Hurricane Dean: Just A Category 3 Storm 50 Years Ago?

On Wednesday I appeared on NPR's "On Point" with Tom Ashbrook to discuss Hurricane Dean and the hurricane-global warming relationship -- you can listen to the whole thing here. On the air with me were two scientists who vigorously disagree about the subject, Peter Webster of Georgia Tech University and Chris Landsea of the National Hurricane Center.

Most of the back and forth I'd heard before, but I was particularly struck by something that Landsea -- a skeptic of any strong hurricane-climate linkage -- said: He thinks that 50 years ago, Hurricane Dean would have been recorded as a mere category 3 storm. As Landsea put it at around minute 23:30:

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if we look back at how this hurricane would have been observed a hundred years ago, or fifty years ago, we would not have known it was a Category 5. We knew that because we were flying aircraft into it, we've got satellites peering down on it. But if it was fifty years ago, back then the aircraft did not fly into the eye of hurricanes, they were worried about crashing. They were kind of flimsy aircraft at that time. And where it made landfall, in a fairly unpopulated area of Mexico, we probably wouldn't have any ground measurements that it was a Category 5. If Dean occurred fifty years ago, we would think it was a Category 3 probably.

After the show I asked Landsea a question about 1955's Hurricane Janet, which just like Dean hit near Chetumal, and which was also a Category 5 at landfall. <-- Hurricane Janet, 1955

Hurricane Dean, 2007

So I asked Chris, how do we know that Janet was a Category 5? After all, Wikipedia lists a 914 millibar pressure measurement for Janet; indeed, Dean, at 906 millibars, helped knock Janet out of the top 10 strongest Atlantic hurricanes list. So where did that Janet measurement come from if we weren't flying planes into storms back then?

Landsea replied by sending me this contemporary scientific paper (PDF) on the 1955 hurricane season, including an explanation of how Hurricane Janet was measured. It turns out that unlike Dean, the storm's slightly different track brought it directly over Chetumal, and the 914 millibar pressure measurement (27.00 inches of mercury) was taken from the ground at Corozal, which was in the southern edge of the eye.

At the time, the paper adds, this pressure reading for Janet was tied for the third place among the lowest such readings ever taken at the earth's surface. By contrast, today it's not even in the top 10 in the Atlantic. And so this ends up considerably reinforcing Landsea's point: Since 1955, we keep measuring lower and lower pressures in hurricanes. That has to be at least in part due to the fact that we're getting better and better at taking these measurements. In other words, it's a human-induced change but not a climate change -- it has to do with our changing techniques and technologies for sampling hurricane strength.

I fully accept Landsea's argument: Measurement has a lot to do with how strong we pronounce hurricanes, and measurement has most assuredly changed over time. But whether that alone accounts for the dramatic recent uptick in Atlantic hurricane activity, I'm not entirely sure -- and other scientists would disagree that measurement changes, on their own, can account for what we're seeing. As the 2007 hurricane season continues, you can be assured that this debate will as well!