|
 |
|
A new climate study indicates
that hurricanes and tropical storms became more frequent in the
Atlantic Ocean during three distinct periods over the last century,
as shown in this graphic. The first part of the 20th century (in
white) was relatively quiet, with an annual average of 6 observed
hurricanes and tropical storms. The annual average increased to 10
after 1930, and then reached 15 from 1995 to 2005 (in darkest
shading). This graphic shows both the total number each year (blue
line) and the nine-year running average, calculated from four
years back through four years ahead of a given year. Called a
running mean, this method smoothes out year-to-year variability to
reveal the long-term trend. The new research associates the
increasing storms with rising sea-surface
temperatures.(Illustration by Steve Dey |
Uit: UCAR (University
Corporation for Atmospheric Research0
Frequency of Atlantic Hurricanes Doubled Over
Last Century; Climate Change Suspected
July 29, 2007
BOULDER—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
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 London.
"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 and 1995.
"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 Pacific Ocean.
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.
About the article
Title: "Heightened Tropical Cyclone
Activity in the North Atlantic: Natural Variability or Climate Trend?"
Authors: Greg J. Holland and Peter J. Webster
Publication: Philosophical Transactions of the Royal Society of London |
