Frequency Of Atlantic Hurricanes Doubled Over Last Century And Climate
Boulder CO (SPX) Jul 31, 2007
About twice as many Atlantic hurricanes form each year on average than a
century ago, according to a new statistical analysis of hurricanes and
tropical storms in the north Atlantic. The study concludes that warmer
sea surface temperatures (SSTs) and altered wind patterns associated with
global climate change are fueling much of the increase. The study, by
Greg Holland of the National Center for Atmospheric Research (NCAR) and
Peter Webster of Georgia Institute of Technology, will be published
online July 30 in Philosophical Transactions of the Royal Society of
"These numbers are a strong indication that climate change is a major
factor in the increasing number of Atlantic hurricanes," says Holland.
The analysis identifies three periods since 1900, separated by sharp
transitions, during which the average number of hurricanes and tropical
storms increased dramatically and then remained elevated and relatively
steady. The first period, between 1900 and 1930, saw an average of six
Atlantic tropical cyclones (or major storms), of which four were
hurricanes and two were tropical storms.
From 1930 to 1940, the annual average increased to 10, consisting of five
hurricanes and five tropical storms. In the final study period, from 1995
to 2005, the average reached 15, of which eight were hurricanes and seven
were tropical storms.
This latter period has not yet stabilized, which means that the average
hurricane season may be more active in the future. Holland and Webster
caution, however, that it is not possible at this time to predict the
level at which the frequency and intensity of storms will stabilize.
The increases over the last century correlate closely with SSTs, which
have risen by about 1.3 degrees Fahrenheit in the last 100 years. The
changes in SSTs took place in the years prior to the sharp increases in
storm frequency, with an SST rise of approximately 0.7 degrees Fahrenheit
leading up to 1930 and a similar rise leading up to 1995 and continuing
even after. The authors note that other studies indicate that most of the
rise in Atlantic SSTs can be attributed to global warming.
Natural cycles and global warming
The unusually active hurricane seasons of 2004 and 2005 have spurred
considerable research into the question of whether more intense tropical
cyclones are correlated with natural cycles, global warming, or some
other cause. The new study indicates that natural cycles are probably not
the entire cause because the increase has happened across the last
century rather than oscillating in tandem with a natural cycle.
The study also finds that enhanced observations in recent decades cannot
account for all of the increase. To observe storms in the Atlantic more
systematically, meteorologists began relying on data from aircraft
flights in 1944 and satellites about 1970. The distinct transitions in
hurricane activity noted by Holland and Webster occurred around both 1930
"We are of the strong and considered opinion that data errors alone
cannot explain the sharp, high-amplitude transitions between the climatic
regimes, each with an increase of around 50 percent in cyclone and
hurricane numbers, and their close relationship with SSTs," the authors
While the number of storms has steadily increased, the proportion of
hurricanes to all Atlantic tropical cyclones has remained steady.
Hurricanes have generally accounted for roughly 55 percent of all
tropical cyclones. However, the proportion of major hurricanes (those
with maximum sustained winds of at least 110 miles per hour) to less
intense hurricanes and tropical storms has oscillated irregularly, and
has increased significantly in recent years.
Last year's storms
The 2006 hurricane season was far less active than the two preceding
years, in part because of the emergence of an El Nino event in the
However, that year, which was not included in the study, would have
ranked above average a century ago, with five hurricanes and four other
named storms. "Even a quiet year by today's standards would be considered
normal or slightly active compared to an average year in the early part
of the 20th century," Holland says.