July
29, 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 "These numbers are
a strong indication
that climate change is a major factor in the increasing number of
Atlantic
hurricanes," says 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.
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 "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 state. 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 "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," ##
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