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Environment

AMS Dispatch One: Emanuel Talk

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Yesterday evening here in San Antonio, Kerry Emanuel gave his first of two presentations. Having seen Emanuel talk perhaps half a dozen times over the past year or more, I was particularly interested in seeing how this talk compared to others.

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Emanuel began by explaining that we still remain essentially ignorant about how to explain a central fact: On average, about 80 to 90 tropical cyclones--like 2005's Wilma, pictured at right--form globally each year. Emanuel, and a number of other scientists, wonder whether we'll really understand the relationship between hurricanes and climate change unless we first understand what controls this number. Emanuel plans to further discuss the role of hurricanes in the climate system in his Thursday talk, which I probably won't be around for. But most of the talk yesterday was devoted to laying out, instead, where he stands on how global warming is currently influencing hurricanes. The result was a fascinating glimpse at science happening "in the wild," so to speak. A chief reason I like to travel to attend scientific conferences, rather than simply reading published papers, is that at conferences you see scientists interact and watch how their views develop in response to challenges and to new data. Papers are merely the end product of all that. Conferences, by contrast, tell you which way the wind is blowing (pun quasi-intentional). As many readers of this blog will already know, in 2005 Emanuel published a now-famous paper (PDF) showing an apparent dramatic increase in the power dissipation of hurricanes in the Atlantic and the Northwest Pacific (where they're called typhoons) over the past several decades. In other words, the storms were getting stronger and lasting longer, and thus potentially becoming more destructive. This paper (and another showing similar findings) touched off a huge and ongoing debate, most recently summarized in a we-just-don't-know-what's-up-yet consensus statement (PDF) by a group of experts assembled under the auspices of the World Meteorological Organization. Emanuel was a party to that consensus statement, but of course he has his own views. And it's clear that although at present he hasn't won over all of his fellow scientists, he still thinks global warming is playing a significant role in increasing the power dissipation of hurricanes, especially in the Atlantic. (Emanuel showed yesterday that the data are much more contested for the Northwest Pacific.) Emanuel spent a considerable amount of time in his talk trying to deconvolve the factors that he believes are underlying an Atlantic trend. For lay audiences, let me simplify: His view is that rising sea surface temperatures, linked to global warming, must be invoked to explain what we're seeing. But they're not the sole explanation. Other factors, including a cooling lower stratosphere and an increase in the air's low-level vorticity (don't ask), are also part of the picture in Emanuel's view. He rejects, however, the notion that a natural cycle, the so-called Atlantic Multi-Decadal Oscillation, can explain tropical Atlantic temperature trends or (by implication) recent hurricane activity. Other scientists contest these stances by Emanuel, and the issues raised here aren't firmly resolved yet. But for the lay reader, there's a key takeaway point from all this. At least one leading hurricane specialist (in fact, it's more than one) thinks global warming is contributing to a marked uptick in Atlantic hurricane activity. So if you hear a journalist, a politician, or a government agency categorically reject the notion that there's any relationship between the two, e.g. here, that perspective doesn't reflect current knowledge. A far more accurate way of saying things would be to state that knowledge is in a state of flux right now. As I understand it, part of the philosophy of Unitarian Universalism is to "live in the questions"; you might say that applies to the current hurricane-global warming debate as well. I expect to see a lot more questions raised later today at the next major event to address this subject: a panel discussion in which Greg Holland and Kevin Trenberth of the National Center for Atmospheric Research will presumably present views similar to those of Emanuel, even as Chris Landsea of the National Hurricane Center will presumably contest them. I'll be there on the scene and report back on how it goes....

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