Despite a number of conflicting research findings, the general consensus among weather and climate researchers is that global warming, whether natural or man-made, is unlikely to increase the frequency of hurricanes in the years to come.

In consensus statements found on the Web site of the Geophysical Fluid Dynamics Laboratory (GFDL), part of the National Oceanic and Atmospheric Administration (NOAA), scientists involved noted, "Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point. No individual tropical cyclone can be directly attributed to climate change."

Indeed, according to Stanley Goldenberg, meteorologist with the Hurricane Research Division of NOAA, based in Miami, "Numerous hurricane meteorologists agree that the historical data has not produced any evidence of changes [due to climate change] in the number or intensity of hurricanes, particularly in the Atlantic Basin, and even globally.'

"There are some who have done studies that do claim a link, [but] virtually all those studies have been heavily rebutted by others in the hurricane community," he noted. "In my opinion, the flaw in those studies is an improper utilization of historical databases. I have been a specialist in hurricane climate data for close to three decades, and others who know the databases well agree with what I am saying."

Mr. Goldenberg pointed to a number of confounding problems in such studies, including the timeframe chosen, the techniques available now and in the past to measure hurricane activity, the ways in which such activity was recorded, and the availability of satellite data--or lack thereof.

"The biggest fallacy is that people think that a hurricane feeds off a warm ocean, and if the ocean gets warmer, we will have more intense hurricanes," he explained. "But there are other factors involved, such as vertical wind shear, which is the difference between the upper and lower layers of the atmosphere. You could also have drier air. These are far more critical factors than the ocean being warmer."

"Everything else being equal, if you warm the ocean under a storm, you might get a stronger storm--but everything else is not equal," said Mr. Goldenberg. "Warming may increase vertical shear and therefore inhibit storms. The ocean itself warming is such a little effect."

He added that while many of today's forecasts for future climate and hurricane activity are based on computer climate models, "We can't even reliably forecast El Ni?o. When you look 50 years into the future, you're getting a picture, but could you be totally wrong? Yes!"

Some climate models, he noted, say that if there is continued warming, higher vertical shear would reduce the frequency of hurricanes but might result in stronger storms. "I am skeptical of those who would state those results as something like an undeniable fact for the future," he stated. "It is a possible guide for the future. But certainly the models are not pointing to increased activity.

"I did not say if there is global warming, it would be man-made," Mr. Goldenberg emphasized. "Not all scientists agree that the warming we've seen is necessarily anthropogenic. It is a blatant lie put forth in the media that makes it seem there is only a fringe of scientists who don't buy into anthropogenic global warming."

According to Peter Dailey, director of Atmospheric Science at AIR Worldwide, based in Boston, "There is now a near consensus that global air temperatures are increasing; however, there is no consensus on how this has affected the temperature of the world's oceans, and in particular in the Atlantic Ocean, or how much of the recent warming trend is attributable to man's activities."

"This is critical for scientists to understand the impact of climate change on land-falling hurricanes that affect North America," he said.

"Recent scientific research indicates that in a future warming world, the Atlantic may experience two primary effects related to hurricane development," he explained. "First, a warmer environment may continue to elevate sea surface temperatures (SSTs), thereby providing more fuel for tropical cyclones to develop and intensify. Second, there may be a trend for more frequent or more intense El Ni?o events which in turn increase wind shear in the Atlantic--an unfavorable environment for tropical cyclones to develop.

"So while it is true that warmer SSTs may lead to more frequent hurricane activity, elevated wind shear may counteract, or possibly even overturn this effect," he continued. "Which of these factors critical to the development of tropical cyclones will ultimately win out is the subject of lively debate within the scientific community and beyond."

Mr. Dailey also reported that recently published studies indicate that hurricane activity could decrease as a result of other competing factors. "For example, simulations of tropical cyclone activity carried out at the GFDL using climate conditions projected for the 21st century indicate the potential for decreased hurricane activity under more pronounced global warming conditions and caution against a reliance on statistical extrapolations of recently elevated activity levels through the end of the century," he said.

Although she asserted that global warming "is driven primarily by human activities," Christine Ziehmann, director of model management for Risk Management Solutions in Newark, Calif., cautioned that, "It is not clear what effect global warming is having, and will have, on the frequency, intensity and geographical distribution of hurricanes in the Atlantic basin."

According to Ms. Ziehmann, computer models of the global climate tend to suggest that global warming should, in the long term, lead to less frequent but more intense tropical cyclones globally. "However, models are less clear about hurricane activity in individual ocean basins," she noted. "For instance, in the Atlantic, some models suggest a long-term increase in frequency, with others suggesting a decrease."

She added, "The evidence shows that there has been an increase in the average intensity of hurricanes in the Atlantic since the 1970s and an increase in frequency since 1995, but it is not clear what contribution, if any, global warming has made to these changes."

In an interview in 2001, renowned hurricane researcher Prof. William Gray told National Underwriter that a complex combination of factors including ocean temperature, ocean currents and the salt content of the ocean at a given time is most likely to affect the frequency of hurricanes. He emphasized that changes in these factors are "natural" and not man-made.

"Professor Gray is absolutely correct that hurricane formation and development depend on factors such as sea surface temperature," said Ms. Ziehmann. "However, it would be wrong to suggest that these factors could only be affected by natural changes in climate and not by man-made global warming.

"Both natural climate variability and man-made global warming influence hurricane formation and development," she stated. "The real question is to what extent they contribute over the time-scale of interest. This is still an open scientific question."

The researchers also addressed the political debate that has attached itself to aspects of climate change.

"For the layman, there is sometimes a tendency to regard every new 'discovery' or scientific finding from the latest published paper as an inviolate fact," said Mr. Dailey. "In reality, rarely is there ever a last and final word in studies of complex systems such as earth's environment. Rather, science is a dynamic process based on the scientific method in which researchers test hypotheses leading to new discoveries but also reexamine earlier theories and try to improve, build upon, or extend them."

Mr. Goldenberg of NOAA added, "There are those who want to attribute any perceived increase in natural disasters to anthropogenic global warming. I predict that if we have an active hurricane season, someone will attribute it to AGW. They're not really looking at the science; they're looking at the disaster."

Special Reports /

Global Warming Not Linked To Increased Hurricane Activity

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