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"Rapid Arctic warming since 2000 may be reshaping and rerouting the narrow current of high winds across the Northern Hemisphere known as the jet stream, forcing it to act like a giant stop light at 30,000 feet. \nThis leads to stalled and prolonged weather systems that can bring deadly extreme heat and rainfall events, a new study has found. \nThe study, published Monday in the journal Proceedings of the National Academy of Sciences, is the latest in a wave of research focused on the potential ties between melting sea ice and skyrocketing air temperatures throughout the Arctic, and extreme weather events in the northern midlatitudes, including the U.S., Europe and Asia. \nThe study adds to growing evidence that manmade global warming may already be reshaping today's weather patterns in ways that favor dangerous storms.\nSee also: Spectacular 'Mothership' Thunderstorm Wins National Geographic Photo Contest\nExtreme weather events have killed hundreds of thousands of people during the past decade, including the Russian heat wave of 2010, which killed an estimated 60,000 and decimated the Russian wheat crop that year, leading to a spike in global food prices.\nThe new study, by a group of researchers at the Potsdam Institute for Climate Impact Research in Germany, focuses on the contortions in the jet stream as it flows around the Northern midlatitudes, steering weather systems generally from west to east in the process. \nLarge north to south bends in the jet stream winds, known as Rossby waves, can lead to strong high and low pressure areas that are difficult to dislodge. The study found a statistically significant increase in the frequency of certain Rossby waves since the year 2000. \n\nVisualization of the polar jet stream, with the highest wind speeds in red.Image:  NASARossby waves are named after atmospheric scientist Carl-Gustav Rossby, who discovered them and explained them in the late 1930s. He made the cover of Time Magazine for his weather insights in 1956.\nThese particular Rossby waves, which fit seven times around the globe and have seven crests or \"ridges\" and seven troughs, now have a greater tendency to become more elongated and nearly stationary. This can lead to extreme weather events from stuck weather patterns, with some areas condemned to weeks of high heat and humidity while others are doused with heavy rains.\nThe study provides evidence of a direct physical connection between Arctic warming and mysterious jet stream behavior, while previous work had only found statistical correlations between such changes. The new study says previous research proposing that the reduced temperature contrast between the north pole and the equator was slowing east-to-west winds, and causing the jet stream to meander more and get stuck more frequently, is only part of the story. \nJennifer Francis, a researcher at Rutgers University who first proposed the hypothesis that Arctic warming is altering the jet stream, told Mashable the new paper \"is a big step forward in understanding how rapid Arctic warming affects the large-scale atmospheric circulation, and the types of extremes that will likely occur more often as global warming continues unabated.\"  (Francis served as an expert reviewer of the paper on behalf of the scientific journal.)\nOne of the ways the jet stream may be changing has to do with the altered temperature balance between the North Pole and the equator \u2014 since that temperature difference is what drives the jet stream in the first place. \nThe study found significant changes in the frequency of nearly stationary, steep jet stream ridges and troughs (known to scientists as \"resonance events\") during the July to August period since the year 2000. The post-2000 period has been marked by rapid Arctic warming at about twice the rate of the rest of the northern hemisphere. The extent of sea ice has plummeted, reaching a record low in 2012, but recovering slightly since then.\n\nTemperatures in the Arctic have increased at about twice the rate of the global average.Image:  NASA\"Since 2000 the Arctic has been warming much faster than the rest of the Northern Hemisphere,\" said lead author Dim Coumou, in an email conversation with Mashable: \n\nWe argue that this has led to changes in the equator-to-pole thermal gradient favorable for resonance conditions ... since 2000 there has been a cluster of resonance events, and such events are characterized by high-amplitude quasi-stationary waves and surface weather extremes.\nUnder normal conditions, there will only be few regions in the mid-latitudes which happen to experience unusual weather. But during periods of slowly propagating high-amplitude Rossby waves, some regions will experience unusually high temperatures while others experience unusually low temperatures. Thus, we see many (both high and low) extremes occurring simultaneously, i.e. a synchronization of extremes around the hemisphere.\n\nExamples of extreme weather and climate events that are linked to stuck weather patterns include the Russian heat wave and Pakistan flooding of 2010, heat waves in the U.S. in 2011 and 2012, and the European heat wave of 2003, when nearly 70,000 people are thought to have perished due to heat-related illnesses, Coumou said. \nGlobal warming is already leading to more heat and precipitation extremes simply by increasing average air temperatures and putting more water vapor into the air. \nWhat the new study shows is that the odds of such extremes may be increasing even further \u2014 due to changes in atmospheric circulation that are related to global warming. \nOther researchers who were not associated with the new study were cautious about its conclusions. Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research (AER) in Massachusetts, says the science is still just emerging when it comes to links between Arctic warming and weather patterns:\nThis is an area of study that is very new and potentially exciting. Our knowledge of how the Arctic may be forcing mid-latitude weather including extremes is much less mature than our knowledge of how for example the tropics forces mid-latitude weather. But it is deserving of our attention and I think could potentially provide significant improvements in both seasonal climate predictions and decadal-scale climate projections.\nAnd Randall Dole, a senior scientist at the National Oceanic and Atmospheric Administration (NOAA), says a multi-year effort is needed to sort through the evidence. Dole said the new study is \"simply one more hypothesis that requires more careful scrutiny.\""^^<http://www.w3.org/2001/XMLSchema#string>
<http://mashable.com/2014/08/11/warming-arctic-causing-deadly-extreme-weather-events/>