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The truth behind wild weather


As Hurricane Irma batters Florida, with Anguilla, Barbuda and Cuba clearing up and Houston drying out after Harvey, it is reasonable to ask whether such tropical cyclones are getting more frequent or fiercer.

The answer to the first question is easy: no. As the Intergovernmental Panel on Climate Change put it recently: “Current datasets indicate no significant observed trends in global tropical cyclone frequency over the past century.” The trend in numbers of major hurricanes making landfall in the United States has been slightly downward over the past century. Harvey and Irma have ended an unprecedented 12-year hurricane drought, in which not a single category 4 or 5 hurricane made American landfall. So whatever global warming is doing or will do, it is not so far increasing the frequency of such storms.

The answer to the second question is less certain. Hurricane Irma is certainly breaking records: probably the strongest storm in the Atlantic outside the Gulf of Mexico, almost rivalling Hurricane Allen (1980) for the strongest hurricane ever to make landfall, wider in its impact than Hurricane Charley (2004) or Andrew (1992). Last week it sustained its 297kmh (185mph) winds for 37 hours, comfortably beating a record set by Typhoon Haiyan in 2013.

But how much of this is down to better measurement? We will never know exactly how ferocious the winds of the Labor Day Hurricane of 1935 were, or the great Barbados hurricane of 1780. An analysis published last month by the American government’s Geophysical Fluid Dynamics Laboratory stated: “It is premature to conclude that human activities, and particularly greenhouse gas emissions that cause global warming, have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity.”

It remains possible that tropical cyclones are becoming slightly fiercer, but slightly less frequent, which would be consistent with some predictions of climate-change theory.

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