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Hurricanes to Unleash Dormant, Hidden Power
By Ker Than
LiveScience Staff Writer
posted: 31 May 2006
01:07 am ET

Kerry Emanuel sparked a debate among his colleagues last year when he published a paper that linked global warming to the trend of increasingly stronger Atlantic Ocean hurricanes observed in recent decades.

In a study to be published soon, the Massachusetts Institute of Technology climatologist will make another bold claim: The cycling of hurricane activity from high to low, which some scientists have attributed to a natural cycle in global weather patterns, is in fact caused by the rise and fall of pollution released by humans.

Furthermore, Emanuel, along with Michael Mann of Pennsylvania State University, contend that the microscopic aerosol particles, which reflect sunlight and cool the atmosphere, have been masking the effect of global warming on Atlantic Ocean hurricanes for several decades. The researchers say that it is only in recent decades, as aerosol emissions from North America and Europe have declined due to clean air standards, that the full impact of greenhouse gas emissions on hurricane strength has been realized.

Meanwhile, other new research by Purdue University scientists supports Emanuel's original finding and extends it to the entire globe.

Together, the two new studies suggest that hurricanes, known as cyclones elsewhere, are getting stronger all over the planet and that humans play a role in the change.

Stronger cyclones worldwide

Research done by Matthew Huber and Ryan Sriver at Purdue University in Indiana independently verifies and expands upon Emanuel's 2005 study, which showed that hurricanes in the Atlantic and Pacific Oceans had increased in duration and intensity by about 50 percent since the 1970s. Emanuel linked the trend to rising sea surface temperatures, or SSTs, caused in part by global warming.

"We used a different technique and different data than Dr. Emanuel, who looked specifically at the Atlantic and western Pacific oceans, whereas we looked at the entire world," Huber said. "Nevertheless, we got the same results that he did, the same basic trends."

The researchers used surface wind and temperature records from the European Centre for Medium-Range Weather Forecasts 40 Year Reanalysis Project to estimate the total wind output of tropical cyclones worldwide from 1958 to 2001. Called the "globally integrated tropical cyclone power dissipation," this value represents the potential damage that a storm can cause.

The Purdue study marks the first time this value has been calculated on a global scale. It found that tropical cyclone activity has doubled over the past 40 years with only a quarter degree Celsius of tropical ocean warming. This is cause for concern, the researchers say, because scientists expect a two-degree warming over the course of the next century.

"The signal that we looked at is a measure of not only the intensity but also the duration of the storm," Sriver told LiveScience. "What we've seen is an increase in strength and duration but not necessarily in the number of storms."

Huber and Sriver's study will be published in an upcoming issue of the journal Geophysical Research Letters.

Polluting the issue

Although Huber and Sriver's study did not examine whether the increase in cyclone activity was due to human-caused global warming, a number of recent studies suggest that this is the case.

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