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.
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.
He is a leading critic of studies that link increased storm intensity to global warming, saying historical data are too crude to determine long-term trends. The same, he argues, is true for storm frequency.
"The doubling in the number of storms that they find in their paper is just an artifact of technology, not climate change," he wrote.
Airplanes and satellites outfitted with sensitive instruments to monitor tropical storms did not exist a hundred years ago. Aircraft began monitoring hurricanes in 1944, satellites about 1970.
Recent research by Landsea finds three to four tropical storms were missed before the mid-1960s. And one storm a year was missed from the 1970s to the 1990s.
"When one takes into account these missing storms, the upward trend disappears and large multiyear variations remain," he said.
According to Holland, however, the technological improvements cannot account for all the observed increase in tropical storms.
His new study accounts for three missed storms per year in the pre-satellite era, and a statistically significant increase is still present, he said.
"So, yes there's uncertainty, there's no doubt about that," Holland said. "But it doesn't change our conclusions."
According to the study, the period from 1900 to 1930 saw an average of six Atlantic tropical storms a year, with four of those storms growing into hurricanes.
From 1930 to 1940, the annual average increased to ten, with five hurricanes and five tropical storms.
Then, from 1995 to 2005, the average number of storms increased to 15, with 8 hurricanes and 7 tropical storms.
Even 2006, which seemed mild in the wake of the record-breaking 2005 season, saw ten tropical storms, the authors note.
Holland likened the transition phases to a thunderstorm in the Rocky Mountains—the sun rises and warms up the clear blue skies for hours, and then suddenly a thunderstorm develops.
"That's a very good example, but what we're looking at is happening on the global scale," he said.
The most recent period has yet to stabilize, leading the authors to conclude there may be even more active hurricane seasons in the future, though other factors may dampen the storm activity.
"What I will say is there is no evidence that I'm aware of and nothing I can think of which indicates they are going to go back down," Holland said.
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