From so simple a beginning…

As Hurricane Dean threatens to hit the Gulf Coast and cause damage, the BBC today has an especially nice animation on how hurricanes form. Hurricane formation is largely a matter of basic physics; as the sea surface warms, the warm air above it rises upwards, drawing more warm air from the surroundings, with the cool condensed air on top coming down and drawing more air along with it, setting up a largely self-sustaining cyclical system. The tricky part as usual is putting numbers on the many different factors affecting the formation, duration, and intensity of the hurricane. The BBC animation shows the basic details pretty well. However, the animation misses a key point; that the major source of the destructive energy of a hurricane comes from the condensation of the warm air that has risen up in the higher atmosphere. This affects the energy output and the strength of the resulting winds, and also leads to some kind of positive feedback loop, with the faster winds and lower pressures causing even more evaporation. The consequent energy in a hurricane dissipated per day is staggering beyond imagination; as the Wikipedia article says, “equaivalent to 200 times the world’s electricity production per day, or to exploding a 10 megaton thermonuclear bomb every 20 minutes”.

It has been a hotly contested issue whether global warming causes hurricane activity. I am as much of an expert on hurricanes as an orchid breeder, but the best I can tell from general reading is that while the frequency of hurricanes seems to be poorly correlated, if at all, to global warming, the intensity of hurricanes seems to be more or less directly dependent on the sea-surface temperatures. This correlation seems to be better established than many others in the complex realm of climate “prediction”. On the other hand, that global warming increases the mean sea-surface temperature seems more likely to be a direct consequence of the laws of physics and much less tenuous. In light of this, I can largely see three statements that seem to add up to a correlation between global warming and increased hurricane intensity:

1. All other factors being “reasonable”, increase in mean sea-surface temperatures is definitely correlated with increased hurricane intensity.
2. Global warming leads to increases in mean sea-surface temperatures (so does the sun’s activity, but this variable, often touted as being responsible for global warming, is in serious trouble)
3. Man’s activities are very likely to be responsible for global warming

In view of these three statements, and having a basic dictionary understanding of the phrases “average” and “very likely” as enunciated in the 2007 IPCC report, it seems pretty likely to me that fossil fuel emissions can cause increased hurricane activity. Of course, this is not “certain”, but if we want certainty, why bother about educating people about the hazards of smoking, or vaccinate people when we are reasonably sure about an impending epidemic? Even these are not “certain”.

Another factor that hurricane intensity depends upon is the strength of prevailing means. If the winds are strong, then the hurricane’s organized structure cannot persist very well. Apparently, this was one of the factors responsible for a relatively mild hurricane season last year. No such thing seems to be in the offing during this season. So we can only wait and see what happens in the next one month.

Some resources:
1. The Wikipedia article is pretty good
2. Kerry Emanuel (MIT) and Peter Webster (Georgia Tech) were two of the principal scientists who showed the correlation between warming and intensity. Emanuel’s book, Divine Wind, is informative and insightful
3. Chris Mooney, who wrote the readable The Republican War on Science, has come out with his latest timely book on hurricanes, Storm World, which I am starting to peruse. By many accounts, it is quite accurate and non-partisan (in contrast with the earlier one which was rightly partisan). More on it after I finish reading it.