Tomorrow I am off to New York for this meeting on hurricanes and climate, sponsored by Columbia University's International Research Institute for Climate and Society. I'm very appreciative that Columbia-IRI has allowed me to attend the event, which will feature presentations from Kerry Emanuel, Chris Landsea, Roger Pielke, Jr., Thomas Knutson, and numerous others.
I will be back late Wednesday; blogging will probably be impaired during the trip. So let me leave you with something substantive before I go:
It turns out that another presenter at the Columbia meeting will be Johnny C.L. Chan, an expert on hurricanes in the western North Pacific ocean who just published this technical comment (1) on the subject in the latest issue of Science. Chan's comment responds to the Webster et al paper (2) from last year showing a marked increase in global hurricane intensity. Chan challenges the Webster group's analysis of the western North Pacific, and Webster et al respond (3) to the criticism in the same issue. (In fact, Chan's challenge apparently led to the Webster group's new paper on hurricanes and global warming (4) published last week Science Express, which I remarked upon here.)
Before discussing the new papers, let me begin by sketching a little context here. The western North Pacific is the most happenin' hurricane basin--accounting for fully a third of global storm activity--and has produced the largest and strongest storm ever recorded (Super Typhoon Tip of 1979). So understanding what's going on there is obviously of major importance. However, in the context of the global picture, it is just one basin out of many.
Solely addressing the western North Pacific, Chan argues that the apparent trend towards stronger storms in this basin isn't real: What's in fact being observed is natural variability. This Chan shows by examining wind speed data back to 1960 (rather than starting in 1970 with satellite records, as the Webster group did). Chan also argues that what's really important to storm strength is not sea surface temperature but rather other parameters such as "atmospheric rotational flow, vertical wind shear, and thermodynamic energy." He wraps up by stating that "Because these factors go through such large interdecadal variations, it is difficult to conclude that more intense typhoons are likely to occur in a warmer world."
Webster et al defend their study, saying that Chan makes some valid points but misses the big picture. Although there's natural variability, they say, their paper nevertheless successfully detected a long term trend with serious implications for the globally warmed future: "Should SSTs continue to rise under anthropogenic forcing, it is reasonable to expect that this relationship will be maintained and that there will be an associated increase in the intensity of typhoons."
I'm not going to pretend to have a deep grasp of the technical issues at stake in the latest exchange. I cannot competently pronounce "right" and "wrong" here; it would be great if some commenter with more aptitude than I could help us to sort matters out. Still, a few things are apparent: Stretching the data back to 1960, as Chan does, clearly changes the analysis--but the question is whether the data before satellite recordings are reliable. Furthermore, Chan has contested the original Webster study in one major hurricane basin (the western North Pacific), but the findings of that study covered all of the hurricane basins. So whoever wins the battle over the western North Pacific, that does not necessarily change the overall scientific outcome.
Meanwhile, as the battle over global warming and hurricanes continues in the scientific literature, we have 68 days to go (counting the rest of today) until the Atlantic hurricane season officially begins....
1. J. C. L. Chan, Science 311, 1713 (2006). [Link]
2. P. J. Webster, G. J. Holland, J. C. Curry, H.-R. Chang, Science 309, 1844 (2005). [Link]
3. P. J. Webster, G. J. Holland, J. C. Curry, H.-R. Chang, Science 311, 1713 (2006). [Link]
4. Hoyos et al, "Deconvolution of the factors contributing to the increase in global hurricane intensity," Science, Published online 16 March 2006; 10.1126/science.1123560. [Link]
As far as I can tell there's nobody from NCAR or the Webster group, which is interesting.
Yeah, not sure what to make of that--but their side of the debate will clearly be represented at the conference by Kerry Emanuel.
While there are vociferious people on both sides of the debate, I think that most scientists are unsure about whether we can detect an increase in hurricane intensity over the last few decades. The Emanuel and Webster et al. papers are provacative, but the uncertainties are large. Scientists always look for multiple verification of important claims before the community accecpts a claim ("we can observe hurricanes getting stronger") as true.
On the other hand, most scientists agree that hurricanes should be getting stronger as the Earth warms, so I think it's just a matter of time before we get the data that unambiguously shows this. Of course, Bill Gray will never be convinced.
These kinds of debates are common in science --- convincing the scientific community that a particular claim is true requires the claim be demonstrated to be true to a high degree of confidence. This has not yet occurred for this claim. I would not expect this to change any time soon; it will probably take a few years before one can say that the "scientific community is convinced that we have observed hurrican intensity increasing."
Thanks Andrew. I agree these kinds of debates are common in science--what makes the hurricane/GW debate different, however, is that the whole thing is playing out under the klieg lights, with massive media attention, for obvious reasons. This can be polarizing.