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Climate scientists link global warming to extreme weather

Blame giant, increasingly lazy atmospheric waves

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There has been a healthy debate in the scientific community about the linkage, if any, between the recent spate of extreme weather events and the ongoing increase in global temperatures. Now a group of researchers reports they've uncovered an atmospheric mechanism that lends credence to the argument that, yes, global warming is to blame for at least some of our increasingly mucked-up weather.

Global weather is normally influenced by waves of air that oscillate between the Earth's tropical and Arctic regions, alternately pulling warm air up from the tropics to northern climes, then bringing cold air down from the Arctic. As a result of uneven global warming, however, those waves are now getting stuck.

"What we found is that during several recent extreme weather events these planetary waves almost freeze in their tracks for weeks. So instead of bringing in cool air after having brought warm air in before, the heat just stays," explained Vladimir Petoukhov of the Potsdam Institute for Climate Impact Research (PIK), lead author of a paper to be published in the Proceedings of the National Academy of Sciences.

The mechanism that Petoukhov and his coauthors studied is, at its core, rather straightforward. As global temperatures have increased, the Arctic is warming to a greater extent than the rest of the northern hemisphere, and the reduced temperature differential between Arctic air and more southerly air is decreased, which reduces air flow between the regions.

In addition, global land mass is concentrated in that hemisphere, and land masses warm and cool more quickly than do oceans. "These two factors are crucial for the mechanism we detected," Petoukhov said. "They result in an unnatural pattern of the mid-latitude air flow, so that for extended periods the slow synoptic waves get trapped."

And when they get trapped, so does the air and heat that they would otherwise be redistributing. Heat thus trapped contributes to such weather events as the 2012 US drought – 2012, by the way, was warmest year ever recorded in the US.

The researchers tested their hypothesis by creating a set of assumptions about atmospheric wave behavior and testing them against 32 years of daily weather data from the US National Centers for Environmental Prediction (NCEP). Sure enough, during recent periods of extreme weather occurrences, their model showed stalled wave formations, as predicted.

"Our dynamical analysis helps to explain the increasing number of novel weather extremes," said coauthor and PIK director Hans Joachim Schellnhuber. "It complements previous research that already linked such phenomena to climate change, but did not yet identify a mechanism behind it."

Schellnhuber is not, however, ready to call case-closed on the connection between global warming and extreme weather. "This is quite a breakthrough," he said, "even though things are not at all simple – the suggested physical process increases the probability of weather extremes, but additional factors certainly play a role as well, including natural variability."

The PIK study may not be anywhere near a final "smoking gun" proof of how the long-term phenomenon of rising global temperatures affects the short-term phenomena of extreme weather events, but with the World Meteorological Organization (WMO) recently announcing that greenhouse gases increased to record levels in 2011, and with the recent cluster of extreme weather events being called "highly unlikely" to be mere statistical outliers, this recent study by the PIK researchers deserves – at minimum – dispassionate consideration. ®

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