for National Geographic News
The number of hurricanes that swirl across the Atlantic Basin has shot up in the past decade, but the increase may just be a return to normal activity after a long lull, suggests a new study.
The findings throw additional fuel onto the debate over the effects of global warming on hurricane frequency and intensity. (Related: "Global Warming Link to Hurricane Intensity Questioned" [July 28, 2006].)
Some researchers have suggested the increase in hurricane activity since 1995 is a result of climate change, whereas others contend the trend is part of natural variability.
A problem plaguing researchers is the lack of a long-term hurricane record for comparison. Reliable observations stretch back only about 50 years.
Now a team of scientists led by Johan Nyberg at the Geological Survey of Sweden in Uppsala has reconstructed the hurricane record of the past 270 years by studying the growth patterns of coral skeletons and the abundance of tiny fossils in a marine sediment core.
The record shows that hurricane activity goes up and down in decadal cycles, with storms gradually decreasing overall from the 1760s until the early 1990s. An abnormal lull of just 1.5 major storms a year was reached between the 1970s and early 1990s.
A major Atlantic storm is defined as any Category 3 or higher hurricane, with sustained winds of at least 111 miles an hour (179 kilometers an hour). (See hurricane videos and get more information about the disasters.)
Since 1995, however, an average of 4.1 major hurricanes have formed, which is comparable to other periods of high hurricane activity in the 270-year long record, according to the researchers. (Related: "2007 Hurricane Season Begins, Will Be Busy, Forecasters Say" [June 1, 2007].)
Writing in this week's issue of the science journal Nature, Nyberg and colleagues say the current trend "thus appears to represent a return to normal hurricane activity."
While the hurricane activity of the past decade appears normal according to the new findings, global warming may still be influencing how many storms form each year, the researchers say.
For example, "the relatively low 'low level' phase from 1970 to 1990 could have been caused by a faster warming of the atmosphere than of the ocean," lead author Nyberg wrote by email.
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