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General Interest
Is global warming behind recent hurricanes?
UA scientists say it's not that simple
October 3, 2005
By Stephanie Doster
Fed by warm ocean temperatures and the right set of atmospheric conditions, Hurricane Katrina blitzed the Gulf Coast on August 29, unleashing storm surge waters, 145-mph winds, and debate over whether global warming contributed to the strong Category-4 storm that swept desperation, destruction, and death over the New Orleans area and parts of Mississippi.
While some research suggests a connection between global warming and the recent severe tropical storms in the Atlantic Ocean, scientists at the University of Arizona say it is not that simple, and caution against linking Katrina, or any single hurricane, to climate change.
Katie Hirschboeck, chair of the Global Change Graduate Interdisciplinary PhD Minor Program and associate professor of climatology, said scientific evidence tying hurricane activity to global warming is, thus far, inconclusive.
“The Atlantic has experienced heightened hurricane activity recently and Katrina seems to be part of that,” Hirschboeck said. “But such periods have also been common in the past and the long term record shows this could be part of natural variability.”
Steven Mullen, head of the Department of Atmospheric Sciences, agreed. “For one event, it’s impossible to draw conclusions on whether it’s related to global warming or if it’s some indicator that we’re getting into a more frequent occurrence of intense hurricanes due to some sort of natural cycle,” Mullen said. “If such occurrences are seen to be picking up over the next decade or so, that will start raising a red flag—either there’s a long-term cycle involved or it’s an artifact of global warming.”
Likening hurricane activity to Wall Street, Andrew Comrie, a climatologist at the University of Arizona, said Katrina cannot be considered characteristic of all hurricanes to come. A particular blue chip stock that takes a hit is not necessarily indicative of an overall change in the stock market, he said.
“You can’t compare today’s stock price to this year’s economy,” Comrie said. “But if you have a run of these things over a period of time, you start to wonder what’s going on.”
It might be tempting to link global warming to hurricane activity because hurricanes build over warm tropical oceans. They require sea surface temperatures of at least 80 degrees Fahrenheit and sea surface temperatures are on the rise.
The temperature of surface waters in the North Atlantic tends to fluctuate sluggishly in what is known as the Atlantic Multidecadal Oscillation (AMO), said Julio Betancourt, Adjunct Associate Professor with the Desert Laboratory and project chief for the Water Resources Division at the U.S. Geological Survey.
Relatively cool water at the beginning of the 20th century heated up between the 1920s and 1960s. But during the relatively quiet years of the 1970s to mid-1990s, the surface waters cooled and vertical wind shear cut down tropical storms before they could form into major hurricanes. Betancourt said the recent increase in major hurricane activity is most likely due to a natural shift towards a warmer North Atlantic since 1995, the first year in a recent string of unusually active hurricane seasons.
“While the total number of Atlantic tropical storms doesn’t really change, major hurricanes form more often with North Atlantic warming, in other words during the 1920s to 1960s and since 1995,” Betancourt said.
Last year, Betancourt and Stephen Gray, a plant ecologist with the Desert Laboratory, published a 500-year reconstruction of the AMO using tree-ring chronologies from the southeastern United States, Scandinavia, Europe, and the Middle East. They found that North Atlantic warming periods tend to average 23 years, with a range of 9 to 53 years, while the current period has lasted 10 years, Betancourt said.
A recent study suggesting that the intensity and duration of storms, but not the frequency, are likely to increase with global warming, over and above AMO-related fluctuations, makes it difficult to discount a climate change component entirely, both Betancourt and Comrie said.
The study, published in the journal Nature (July 31, 2005) by Kerry Emanuel, a professor of Earth, Atmospheric, and Planetary Science at the Massachusetts Institute of Technology, suggests that hurricanes have become notably more powerful in the past 30 years, and correlates the increasing intensity to rising sea temperatures.
Responding to the study, Betancourt said warm AMO periods could amplify the effects of global warming, while cold AMO phases could be sufficient to mask them.
“You can’t attribute Katrina to global warming, but you are loading the dice by increasing temperatures. And you’re probably loading the dice even more during warm AMO periods than during cold AMO periods,” Betancourt said. “It’ll be really interesting to see how hurricanes behave once the North Atlantic shifts back to its cool mode, which is bound to happen. It’s only a matter of time.”
With parts of Texas and Louisiana reeling from the punch of yet another Atlantic hurricane—Rita. some might wonder why the Southwest isn’t also seeing major storms, like the one carried on the winds of Pacific-born Nora in 1997.
John Glueck, a meteorologist with the National Weather Service in Tucson, said there seems to be an inverse relationship between activity in the Atlantic and activity in the Pacific because of certain climate factors. When the Atlantic is busy, the eastern Pacific is quiet, and vice versa, he said.
Tropical storms and depressions that wash over the Southwest tend to be the remnants of eastern Pacific hurricanes that travel south of Cabo San Lucas before they re-curve and are carried by a trough toward northern Mexico and Baja and into the desert Southwest, Glueck said. The most recent hurricane to spin near the Southwest was Hurricane Otis, which forced evacuations along Mexico’s Baja California peninsula before weakening into a tropical depression by October 3. Otis, which continues to crawl off the Mexican coast, was expected to weaken further, according to the National Oceanic and Atmospheric Administration's National Hurricane Center.
The Southwest also “can get residual moisture from storms hitting the southern part of Texas,” Glueck said. “But it won’t be the type or depth it was in the Gulf of Mexico.”