Aug. 4, 2008 -- The question of whether global warming is affecting hurricanes that form over the Atlantic Ocean -- and if so, how -- has sparked much debate in recent years. Climatologists believe that rising ocean temperatures can lead to stronger hurricanes, but the jury is still out on whether the storms will form more often in a warmer world.
A new study strengthens the argument that they won't.
Hurricane frequency has increased at a rate of about six storms per century since 1965, when weather satellites began keeping close watch over the Atlantic. Measurements are sketchy before then, and before the first storm reconnaissance flight in 1944 they're even worse -- the only source of information being ships' log entries.
When the record is extended back to include 1878-1900, however, storms appear to be on a slower rise, about 3.8 per century.
To correct for inaccuracies in the record before 1965, Gabriel Vecchi and Thomas Knutson of the National Ocean and Atmospheric Administration in Princeton, N.J., performed a thought experiment: What if modern-day storms had occurred back then? How many storms would ships traveling the Atlantic have simply sailed past without noticing?
When the researchers superimposed known storm tracks from 1965 onward over ship tracks between 1878 and 1965, they found the ships indeed missed many storms.
"In the late 1870's, ship traffic was sparse, and they may have missed about a third of the storms," Knutson said, or about four storms per year. By the 1960s, traffic was dense enough to catch all of the storms.
Adding the number of missed storms to the official record yields a nearly flat trend in storm frequency from 1878-2006, despite a clear rise in ocean surface temperatures of 0.7 degrees Centigrade during the same time period. Their work was published in the July issue of Journal of Climate.
"The unadjusted record has dire implications for the future, with maybe three more doublings of storm frequency by 2100," Knutson said. But once the missing storms are counted, he added, "we're not finding much evidence that global warming is going to lead to a large increase in tropical storms in the Atlantic."
Vecchi and Knutson's results agree closely with a similar study in 2007 by Edmund Chang and Yanjuan Guo of the State University of New York, Stony Brook. Together, the groups' work suggests that the relationship between warming temperatures in the oceans and storm frequency isn't well understood.
"We have a very solid theory that links storm intensity to sea surface temperature," said James Kossins of the University of Wisconsin, Madison. "But although an increase in storm counts is sometimes implicitly associated with global warming, it doesn't stand up to re-analysis, and we don't really have a solid scientific theory for why that is."
Related Links:
Michael Reilly's blog: Strike Slip
Treehugger.com: Interview with Chris Mooney, author of "Storm World"
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