“Because hurricanes form over warm ocean water, it is easy to assume that the recent rise in their number and ferocity is the result of global warming.
But that is not the case, scientists say. Instead, the severity of hurricane seasons changes with Atlantic Ocean temperatures in cycles that take several decades. The recent onslaught “is very much natural,” said William Gray, a professor of atmospheric science at Colorado State University who issues forecasts for the hurricane season.
From 1970 to 1994, the Atlantic was relatively quiet, with 38 major hurricanes, but no more than three ofthem in any year and none at all in three of those years. Cooler water in the North Atlantic strengthened wind shear, which tends to tear storms apart before they turn into hurricanes.
In 1995, hurricane patterns reverted to the active mode of the 1950s and ’60s. From 1995 to 2003, there were 32 major hurricanes, with sustained winds of 111 miles an hour, or 178 kilometers an hour, or more, in the Atlantic. It was chance, Gray said, that only three of them struck the United States at full strength.
Historically, the proportion hitting the United States has been 1 in 3. Then last year, three major hurricanes, half of the six that formed during the season, hit the United States. A fourth, Frances, weakened before striking Florida.
of the six that formed during the season, hit the United States. A fourth, Frances, weakened before striking Florida.
“We were very lucky in that eight-year period, and the luck just ran out,” Gray said. “We always thought it would run out.”
In an article this month in the journal Nature, Kerry Emanuel of the Massachusetts Institute of Technology, wrote that global warming might already have had some effect. But he added, “What we see in the Atlantic is mostly the natural swing.”
“– Hurricane Katrina’s fury has reignited the scientific debate over whether global warming might be making hurricanes more ferocious.
At least one prominent study suggests that hurricanes have become significantly stronger in the past few decades during the same period that global average temperatures have increased. Katrina blew up in the Gulf of Mexico to a Category 5 hurricane with winds of 175 mph before slackening a bit Monday when it hit, swamping New Orleans and the Mississippi coast.
Other leading scientists agree the Atlantic Basin and Gulf Coast regions are being battered by a severe hurricane phase that could persist for another 20 years or more. But they believe that a natural environmental cycle is responsible rather than any human-induced change, and they point to what they consider to be large gaps in the global warming analysis conducted by a climatologist at the Massachusetts Institute of Technology.
Roger Pielke Jr., who studies the social impacts of natural disasters and climate change at the University of Colorado, said any link between the intensity of Katrina and other recent hurricanes and global warming is “premature.” Most forecasts suggest climate change would increase hurricane wind speeds by 5 percent or less later in this century.
Pielke’s analysis will be published later this year in the Bulletin of the American Meteorological Society.
“There are good reasons to expect that any conclusive connection between global warming and hurricanes or their impacts will not be made in the near term,” he said.
In August, MIT climatologist Kerry Emanuel reported in the journal Nature that major storms spinning in both the Atlantic and the Pacific have increased in duration and intensity by about 50 percent since the 1970s. During that period, global average temperatures have risen by about one degree Fahrenheit along with increases in the level of carbon dioxide and other heat-trapping pollutants from industry smokestacks, traffic exhaust and other sources.
Hurricanes rely on huge pools of warm water at the surface of the ocean to grow for several days. As trade winds spin the storm, it pulls more heat from the ocean and uses it as fuel. Typically, large storms require sea surface temperatures of at least 81 F.
Scientists say rising global atmospheric temperatures have been slowly raising ocean temperatures, although they still vary widely from year to year.
On Web logs, scientists and environmentalists in the United States and Europe sparred over the possible connection.
The evidence linking global warming and hurricane intensity might be fuzzy, but it highlights a potential issue worth examining right away, some say.
“Maybe a connection here is yet to be clearly established, but it is also yet to be ruled out,” said Terry Richardson, a physicist at the College of Charleston in South Carolina on CCNet, a British climate blog.
Pielke and other researchers, including meteorologists at federal weather laboratories, say Emanuel’s evidence is too slim at this point.
The past 10 years have been the most active hurricane seasons on record, and many researchers say the trend could persist for another 20 years or more. They believe it’s a consequence of natural salinity and temperature change in the Atlantic’s deep current circulation elements that shift back and forth every 40-60 years.”
“Sir David King, the British Government’s chief scientific adviser, has warned that global warming may be responsible for the devastation reaped by Hurricane Katrina.
“The increased intensity of hurricanes is associated with global warming,” Professor King told Channel 4 News yesterday. “We have known since 1987 the intensity of hurricanes is related to surface sea temperature and we know that, over the last 15 to 20 years, surface sea temperatures in these regions have increased by half a degree centigrade.
“So it is easy to conclude that the increased intensity of hurricanes is associated with global warming.”