for National Geographic News
The number of Atlantic hurricanes that form each year has doubled over the past century and global warming is largely to blame, according a new study.
The increase occurred in two major steps of about 50 percent each, one in the 1930s and the second since 1995.
"It hasn't been a steady, gradual increase," said Greg Holland, a scientist with the National Center for Atmospheric Research in Boulder, Colorado.
The increases coincide closely with rises in sea surface temperatures in the eastern Atlantic tropics. Previous studies have attributed these rises to human emissions of greenhouse gases.
"That [correlation] implies there is a substantial contribution by greenhouse warming to the current crop of tropical cyclones," Holland said.
The study also shows that the proportion of major hurricanes to less intense hurricanes has sharply increased in recent years, which agrees with earlier studies showing an increase in stronger storms.
(Related: "Warming Oceans Are Fueling Stronger Hurricanes, Study Finds" [March 16, 2006].)
However, the new study found the proportion of major hurricanes has fluctuated with a "remarkably constant period of oscillation" over the past century, Holland said. The oscillation appears to be a natural variability not associated with warming.
So while the storms' severity seems to fluctuate in a natural cycle, their frequency is on the rise, he explained.
"What we're seeing [now] is a high point of major storms, which has been a very, very stable oscillation, combined with a sharp increase in the frequency of all storms, which has been a trend," Holland said.
Holland and Peter Webster of the Georgia Institute of Technology in Atlanta published their findings online today in the research journal Philosophical Transactions of the Royal Society of London.
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