Human activities trigger extreme weather

Man-made climate changes could be responsible for severe regional weather extremes in recent years, German researchers say. Significant weather events such as the heat wave in the United States in 2011 or the one in Russia 2010 coinciding with the unprecedented Pakistan flood could have a common physical cause, scientists at the Potsdam Institute for Climate Impact Research said. Writing in the journal Proceedings of the National Academy of Sciences, they suggest human-caused climate change repeatedly disturbs the patterns of atmospheric flow around the globe's Northern Hemisphere through a subtle resonance mechanism. "An important part of the global air motion in the mid-latitudes of the Earth normally takes the form of waves wandering around the planet, oscillating between the tropical and the Arctic regions," lead author Vladimir Petoukhov said. "So when they swing up, these waves suck warm air from the tropics to Europe, Russia, or the United States, and when they swing down, they do the same thing with cold air from the Arctic," he said. Climate change caused by greenhouse-gas emissions can upset normal temperature differences that are a main driver of these airflows, he said. "They result in an unnatural pattern of the mid-latitude air flow, so that for extended periods the slow synoptic waves get trapped." "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."