In order to get a tropical cyclone spinning, a lot of things have to go right (or wrong, depending upon your perspective). First, you need a location that's warm but also a certain distance north or south of the equator. In places too close to latitude zero, winds won't swirl inwards towards an area of low pressure to create a cyclonic rotation (a phenomenon known as the Coriolis effect). Second, you need a temperature gradient between the warm ocean surface and the cooler atmosphere above it, a situation that's favorable to what meteorologists call convection (the transfer of heat upward with rising air). And that's not all. You also need a region with low levels of what's known as vertical wind shear. When winds blow in different directions at different altitudes, they can tear apart a hurricane's structure. Furthermore, you need high sea surface temperatures--above 80 degrees Fahrenheit. That's particularly crucial, since the heat energy stored in the ocean fuels the hurricane, first entering the air through evaporation and later being released higher in the atmosphere when water vapor condenses into rain and clouds. This upward transfer of energy explains how a hurricane's heat pump--or heat engine, depending upon the metaphor you prefer--really gets chugging. And even with all of these conditions in place, hurricanes still won't form without some initializing disturbance or "trigger," such as an African easterly wave. No, these aren't the kind of waves that that surfers catch. It's better to think of them as wave-like patterns of air, often accompanied by thunderstorm clusters, that ride easterly winds across the Atlantic and often stir up hurricanes during their passage. All of these factors--and others as well--help determine whether or not a hurricane comes to exist in the first place. However, when scientists suggest that global warming might be intensifying hurricanes, it's generally changes in sea surface temperature (SST) that interest them most. The basic argument runs like this: All other factors being equal, higher SSTs will strengthen storms and lengthen their duration by providing a greater energy source for them to draw upon. This argument is rooted in our modern thermodynamic understanding of hurricanes, but it also makes a kind of intuitive sense. After all, we know that storms weaken over cold water, or over land, because they lose their energy source. No wonder running over warm water, and especially deep warm water, strengthens them. So when two scientific papers came out last year linking global warming to an increased number of hurricanes reaching Category 4 and 5 intensity levels (1, 2), it's no surprise that the suspected mechanism driving this outcome was a global increase in sea surface temperature (an increase linked to global warming). However, neither of the papers did an in-depth analysis to demonstrate that SST change was, in fact, causing the observed change. They merely noted correlations between hurricane strength and increasing SST. But now, in the latest issue of Science, several members of the Webster group at Georgia Tech--which produced one of those initial papers (2)--have taken the next logical step (3). Their new paper seeks to determine whether higher SSTs are, indeed, causing a growth in hurricane strength--or, alternatively, whether that change can be attributed to changes in other important factors like vertical wind shear, atmospheric heat and humidity, or something called "zonal stretching deformation" (which I don't completely understand, but which apparently relates to the Coriolis effect again). And sure enough, they found that all the evidence pointed towards SSTs. The Georgia Tech scientists report that of the four factors they studied, only SST had been trending upward consistently in all six of the ocean basins considered. For the other three factors, no consistent global trends were observed (though there were trends in some of the individual basins). And so, after some complicated statistical/"information theory" jazz designed to figure out the relative importance of these variables--there should be an award to the journalist or blog denizen who can explain this part of the study in layman's terms--the paper (of which I have a copy, not sure it's available online yet) concludes:
The implication of these results is that the strong increasing trend in [the number of Category 4 and 5 hurricanes] for the period 1970-2004 is directly linked to the trend in tropical SST and that other aspects of the tropical environment, while influencing shorter term variations in hurricane intensity, do not contribute significantly to the global trend of increasing hurricane intensity.
So what's the take-home point here? Well, this paper would certainly seem to strengthen the conclusions of previous studies about a link between hurricane strength and global warming. More specifically, the new study, by attempting to rule out other variables that affect hurricanes, adds support to the contention that the link between global warming and hurricane intensity hinges upon rising sea surface temperature. Hurricane theory had long pinpointed SST as the chief source of storm energy, but now a statistical test seems to back that up. To be sure, we'll have to see whether these results hold up after criticism. My guess is that they will gain ample media coverage, and that this coverage will contain critical reactions from scientists who remain skeptical of the initial studies linking hurricanes to global warming. But of course, that's how science is supposed to work. Indeed, I'm told that the original idea for the new study actually arose because the authors were responding to criticism of their previous study published in Science (2). That exchange, I understand, is currently in press. In any case, I'm no scientist, but allow me to just wade in here with one slight comment of my own (I hope I can explain this right). As I've noted above, many factors control whether hurricanes develop and how strong they become, and it's very hard for scientists to say precisely how some of those could change in the future as a result of global warming. Take one of the parameters examined in the latest study--atmospheric temperatures, which, if rising, could suppress hurricanes. The latest study suggests that this isn't a major factor yet, but it certainly could become so. After all, the lower the temperature difference between the ocean and the upper troposphere, the harder it is to get the upward heat transfer going through convection. And there are good reasons to think that global warming can or will trigger upper tropospheric warming. So if that happens, it might offset some of the importance of the increase in SST on hurricane strength--although I'm not sure anyone thinks it would totally offset the significance of the SST increase. But it does go to show you how complex this subject is. In any case, that's my two cents based upon some reporting that I've been doing. From a political perspective, the upshot is that we can expect the rather testy hurricane-global warming debate to flare up again over this paper, so that will be interesting to watch. Meanwhile, if anyone can explain the new paper's stuff about information theory in English and without equations, I strongly urge you to do so in the comments. Moreover, if I've explained any of the above science wrong, which is likely, please correct me--that's what blogging is for.... References: 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) 3. Hoyos et al, "Deconvolution of the factors contributing to the increase in global hurricane intensity," Science, March 17, 2006, Vol. 312. (abstract)