It may seem a strange question to be asking in a season that, so far, hasn't yet seen an Atlantic hurricane. But while the weather in any given year can be tricky and unpredictable, there's no doubt that we're currently in an active period for Atlantic storms in general--and now, a new paper (PDF) from Greg Holland of the National Center for Atmospheric Research and Peter Webster of Georgia Tech explicitly ties this period of heightened activity to global warming by asserting that the total number of Atlantic storms has increased markedly over the past century, in correlation with rising sea surface temperatures. At the outset, it's worth noting that National Hurricane Center specialist Chris Landsea's rebuttal (PDF) to the Holland/Webster paper was published in EOS back in May, well before the Holland/Webster paper itself officially came out (in the Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences). This is simply a quirk of the scientific publication process: Although they haven't appeared in a journal until now, the arguments about storm numbers made by Holland and Webster have been kicking around for some time. Indeed, back in January on this blog I reported on a public back-and-forth between Holland and Landsea over Atlantic storm numbers that occurred at the annual American Meteorological Society meeting in San Antonio. Still, as the Holland-Webster paper is now garnering major media coverage, it seems an appropriate time to provide some commentary on it. Allow me to start out by providing the abstract of the paper so that you can see what is being argued (you'll find that abstract as well as my further analysis after the jump):
We find that long-period variations tropical cyclone and hurricane frequency over the past century in the North Atlantic Ocean have occurred in the form of three, relatively stable regimes separated by sharp transitions. Each regime has seen 50% more cyclones and hurricanes than the previous regime and is associated with a distinct range of sea surface temperatures (SSTs) in the eastern Atlantic Ocean. Overall, there appears to have been a substantial 100-year trend leading to related increases of over 0.7 C in SST and over 100% in tropical cyclone and hurricane numbers. It is concluded that the overall trend in SSTs and tropical cyclone and hurricane numbers is substantially influenced by greenhouse warming. Superimposed on the evolving tropical cyclone and hurricane climatology is a completely independent oscillation manifested in the proportions tropical cyclones that become major and minor hurricanes. This characteristic has no distinguishable net trend and appears to be associated with concomitant variations in the proportion of equatorial and higher-latitude hurricane developments, perhaps arising from internal oscillations of the climate system. The period of enhanced major hurricane activity during 1945-1964 is consistent with a peak period in major hurricane proportions.
Let's translate this into some numbers. Looking at the HURDAT or "best track" database maintained by the National Hurricane Center, Holland and Webster find that between 1855 and 1900 we had about 7-9 storms (tropical storms or stronger) per year; between 1905 and 1930 there was a decrease down to about 6 storms per year; and then between 1931 and 1994 there were around 6 to 14 storms per year with an average of 10. Since 1995, however, we're seeing dramatically more activity--so far, there have been 15 storms per year on average. All of this has occurred as sea surface temperatures, particularly in the eastern Atlantic, have warmed: "There has been an average of one additional tropical cyclone for each 0.1 C increase in SST and one hurricane for each 0.2 C increase," write Holland and Webster. These scientists know very well that the hurricane database they're using to derive these numbers isn't perfect, and indeed, that it surely gets worse the further you go back in time (because more storms would have been missed in earlier eras). So they define the uncertainties thusly: "Our conclusion is that the number of earlier missed storms most likely lies between 1-3 per year prior to 1900, less than 2 in the early [20th] century and dropping off to essentially zero by 1960." Using these assumptions, Holland and Webster show a strong and very troubling trend towards increasing Atlantic storm numbers--and we're getting more intense storms, they say, largely because this increase in numbers gives more storms the opportunity to achieve their full potential. As they put it: "We are of the strong and considered opinion that data errors cannot explain the sharp, high amplitude transitions between the climatic regimes in the NATL, with an increase of around 50% in cyclone and hurricane numbers, and the close relationship of these regime changes with SSTs." But, here's where the National Hurricane Center's Chris Landsea--quoted in recent news stories calling the latest work "sloppy science"--comes in with a counterpoint (PDF). Landsea says data errors can explain the apparent trend. As his paper puts it, referring both to the Holland/Webster study and another piece of work: "both analyses, with no indication of uncertainty or error bars, presumed that tropical cyclone counts are complete or nearly complete for the entire basin going back in time for at least a century...this presumption is not reasonable...improved monitoring in recent years is responsible for most, if not all, of the observed trend in increasing frequency of tropical cyclones." In fact, Landsea suggests that we should probably assume something more like 3.2 missed storms per year from 1900-1965 and 1 per year from 1966 to 2002--which, he explains, would be enough to essentially nullify the apparent trend. Landsea further notes that the percentage of total storms that make landfall has been decreasing in recent years, and from this he deduces that in earlier eras, storms remaining at sea weren't detected nearly so frequently as they are today, when we have a much denser observing network. To illustrate this point, in his paper Landsea includes the graphic below, which compares the tracks of storms during the 1933 hurricane season (which saw 21 officially recorded storms) and the 2005 season (which set the record with 28). As you can see, landfall proportions were far higher in 1933 and far fewer storms out at sea were detected:
Let me be completely frank: I have no idea who's right in the current argument between Holland/Webster and Landsea. Indeed, in some sense it's probably unknowable--we're talking about missed storms, after all, and now that they've been missed of course we don't know how many of them there were. From a policy perspective, though, we don't have to remain completely agnostic regarding this debate. There are several important points to take away from this latest dustup, and in a follow-up post, I will tease out those implications. But for now, if I've left everyone scratching their heads about who to trust in the current argument, all I can say is, I'm scratching my head too....
