As I noted in a previous "Storm Pundit" item, there have been misleading attempts to claim that the 2007 Atlantic hurricane season has been dramatically inactive, and to at least infer that this in some way refutes the idea that hurricanes are changing due to global warming.
The truth of the matter is that we can't conclude anything definitive from a single year in a single hurricane basin -- there's far too much noise in the system. Now, leading meteorology blogger Jeff Masters explains further how we ought to think about this season (which still isn't over yet):
We've gotten very lucky this hurricane season since the departure of Hurricane Felix in early September. We had a record eight named storms form in September, yet we had only four hurricane days that month. Wind shear has been strategically high at the right time and right place, and storms have tended to form too close to land to develop... There are still several low-shear periods ahead for the Western Caribbean this hurricane season, and I expect our luck may not hold for one of these periods. There is still one hurricane likely to form this season, possibly a major hurricane.
Masters is shooting straight here: Lots of storms developed, and yet wind shear always seemed to be right there at the ready, preventing them from intensifying much. That can happen in any given season, but over the longer term, wind shear will have to average out, and strong hurricanes will have to occur -- and scientists will continue to analyze their intensities and numbers.
Hurricanes, Global Warming and Humidity
In the meantime, as we wait for more data to come in, other kinds of evidence can also point the way forward. In particular, last week USA Today reported on two new studies, which I have not yet read, suggesting that global warming is increasing atmospheric humidity -- or, in other words, warmer air is allowing the atmosphere to retain more water vapor.
The idea that humidity should increase under global warming has been around for quite a long time. As I explain in my book Storm World, it all goes back to a fundamental law of physics, the Clausius-Clapeyron equation, which stipulates that the amount of moisture that the air can carry increases dramatically with rising temperature. It is this relationship between humidity and temperature that has led scientists to expect a "water vapor feedback": As temperature rises due to carbon dioxide concentrations in the atmosphere, warmer air should also retain more water vapor -- and water vapor is another greenhouse gas! That should, in turn, greatly increase the rate of warming.
There's also an implication for hurricanes: If there's more water vapor in the air, then among other things, these storms ought to be able to rain more. And indeed, this very year we saw a new rainfall record: Cyclone Gamede dumped 12.9 feet of rain over a 72 hour period in the French island of La Reunion!
So even as we wait for more data on how hurricanes may be changing, we shouldn't ignore what theory tells us either. Rainfall can be a particularly devastating aspect of a hurricane's impact, especially in mountainous regions where it can create flooding and mudslides. And it ought to be on the rise.
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