Hurricane Dean: 1 Of 10 Most Intense Atlantic Hurricanes Ever Measured

Early this morning Hurricane Dean, after mercilessly intensifying over deep warm waters in the western Caribbean, slammed the Yucatan Peninsula around Chetumal as a Category 5 hurricane with winds upwards of 165 miles per hour and a minimum central pressure of 906 millibars. And those are just the official figures. Dean may have been stronger. It kept intensifying right up to landfall, and struck at peak intensity.

Dean was officially the most powerful hurricane that we've seen globally so far in 2007, and was by far the strongest at landfall. It was also the first Category 5 Atlantic hurricane seen since the record-setting Hurricane Wilma of October 2005. In fact, Dean set some records of its own. Its pressure was the ninth lowest ever measured in the Atlantic, and the third lowest at landfall. Indeed, there hasn't been a full Category 5 landfall in our part of the world since 1992's Hurricane Andrew. Dean was in all respects a terrifying storm, and we can only hope that the damage will somehow be less than expected as it tears across the peninsula and then, after crossing the Bay of Campeche, moves on to a presumed second Mexican landfall.

No one storm says anything about climate change; but nevertheless, climate change may affect weather in the aggregate. So as we wait for news about just how destructive Dean has been, let's consider

the storm from a climate perspective, bearing in mind the scientific expectation that global warming ought to intensify the average hurricane (by how much remains hotly disputed). How does Dean fit into that ongoing scientific argument?

Well, first of all, Dean now takes its rank among the top ten most intense Atlantic hurricanes. If you look at that list you'll see that six of the strongest (Wilma, Rita, Katrina, Mitch, Dean, and Ivan) have been in the past ten years. That's not the kind of statistic that's easy to overlook. According to these data we are getting more super-strong storms in the Atlantic basin than we ever have before.

To be sure, there's a counterargument here: Data wasn't as good on hurricane intensity in previous eras as it is today, when our measuring equipment is better than ever. Stronger storms may well have existed in the past, but we were simply incapable of detecting their true strength. This is a serious objection, although it's hard to know precisely how serious. Nevertheless, the fact remains that if you look at the official records, Dean now fits in to a staggering hurricane decade. That's highly suggestive, if not definitive. And this staggering decade has occurred in part because of anomalously warm ocean temperatures in the hurricane-prone regions. Many scientists question whether you can explain these warm anomalies without invoking global warming as at least part of the cause.

So once again, even though Dean was not "caused" by global warming, when considered in its Atlantic context the storm is certainly consistent with the argument that there's something going on out there that's new -- and more than a little scary.

Typhoon Gonu

And what about the global perspective? Well, according to my ongoing "Storm Pundit" count of mega-hurricanes, Dean is the 10th Category 4 or 5 tropical cyclone observed globally this year. There have also been two borderline Category 3/Category 4 storms in my estimation. Here's my tally of the data on intense storms in all the global hurricane basins, which is based upon the Unisys database of so-called hurricane best track records and supplemented by other data where I've found this database incomplete: South Indian Ocean:

• Cyclone Dora, Cat 4
• Cyclone Favio, Cat 4
• Cyclone Indlala, Cat 4
• Debatable: Cyclone George, Cyclone Jaya; see these two posts for more information about these storms)

North Indian (Bay of Bengal/Arabian Sea):

• Cyclone Gonu, Cat 5

Northwest Pacific:

• Typhoon Yutu, Cat 4
• Supertyphoon Man-Yi, Cat 4
• Typhoon Usagi, Cat 4
• Super Typhoon Sepat, Cat 5

Northeast Pacific:

• Hurricane Flossie, Cat 4

Atlantic:

• Hurricane Dean, Cat 5

Now first some caveats: Nature doesn't really care about our hurricane classification systems, and a storm that is a weak Category 4 for a few hours isn't nearly equivalent to one that's a strong Category 5 for several days. But that said, we have to work within the frameworks we have, and this analysis proceeds in that spirit.

And one additional note: In any given year there are always a few storms where you can argue about the precise intensity category due to limitations of, and inhomogeneities in, our measuring systems across different hurricane basins of the globe.

But with these warnings out of the way, what can we make of the data above?

Well, to repeat -- and as I discuss in detail in my book Storm World -- some scientists argue that the total number of the most intense hurricanes occurring annually is on the rise due to global warming and its heating of the oceans. 2007 isn't over yet, but in this ongoing debate, this year's complete tally of intense storms will serve as an important data point. That's where Dean and its cousins fit in.

At least by my own count, there were 19 of these intense storms in 2006, 22 in 2005, and 23 in 2004. Hurricane specialist Jeff Masters says the long term average is 17 -- in which case all of these years would be above it and we might indeed be looking at a trend.

It remains to be seen how 2007 will ultimately look when compared with these global totals for the past 3 years. But we can count on some more very strong storms this year. Not only do we expect more action from the Atlantic, Northeast Pacific, and especially the highly active Northwest Pacific, but we could also get a strong North Indian storm later this year. Finally, once the southern hemisphere summer rolls in around November and December, we might even get an intense storm or two rotating the other direction on the other side of the equator.

So in short: We're still collating data on how many intense storms there are per year and whether we see a robust trend in the number of the strongest hurricanes. Some scientists think we do, others disagree. However, theoretical expectations -- rooted, essentially, in the laws of physics -- say that storms ought to be intensifying as global warming changes the thermodynamics. Right now scientists are struggling to find harmony between those expectations and the ever increasing store of hurricane intensity data. In this context, Dean is just one tiny data point, but a troubling one nonetheless -- and we can say that even before all the evidence is in.