Global Warming Storm Link Probed : Predicted Rise in Ocean Temperatures May Increase Winds
Hurricane Hugo is a harbinger of the type of high-intensity storms that can be expected more often in the future due to the greenhouse effect and the resulting global warming, researchers said Tuesday.
Warming ocean and air temperatures will feed extra energy into the hurricanes, increasing their wind speeds by 20% to 25% and their maximum intensity by as much as 60%, according to computer models.
“We can say with some confidence that the frequency of Category 4 and Category 5 hurricanes (the most severe) will increase, but we can’t say by how much,” said meteorologist Richard Anthes, head of the university consortium that operates the National Center for Atmospheric Research in Boulder, Colo.
Gilbert Sets Record
In fact, the most intense hurricane ever observed in the Atlantic Ocean, Hurricane Gilbert with wind speeds of more than 200 m.p.h., occurred last September. But neither Gilbert nor Hugo can be definitely linked to global warming, researchers noted. “It’s impossible to link one event to climatic change,” Anthes said.
Researchers are generally cautious in their predictions about the effects of global warming.
“It’s difficult to take something as complex as a hurricane and attribute changes to a single cause like warmer temperatures, simply because the atmosphere is such a complex mixture and there are so many physical processes going on,” said Ohio State University atmospheric scientist Jay Hobgood, one of the researchers whose models are the basis for projections of hurricane strengths.
“There’s an awful lot of uncertainty and an awful lot that is not understood,” added meteorologist Kerry Emanuel of the Massachusetts Institute of Technology, another modeler. “But if ocean temperature goes up, the intensity of extreme hurricanes will go up.”
‘Seat-of-the-Pants Feeling’
And, said meteorologist Rainer Bleck of the University of Miami, there is a growing consensus among researchers that this is what will happen. But “that conclusion is not based on scientific results yet,” he added. “It is more of a seat-of-the-pants feeling.”
The link to global warming occurs because heat is the engine that drives hurricanes. Strong sunlight heats both air and water near the Equator, causing a greater evaporation of moisture from the ocean. The warm, moist air expands and rises, creating a ring of low pressure and storms around the Equator known as the Intertropical Convergence Zone.
As the moist air rises, it cools, causing its moisture to condense into clouds. But that condensation releases a tremendous amount of heat stored in the gaseous water, accelerating the heating and rising of the air.
Starts With ‘Kicker’
Although scientists do not yet know under precisely what conditions hurricanes form, this condensation and heat release is the “kicker” that starts vast amounts of air climbing upward, drawing in ever-larger quantities of moist air from the ocean surface, Bleck said.
Eventually, the Earth’s rotation causes the fast-moving air masses to spin in a counterclockwise motion in the Northern Hemisphere or clockwise in the Southern.
Hurricanes typically take five to six days to form, eventually spreading over an area of thousands of square miles and developing wind speeds that range from 75 m.p.h. (the official minimum for a hurricane) to more than 200 m.p.h. The storms continue as long as they can draw warm moist air from the ocean. Hurricanes peter out when they pass over cold water or land.
Gases Trap Heat
Climatologists predict that global warming--caused by the accumulation of carbon dioxide and certain other gases in the atmosphere, which trap the Earth’s heat like the panes of a greenhouse--will cause both ocean and air temperatures to increase, perhaps eventually by as much as 10 degrees Fahrenheit.
A warmer ocean causes more water to evaporate, and warmer air can hold more water vapor. Both factors would increase the strength of hurricanes.
“That’s what you see in reality,” Hobgood said. “If you have warmer air, warmer water, you do get a more intense storm.”
‘Need More Research’
The problem, he added, is that warming will not occur to the same extent everywhere, and other conditions caused by global warming, such as stronger winds at high altitudes, might mitigate some of the effects of heating. “We need to do a lot more research,” he said.
The researchers did have one piece of good news. Hurricane Hugo will probably sap the strength of tropical depression Iris, which is following close behind it. It will probably also push Iris northward into the Atlantic, keeping it from striking the United States or islands in the Caribbean.
