BOULDER -- The number of Category 4 and 5 hurricanes worldwide hasnearly doubled over the past 35 years, even though the total number ofhurricanes has dropped since the 1990s, according to a study byresearchers at the Georgia Institute of Technology and the NationalCenter for Atmospheric Research (NCAR). The shift occurred as globalsea surface temperatures have increased over the same period. Theresearch appears in the September 16 issue of Science.
Peter Webster, professor at Georgia Tech's School of Earth andAtmospheric Sciences, along with NCAR's Greg Holland and Georgia Tech'sJudith Curry and Hai-Ru Chang, studied the number, duration, andintensity of hurricanes (also known as typhoons or tropical cyclones)that have occurred worldwide from 1970 to 2004. The study was supportedby the National Science Foundation (NSF), NCAR's primary sponsor.
"What we found was rather astonishing," said Webster. "In the 1970s,there was an average of about 10 Category 4 and 5 hurricanes per yearglobally. Since 1990, the number of Category 4 and 5 hurricanes hasalmost doubled, averaging 18 per year globally."
Category 4 hurricanes have sustained winds from 131 to 155 miles perhour; Category 5 systems, such as Hurricane Katrina at its peak overthe Gulf of Mexico, feature winds of 156 mph or more.
"This long period of sustained intensity change provides anexcellent basis for further work to understand and predict thepotential responses of tropical cyclones to changing environmentalconditions", said NCAR's Holland.
"Category 4 and 5 storms are also making up a larger share of thetotal number of hurricanes," said Curry, chair of the School of Earthand Atmospheric Sciences at Georgia Tech and coauthor of the study."Category 4 and 5 hurricanes made up about 20% of all hurricanes in the1970s, but over the last decade they accounted for about 35% of thesestorms."
The largest increases in the number of intense hurricanes occurredin the North Pacific, Southwest Pacific, and the North and South IndianOceans, with slightly smaller increases in the North Atlantic Ocean.
All this is happening as sea surface temperatures have risen acrossthe globe between one-half and one degree Fahrenheit, depending on theregion, for hurricane seasons since the 1970s.
"Our work is consistent with the concept that there is arelationship between increasing sea surface temperature and hurricaneintensity," said Webster. "However, it's not a simple relationship. Infact, it's difficult to explain why the total number of hurricanes andtheir longevity has decreased during the last decade, when sea surfacetemperatures have risen the most."
"NCAR is now embarking on a focused series of computer experimentscapable of resolving thunderstorms and the details of tropicalcyclones," said Holland. "The results will help explain the observedintensity changes and extend them to realistic climate changescenarios."
The only region that is experiencing more hurricanes and tropicalcyclones overall is the North Atlantic, where they have become morenumerous and longer-lasting, especially since 1995. The North Atlantichas averaged eight to nine hurricanes per year in the last decade,compared to six to seven per year before the increase. Category 4 and 5hurricanes in the North Atlantic have increased at an even faster clip:from 16 in the period of 1975-89 to 25 in the period of 1990-2004, arise of 56%.
A study published in July in the journal Nature came to a similarconclusion. Focusing on North Atlantic and North Pacific hurricanes,Kerry Emanuel (Massachusetts Institute of Technology) found an increasein their duration and power, although his study used a differentmeasurement to determine a storm's power.
But whether all of this is due to human-induced global warming isstill uncertain, said Webster. "We need a longer data record ofhurricane statistics, and we need to understand more about the rolehurricanes play in regulating the heat balance and circulation in theatmosphere and oceans."
"Basic physical reasoning and climate model simulations andprojections motivated this study," said Jay Fein, director of NSF'sclimate and large scale dynamics program, which funded the research."These results will stimulate further research into the complex naturaland anthropogenic processes influencing these tropical cyclone trendsand characteristics."
Webster is currently attempting to determine the basic role ofhurricanes in the climate of the planet. "The thing they do more thananything is cool the oceans by evaporating the water and thenredistributing the oceans' tropical heat to higher latitudes," he said.
"But we don't know a lot about how evaporation from the oceansurface works when the winds get up to around 100 miles per hour, asthey do in hurricanes," said Webster, who adds that this physicalunderstanding will be crucial to connecting trends in hurricaneintensity to overall climate change.
"If we can understand why the world sees about 85 named storms ayear and not, for example, 200 or 25, then we might be able to say thatwhat we're seeing is consistent with what we'd expect in a globalwarming scenario. Without this understanding, a forecast of the numberand intensity of tropical storms in a future warmer world would bemerely statistical extrapolation."
NCAR Tip Sheet: Hurricane Prediction, Behavior, and Impacts -- http://www.ucar.edu/news/releases/2005/hurricanetips.shtml
Georgia Institute of Technology - School of Earth and Atmospheric Sciences -- http://www.eas.gatech.edu/
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