulations called CMIP5 of this past period—as well as into observation data, and it turns out that the temperature distribution favoring planetary wave airstream stalling increased in almost 70 percent of the simulations,” he said.
Interestingly, most of the effects occurred in the past four decades. “The more frequent persistent and meandering jetstream states seems to be a relatively recent phenomenon, which makes it even more relevant,” says scientist Dim Coumou from the Department of Water and Climate Risk at VU University in Amsterdam (Netherlands).
“We certainly need to further investigate this—there is some good evidence, but also many open questions. In any case, such nonlinear responses of the Earth system to human-made warming should be avoided. We can limit the risks associated with increases in weather extremes if we limit greenhouse-gas emissions,” he said.
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