The Intersection

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.


  1. #1 Harris Contos
    February 28, 2006

    My scientific knowledge being on the rather basic side, for reference I go back to grade school, when the science teacher would tell us that ultimately all our energy comes from the sun. (Yeah, I got a star on my forehead for getting one right.) It then seems to me that what our atmosphere/climate does is to modulate that energy coming from the sun in the form of solar radiation, and Wallace’s work seems to make intuitive sense when looked at from that perspective. Wallace posited “density” and “weight”; more recent climatologists are more refined, but what they may be driving at is that the earth’s atmosphere, vast as it is, is quite delicately balanced insofar as its capacity to modulate solar energy is concerned. Whenever that balance is upset, say through the emission and build up of heat-trapping gases, something’s gotta give in the physics and chemistry of the atmosphere’s mechanism to modulate solar energy, which I guess might be a very broad definition of climatology, the precise pathways and logical connections of which the scientists are working on now. Again relying more on intuition than on much scientific knowledge, changes in the earth’s atmosphere’s ability to modulate solar energy would seem to lead to more frequent and more intense hurricanes, but I wouldn’t restrict the overall phenomenon to just hurricanes. Changes in wind patterns, rainfall distribution, snow fall amounts, ocean currents, etc. would all fall into the same rationale, although what the level of scientific modeling and understanding is on these points I don’t know– probably behind that of hurricane dynamics, however. Basically, a lot more energy is being introduced into the earth/atmosphere system, and it’s gotta manifest itself one way or another, that’s my 2 cents.

  2. #2 Oliver
    March 1, 2006

    As it happens, I was writing something about Wallace this morning, and bemoaning my relative ignorance on him, since he is a wonderful mixture of clear scientific thinking (prediction that Martian ice caps must be dry ice) and a religio/spritualist mindset almost irretrievably alien to me. (But then I lack the powers of empathic projection necessary for a good historian, as well as many other desiderata). In general, Wallace is very keen on the idea that humans are the only corporeal intelligences of the cosmos and that this planet is tailored to their needs (hence the proto-anthropic climate stuff) while other planets are not (hence the campaign against Lowell’s Martians). One thing that came out of my work was a determination to reacquaint myself with a guy who wrote a dissertation on him some years ago. If I succeed, I’ll let you know

  3. #3 Chris Mooney
    March 1, 2006

    Great. Sounds fascinating.

  4. #4 Jon Winsor
    March 1, 2006

    Theory is great, but nothing hits you in the gut like hard data…

    And don’t forget simple appeals to common sense. Probably with a controversial area like climate science, there are going to be lots of readers who will be reading with the old adage “figures lie and liars figure” in mind. So if you can ground the subject in common sense and intuition, skeptical readers are more likely to follow you into the more abstract data and theory. The image of a Victorian scientist traveling to the tropics and having this thought over a hundred years ago reinforces the notion that there are strong elements of common sense and intuition lying behind the jargon-laden scientific arguments. (As there are. Everyone who’s boiled a pot of water can see that there’s a relationship between thermal and kinetic energy.)

  5. #5 nolagal
    March 2, 2006

    I agree completely with your analysis of why this has suddenly become a heated and highly visible topic of research and discussion. In the last few years, we’ve seen both a return of frequent, large, powerful hurricanes in the Atlantic, not seen since the 1950’s or earlier. Concurrently, we’ve finally reached a point where the data and models are available to document the correlations. The amazing timing of the release of Emanuel’s paper in August 2005, just weeks before Katrina, ensured the paper would get a wide reception.

    But Emanuel’s 05 paper doesn’t just provide hard data to support his earlier theoretical models. The data absolutely blow his earlier models out of the water! In his earlier papers, Emanuel argued for a link between warming (esp. of sea surface temp) and hurricanes that would lead to a slight (less than 10%) increase in hurricane intensity, and in fact believed that there was an intensity ceiling not much higher than that. But his analysis demonstrated that PDI has more than doubled – and that with only a 0.5 degree C change in SST. If that’s what happens with a half a degree change, what about the predicted 2-5 degree change?

    Even with the modifications recommended by Landsea in his December reply, the correlations and the magnitude of the impact of SST on PDI still stand. I think this has really convinced people – at least within the scientific community – that this is an immediate and serious area of concern.

    As for Wallace – I really wish I knew more about him as well, he’s a fascinating and incredibly intelligent figure. I do know that he spent quite a bit of time travelling, especially in the monsoon areas of Indonesia and SE Asia (e.g., his observations leading to the “Wallace Line”).

    Keep up the great work!

  6. #6 Chris Mooney
    March 4, 2006

    Thanks to all of you for your very helpful comments. Reading more about AR Wallace shows that he didn’t really think storms would actually get stronger. He was just throwing this out there to show that our world is *just right*…

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