Scientists downplay global warming as cause for surge in hurricanes

FORT LAUDERDALE, Fla. _ As hurricane after hurricane strikes the southeastern United States,… FORT LAUDERDALE, Fla. _ As hurricane after hurricane strikes the southeastern United States, many people wonder whether the rash of storms is the result of global warming.

The answer from scientists: Probably not.

Although many experts think global warming could increase the number or the intensity of hurricanes 50 years from now, they say this year’s storms were caused by natural changes in the ocean and atmosphere. These include a multi-decade cycle of warm water moving through the Atlantic Ocean, and the unusual mildness of the hurricane-suppressing patch of warm water in the Pacific called El Nino.

“This isn’t a global-warming sort of thing,” said Hugh Willoughby, senior scientist at the International Hurricane Research Center of Florida International University. “It’s a natural cycle.”

In a letter last week to Sen. John McCain, R-Ariz., who was holding hearings on global warming, 10 climate researchers urged him to be cautious of “the erroneous claim that hurricane intensity or frequency has risen significantly in recent decades in response to the warming trend seen in surface temperature.”

Even environmental groups, which can be counted on to sound the alarm at any sign of global warming, say it probably isn’t a factor in the current storm season.

“The general consensus is it’s unlikely,” said Benjamin Preston, senior research fellow at the Pew Center on Global Climate Change, which advocates reductions in emissions of carbon dioxide and other heat-trapping pollutants. “We can actually explain an active hurricane season using natural variability.”

No one denies that there have been more hurricanes over the past few years. There have been 76 hurricanes since 1995, the most active period on record, according to the National Oceanic and Atmospheric Administration.

Experts attribute the increase to a two-degree rise in water temperature in the Atlantic that occurs every 25 to 40 years, possibly due to the slow movement of deep warm-water currents. This warmer water diminishes the high-altitude winds that can break up young storms, allowing more of them to strengthen into hurricanes.

But if global warming continues, it could inject a new element into these ancient meteorological cycles. Temperatures around the world have risen about 1 degree Fahrenheit in the past century. International panels of scientists have concluded the main cause was emissions of carbon dioxide and other pollutants from such sources as automobiles and power plants, which trap the sun’s heat in the atmosphere.

While most scientists discount the role of global warming in the current hurricane season, many say rising temperatures could eventually lead to more intense hurricanes, with stronger winds and heavier rain.

A study by scientists with the National Oceanic and Atmospheric Administration and other agencies found that maximum wind speeds could increase an average of 5 to 6 percent if global temperatures rise. More significant, the study found that maximum precipitation could go up by about 34 percent, which “could have important future societal consequences.”

In mountainous countries, hurricane rains can be murderous. In 1998 rains from Hurricane Mitch generated landslides and flashfloods that killed about 10,000 people. This week Hurricane Jeanne swept across Haiti as a tropical storm, causing floods that killed almost 700 people.

Worsening the impact of these stronger hurricanes would be a rise in sea levels, also caused by global warming. The Environmental Protection Agency estimates that sea levels have gone up six to 12 inches in the past 100 years, with the rate of increase accelerating over the past decade. As water gets warmer, it expands. And higher temperatures melt glaciers, a smaller but significant cause of sea-level rise.

Scientists with the U.N.’s Intergovernmental Panel on Climate Change, the leading international body on this issue, wrote that the combination of stronger hurricanes and higher sea levels could lead to “coastal erosion, coastal flooding, saltwater intrusion into freshwater habitats and loss of coastal ecosystems.”

Scientists are less certain whether global warming will lead to more hurricanes. The Intergovernmental Panel on Climate Change said there’s little evidence that a small rise in surface temperatures would lead to more hurricanes.

One reason is that global warming is projected to take place primarily in higher latitudes, nearer the poles than the equator. Second, hurricanes need not only warm water but a wide spread between warm and cold water to generate winds. So even if surface water temperatures increase across the board, the spread may be no bigger than it is now. And some scientists, such as Florida State University Professor Jim O’Brien, think a warmer climate would simply lead to more thunderstorms, which redistribute heat in the tropics.