There was another twist to the hurricanes/global warming issue in Science Express on Friday where a new paper from the Webster/Curry team just appeared. This study, lead by Carlos Hoyos, crosses a few t’s and dots a couple of i’s on the connection of increasing numbers of intense hurricanes (Cat. 4 and 5) to sea surface temperatures (SST). Basically, they looked at a number of other key variables for hurricane intensity (like wind shear and humidity) and examined whether there was any pattern to those variables across the different ocean basins that they study. Bottom line? None of the other variables have as much explanatory power for the long term trends as SST which is the only consistently trending constituent in the mix. So far, so un-surprising. However, one interesting aspect of this story is that almost all the key players in the ongoing debate were interviewed by different journalists in various media and those comments are probably more useful for gauging the state of play than the details of the new paper itself.
First off, Chris Landsea, a serious researcher in the ‘dissenting’ camp, in the WSJ story (reprinted here) and in a Newsday article is quoted as saying that the data (particularly from the early 70s and the Northern Indian Ocean) aren’t good enough to come to these conclusions. However, Curry riposts that for the doubling of Cat 4 + Cat 5 storms to be an artefact, half the Cat 1 + Cat 2 storms in the early records would have had to have been mis-classified Cat 4 storms instead and she thinks that unlikely. More oddly, Landsea also makes the point that the sensitivity of the hurricane intensity disagrees with model predictions and theory. This is odd, not because it’s not true (and is the principle reason why the attribution of more intense hurricanes to GW is not yet set in stone), but because Landsea has previously been much more of a champion of favoring observations over modelling.
In the same National Geographic piece, Roger Pielke Jr. is broadly supportive and makes his standard comment that the increasing damages from hurricanes is mostly related to increased development rather than changes in intensity, but that obviously, future potential increases in intensity will be an additional factor.
Fox News interviewed Kerry Emanuel (author of a related study last year, which was slightly modified following comments by Landsea in Nature), who is also unsurprised that the longer term trends are related to SSTs and not to any of the other factors. Kevin Trenberth (also in the Fox News piece) made a good point though. The authors of the study used the NCEP reanalysis as the source of their data. A reanalysis is a re-running of the current state-of-the-art weather forecasting model for all the sources of data that were available in the past (i.e. a hindcast of what all the 6 hour weather forecasts would have been if they had used today’s model). These projects (and there are two main ones – NCEP and ERA-40) have a problem in that the amount of useful data increases as you go along – most significantly around 1979 when satellite data starts to be significant. So estimates of key quantities are likely to be worse prior to 1979. Not mentioned, but conceivably important is that the NCEP reanalysis is tied in some respects to the radiosonde data, which, as we discussed last year, may have some spurious trends. This doesn’t obviously affect the results significantly, but it does suggest that doing the analyses again using the ERA-40 data might be a useful check.
So where does that all leave us? Basically, although everyone acknowledges that there are data problems early in the record, it seems clear that there has been a global rise of the most intense hurricanes over the last 30 years and the most obvious explanation is that this is related to the contemporaneous increases in tropical SST in each basin. However, the magnitude of the correlation cannot yet be explained in terms of our basic theoretical understanding, and is significantly stronger than some modelling work has suggested it should be. Possibly the theory needs work (hurricanes are a complicated business!) or there are other factors at play that haven’t yet been considered. Since the SST changes are global, and almost certainly tied to greenhouse gas driven global warming, there are the beginnings of a corroborated link between increases in hurricane intensity and GW – however, so far there are only a couple of ducks in a row.