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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