July 2007
Twice as many Atlantic hurricanes
formed each year from 1995 to 2005, on average, than formed during
parallel years a century ago finds a new statistical analysis of
hurricanes and tropical storms in the north Atlantic. The researchers
conclude that warmer sea surface temperatures and altered wind patterns
associated with global climate change are responsible for the increase.
The study, by Greg Holland of the National Center for Atmospheric
Research, NCAR, and Peter Webster of Georgia Institute of Technology, is
published online today by 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.
For the 2007 Atlantic hurricane season, NOAA scientists predict 13 to 17
named storms, with seven to 10 becoming hurricanes, of which three to five
could become major hurricanes of Category 3 strength or higher. An average
Atlantic hurricane season brings 11 named storms, with six becoming
hurricanes, including two major hurricanes.
The analysis by Holland and Webster identifies three periods since 1900,
separated by sharp transitions, during which the average number of
hurricanes and tropical storms increased and then remained elevated and
relatively steady.
The first period, between 1900 and 1930, saw an average of six Atlantic
tropical cyclones each year, 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 last period has not yet stabilized, which means that the average
hurricane season may be even more active in the future.
Holland and Webster say 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 sea surface
temperatures, which have risen by about 1.3 degrees Fahrenheit in the last
100 years, say Holland and Webster.
The changes in sea surface temperatures took place in the years before to
the sharp increases in storm frequency, with an sea surface temperature
rise of 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 sea surface temperatures can be attributed to global warming.
The unusually active hurricane seasons of 2004 and 2005 have prompted
considerable research into the question of whether more intense tropical
cyclones are correlated with natural cycles, global warming, or some other
cause.
According to the 2007 assessment report of the Intergovernmental Panel on
Climate Change, IPCC, on a global scale, "There is no clear trend in the
annual numbers of tropical cyclones."
But considering the North Atlantic apart from the global pattern give a
different picture.
The IPCC wrote, "the frequency of tropical storms has increased
dramatically in the North Atlantic. Reasons for this increase are
currently subject to intense debate among climate scientists."
"At least two recent peer-reviewed scientific studies indicate a
significant statistical link between the increased frequency and global
warming," the IPCC said earlier this year, "but research to identify a
mechanism explaining this link is ongoing."
"All signs that I've seen show that it's related to natural variability,"
said Eric Blake, a meteorologist at the National Hurricane Center, whose
view is supported by some scientists. "There could be some impact of
global warming, but its role is probably a secondary or tertiary role."
In May, the Woods Hole Oceanographic Institution published research
showing that the frequency of intense hurricanes in the Atlantic Ocean
appears to be closely connected to long-term trends in the El
Niņo/Southern Oscillation and the West African monsoon. Geologists Jeff
Donnelly and Jonathan Woodruff made that discovery while assembling the
longest-ever record of hurricane strikes in the Atlantic basin.
The Holland and Webster study indicates that natural cycles are probably
not the entire cause of the increase in hurricane numbers because the
increase has happened across the last century rather than oscillating in
tandem with a natural cycle.
Better weather observations in recent decades cannot account for all of
the increase either, according to Holland and Webster.
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 documented 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 sea surface
temperatures," the authors state.
Holland says, "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."
When it comes to intensity of hurricanes, the IPCC says there has been an
increase in intensity and it is "more likely than not" that there is a
human contribution to the observed trend of hurricane intensification
since the 1970s."
Future hurricanes will be even more intense, the IPCC predicted earlier
this year, saying, "It is likely that future tropical cyclones (typhoons
and hurricanes) will become more intense, with larger peak wind speeds and
more heavy precipitation" associated with ongoing increases of tropical
sea surface temperatures.
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