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Environment

Climate and Hurricanes: An Idea With a History

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Without a doubt, 2005 was the year that ignited a fierce and lasting debate over the extent to which global warming might be increasing the strength of hurricanes. That's largely thanks to two back-to-back scientific papers, published in the leading journals Nature and in Science, which provided data suggesting that storms have grown considerably stronger over the past several decades:

1. Emanuel, "Increasing destructiveness of tropical cyclones over the past 30 years," Nature, Vol 436, August 4, 2005. (PDF) 2. Webster et al, "Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment," Science, September 16, 2005, Vol 309. (link)

However, it's not as if this idea came out of nowhere. Far from it. In the course of my early work for the book, I've been tracing the intellectual and scientific history of the notion that hurricanes might be connected to climate. What I've found thus far is, I think, pretty interesting. Flashback to 1903, when Charles Darwin's famed rival, Alfred Russell Wallace, published a book entitled Man's Place in the Universe. That book contains an intriguing paragraph that, although certainly not correct in all of its scientific particulars, suggested a relationship between hurricane frequency and intensity and sun-induced climatic changes:

It is outside the zone of the equable trade-winds, and in a region a few degrees on each side of the tropics, that destructive hurricanes and typhoons prevail. These are really enormous whirlwinds due to the intensely heated atmosphere over the arid regions already mentioned, causing an inrush of cool air from various directions, thus setting up a rotatory motion which increases in rapidity till equilibrium is restored. The hurricanes of the West Indies and Mauritius, and the typhoons of the Eastern seas, are thus caused. Some of these storms are so violent that no human structures can resist them, while the largest and most vigorous trees are torn to pieces or overturned by them. But if our atmosphere were much denser than it is, its increased weight would give it still greater destructive force; and if to this were added a somewhat greater amount of sun-heat--which might be due either to our greater proximity to the sun or to the sun's greater size or greater heat-intensity, these tempests might be so increased in violence and frequency as to render considerable portions of the earth uninhabitable.

Scientists I've consulted tell me that Wallace was wrong about much of this--particularly the notion that what makes storms stronger is atmospheric "density" or "weight." Instead, what counts is heat and moisture. Still, it's intriguing that he considered the notion that increasing the sun's heat might strengthen storms. And note in particular Wallace's specific comment that adding "a somewhat greater amount of sun-heat" to the system could intensify hurricanes--this might conceivably occur via the greenhouse effect's trapping of heat, though Wallace of course doesn't mention this possibility. To be sure, Wallace also thought storms would grow in number due to increased "sun heat." But even now, there's no particular evidence of this happening, or clear theoretical reasons for thinking it would. The debate today over global warming's affect on hurricanes centrally turns on the question of storm intensity, not numbers. Now fast forward to 1987. In that year, MIT's Kerry Emanuel (author of one of the two papers cited above) published a study in Nature entitled "The Dependence of Hurricane Intensity on Climate" (PDF). It starts out like this:

Tropical cyclones rank with earthquakes as the major geophysical causes of loss of life and property. It is therefore of practical as well as scientific interest to estimate the changes in tropical cyclone frequency and intensity that might result from short-term man-induced alterations of the climate. In this spirit we use a simple Carnot cycle model to estimate the maximum intensity of tropical cyclones under somewhat warmer conditions expected to result from increased atmospheric CO2 content. Estimates based on August mean conditions over the tropical oceans predicted by a general circulation model with twice the present CO2 yield a 40-50 % increase in the destructive potential of hurricanes.

Once again, I'm sure that science has progressed considerably over the nearly two decades since Emanuel published this paper. (Science generally does.) But at least in a theoretical sense, a publication like this makes it clear that the notion of hurricanes growing stronger because of a warmer climate has quite a long history. This raises the obvious question: Why are people paying a lot more attention to it now than they ever did before? That the debate has heated up recently can be attributed to at least three major factors (although there are certainly many others as well). First of all, When Kerry Emanuel published his paper in 1987, the Atlantic wasn't in an active hurricane phase. Four storms hadn't hit Florida in the space of a single year (as happened in 2004). New Orleans hadn't been wiped out by Katrina (as happened in 2005). In short, the issue's salience was much lower. The global warming issue is also much more high profile now than it was in 1987. History buffs will recall that it was the year after that, 1988, that really put this issue on the map for the first time. Last but certainly not least, Emanuel's older paper discusses the issue in purely theoretical, thermodynamic terms. He's talking about what should happen given what we know of basic physics. Something similar might be said of a number of modeling studies that have been done, which also suggest that hurricanes should be enhanced in a warmer world. (See for example here.) By contrast, the 2005 papers cited above present data about what (they claim) is happening. That's much more immediate. Theory is great, but nothing hits you in the gut like hard data (although both must line up nicely together if we are to achieve true scientific understanding). Stoat elaborates on this last point when it comes to the hurricane-climate issue: "Most people will accept that there *can* be an effect; its the detecting of it that's tricky." And that, in short, is why so much scientific energy is being devoted to the relationship between hurricanes and global warming right now--even if such a relationship has been a subject of speculation, scientific theorizing, and modeling studies for some time. I'd love to hear any comments on this post, particularly relating to other early or historically relevant mentions of this concept that I may not be aware of. Also, if anyone knows much about Alfred Russell Wallace's life, I'm interested in learning what may have led him into the fascinating speculation cited above. I imagine it had something to do with his quite considerable world travels. I've just ordered Wallace's biography, by Michael Shermer, from Amazon, which may or may not cast some light on the matter.

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