GLOBAL WARNING There’s growing evidence that global warming is driving crazy winters It may be the timeliest -- and most troubling -- idea in climate science. Back in 2012, two researchers with a particular interest in the Arctic, Rutgers Jennifer Francis and the University of Wisconsin-Madisons Stephen Vavrus, published a paper called Evidence linking Arctic amplification to extreme weather in mid-latitudes. In it, they suggested that the fact that the Arctic is warming so rapidly is leading to an unexpected but profound effect on the weather where the vast majority of us live -- a change that, if their theory is correct, may have something to do with the extreme winter weather the U.S. has seen lately. In their paper, Francis and Vavrus suggested that a rapidly warming Arctic should interfere with the jet stream, the river of air high above us that flows eastward around the northern hemisphere and brings with it our weather. Sometimes, the jet stream flows relatively directly from west to east; but other times, it takes long, wavy loops, as in the image above. And according to Francis and Vavrus, Arctic warming should make the jet stream more wavy and loopy on average – some have called it “drunk” -- with dramatic weather consequences. Heres the atmospheric physics behind the idea: Warm air expands, and naturally there is much more warm air at the equator than at the poles. Thus, the atmosphere is thicker at the equator, and the jet streams motion is driven by the decline in atmospheric thickness as one moves in a poleward direction -- in effect, its atmospheric river flows downhill, in Francis’s words. However, if the Arctic is warming faster than the mid-latitudes, then the difference in thickness as you move in a poleward direction should decrease. And this should slow the jet stream, leading to more loops and turns -- and consequently, weather of all types getting stuck in place for longer. According to Francis, the extreme U.S. winter of last year and now, the extremes at the beginning of this season, fit her theory. This winter looks a whole lot like last winter, it’s a very amplified jet stream pattern, she says. We know that when we get these patterns, it tends to be very persistent. And it is definitely the type of pattern that we expect to see more often as the Artic continues to warm so fast. To be sure, Francis acknowledges that our recent bout of extreme cold was kickstarted most directly by Typhoon Nuri, which swerved up into the mid-latitudes and exploded into an atmospheric bomb over the Bering Sea. That had the downstream effect of basically taking the jet stream and giving it a whip, whipping a wave into it, says Francis. But she also suspects that the jet stream is more susceptible to these kinds of dramatic influences because it is weaker now. In general, her theory does not say global warming caused any particular weather event, only that it is shifting the overall pattern of jet stream behavior, making certain kinds of persistent weather extremes more likely to occur. Francis isnt the only one to suggest this. The widely read weather blogger Jeff Masters mused yesterday on whether the extreme snowfall in western New York this week might be due to jet stream weirdness. Weve seen an unusual number of extreme jet stream patterns like this in the past fifteen years, which happens to coincide with the period of time weve been observing record loss of summertime Arctic sea ice and record retreat of springtime snow cover in the Arctic, noted Masters -- although he refrained from fully embracing the theory, noting that it still has its detractors. Capital Weather Gangs Jason Samenow also just discussed the evidence behind Franciss idea, which he calls controversial. Francis argues, however, that the evidence in her favor is mounting -- she cites no fewer than five (1, 2, 3, 4, 5) scientific papers published in the last year or so that she considers supportive, and hints that more are coming. We’ve got 5 papers that all look at that particular mechanism in different ways -- different analysis, different data sets, observation and models -- and they all come to the same conclusion and they all identify this mechanism independently, she says. You cant call Franciss idea fully established. You cant say theres a scientific consensus on it. And you cant say that the august U.N. Intergovernmental Panel on Climate Change embraces it. Not yet. But its certainly a very serious idea and one of the most discussed theories in climate science. Call it a contender. And if its right, well...then we all know, already, what global warming feels like. washingtonpost/blogs/wonkblog/wp/2014/11/20/theres-growing-evidence-that-global-warming-is-driving-crazy-winters/ GLOBAL-WARNINGS
Posted on: Sat, 22 Nov 2014 07:23:36 +0000
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