Global warming may strengthen hurricanes

Friday, Sept. 23, 2005 | 9:59 a.m.

Global climate change may not increase the number of hurricanes, but a growing body of evidence suggests that the strength of those storms may be growing because of the warming that scientists have tracked over the last 30 years, climate researchers said at a Las Vegas conference Thursday.

The public's focus has been on the Atlantic and Gulf coasts, which were slammed by Hurricane Katrina on Aug. 29 and now Hurricane Rita today. Climatologists who attended the conference on Climate Change in the West, however, warned that the tropical cyclones that boil up in the summer and fall can affect the weather in the Southwest.

The researchers referred to two studies published in respected scientific journals this month, one in Nature and the other in Science, that said tropical cyclones have been intensifying, and that the trend could be correlated with warmer ocean temperatures. The two journals are highly competitive and are considered among the best scientific journals in the world.

Marty Hoerling, a research meteorologist at the National Oceanic and Atmospheric Administration climate center in Boulder, Colo., said the West can be impacted in two ways by tropical systems. Some storms move through the Gulf of Mexico, across the Mexican plateau and can contribute to summer monsoons in the desert Southwest and Rocky Mountains.

Hurricane and other tropical systems in the Eastern Pacific also can impact weather and precipitation in the West by sending plumes of moisture up through California. Both types of systems bear watching, he said.

"The numbers may not be growing, but they seem to be growing in intensity," Hoerling said.

Kelly Redmond, deputy director of the Western Regional Climate Center in Reno's Desert Research Institute, noted that both Atlantic and Pacific oceans have had active tropical cyclone seasons, with three systems now in the Eastern Pacific.

Pacific storms are "most relevant to the Southwest," he said. "They can cause some pretty heavy precipitation, sort of an extended monsoon season."

Hoerling said tropical cyclones are complex systems, and some factors that contribute to storm development or intensity can also degrade them with wind shear or other mechanisms, he said.

Nonetheless, "We have a warmer ocean. We have more energy for storms."

He noted that the research published this month focuses on the observed record of tropical storms. "It doesn't project into the future."

If the results hold true going forward, however, it could be very bad news for those closest to the storms. Hoerling said that an increase from a Category 2, which tops out at 110 mph, to a Category 4 storm, which begins at 131 mph, doesn't sound like much to most people -- unless they experience the difference personally.

"Even a small increase in the number of Category 4 or 5 storms means a huge increase in the amount of damage," Hoerling said.

"What we see in the record on hurricanes is no clear trend in the number of storms, but a real clear trend in terms of intensity," said Dan Lashof, science director for the Natural Resources Defense Council's climate research center.

The NRDC, which co-sponsored the climate conference with the Southern Nevada Water Authority and the Desert Research Institute, is an environmental group that has argued for steps to control carbon dioxide emissions.

Lashof said the trend toward storm intensification, like that of global warming, has become increasingly clear over the last 30 years.

"You're seeing strong storms, and longer-lasting storms, and those are very strongly correlated with warmer ocean temperatures" he said.

Lashof said that like the issue of human-caused global warming itself, the conclusion that the warming will have an impact on hurricanes is growing within the business and regulatory communities.

"This is pretty new," he said. "It's been the last three or four years that this has sunk in. It has real implications for the way we do business and the way we live."

He said the insurance industry, particularly, is considering how the trend will change its business.

"On the broader issue of whether the Earth is warming, there's almost no honest doubt anymore," he said.

He said more research needs to be done on the relationship on tropical cyclones, global warming and the impact on regional climates. Redmond said the primary driver in the formation of such storms is not just warm ocean temperatures, but the difference in temperature between the tropics and more northern latitudes.

Global warming might play a role, but Redmond cautioned that researchers believe the frequency of storms has a lot to do with decadeslong natural cycles.

"We have to be very careful in this business that we don't confuse long-term variations with climate change," he said.

The twin Category 5 storms of September, with their "rapid, tremendous rates of deepening wind and pressure, it's just incredible," provide anecdotal evidence that storms are getting worse, but Redmond noted that this is not the first time an Atlantic hurricane season has seen two Category 5 storms.

In recorded history, The Atlantic has seen two such storms in a single season twice before: in 1960 and 1961.

Michael Dettinger, a research hydrologist for the U.S. Geological Survey and a researcher at the Scripps Institute of Oceanography in La Jolla, Calif., said the fact that the frequency of hurricanes seems to go up and down in wide arcs has been confirmed through observation, but the evidence of intensification of storms due to global warming is much more recent.

He said measuring the intensity of the storm as opposed to the number of storms is a relatively new way of gauging a season.

"We're all scrambling to decide whether this new measure is in fact the right way to look at this," Dettinger said.

The storm season of human tragedy and catastrophic property damage has provided one benefit, he said. It has shown that "environmental catastrophes have to be taken very seriously."

"There are lessons to be learned. If these are what hurricanes are going to look like in the future, we have to deal with that.