More Hurricanes

The question of whether global warming is bumping up hurricanes gets another good airing in today’s Nature, with two strong arguments against the suggestion that Kerry Emanuel made in August in Nature (sub. req. for the Nature links) of a global warming-hurricane link.

Just as Emanuel’s initial work was seized upon by the left (“As this season’s hurricanes slam into our coastlines, MIT scientists have hit us with a dose of reality: global warming is to blame.”) I expect this new bit of work questioning the global warming-hurricane link to be seized upon by the right. (My prediction: a new Tech Central Station piece coming soon to supplant their previous discussion.)

But this is really too important for cherry-picking. The scientific back-and-forth suggests this is really very unsettled, and the political opportunists on both sides ought to back off. Further study is clearly needed.

Roger Pielke Jr. argues that hurricane damage data, normalized to reflect societal changes (i.e. more people building houses at the beach, etc.) “shows no upward trend.” Roger’s data stands in implied contradiction to Emanuel, who created a “power dissipation index” – a measure of total wind force over the life of storms – and concluded it was on the rise. What Pielke argues, in essence, is that if hurricane destructiveness is on the rise, it’s not showing up in the hurricane damage data:

These loss data indicate two possibilities with respect to Emanuel’s analysis1: if the power-dissipation index metric is an accurate indicator of hurricane destructiveness, then the trend identified by Emanuel could be an artefact of the data and/or methods; alternatively, the trend he identifies is an accurate reflection of trends in the real-world characteristics of storms, but the power-dissipation index is a weak indicator of hurricane destructiveness.

Emanuel replies that there’s far less data in Pielke’s data set to work with, because onshore damage only integrates the time the hurricane is making landfall, while Emanuel’s data includes the whole life of the storm. “It is therefore possible,” Emanuel writes, “that the real trend is detectable in the power dissipation but not in landfalling statistics.”

Chris Landsea offers an alternative explanation: that Emanuel’s manipulations of the data to correct for problems in the 1940s-1960s are unwarranted. Take that away, Landsea argues, and the effect found by Emanuel goes away:

These factors indicate that instead of “unprecedented” tropical cyclone activity having occurred in recent years, hurricane intensity was equal or even greater during the last active period in the mid-twentieth century.

Emanuel accepts Landsea’s argument that he overcorrected, but argues that the effect remains:

I maintain that current levels of tropical storminess are unprecedented in the historical record and that a global-warming signal is now emerging in records of hurricane activity.