Warmer seas creating stronger hurricanes

By Ker Than

updated 2:39 p.m. ET March 17, 2006

A rise in the world's sea surface temperatures was the primary contributor to the formation of stronger hurricanes since 1970, a new study reports.

While the question of what role, if any, humans have had in all this is still a matter of intense debate, most scientists agree that stronger storms are likely to be the norm in future hurricane seasons.

The study is detailed in the March 17 issue of the journal Science.

In the 1970s, the average number of intense Category 4 and 5 hurricanes occurring globally was about 10 per year. Since 1990, that number has nearly doubled, averaging about 18 a year.

Category 4 hurricanes have sustained winds from 131 to 155 mph. Category 5 systems, such as Hurricane Katrina at its peak, feature winds of 156 mph or more. Last year, Wilma packed wind speeds of 175 mph and set a record as the strongest hurricane in terms of barometric pressure.

While some scientists believe this trend is just part of natural ocean and atmospheric cycles, others argue that rising sea surface temperatures as a side effect of global warming is the primary culprit.

According to this scenario, warming temperatures heat up the surface of the oceans, increasing evaporation and putting more water vapor into the atmosphere. This in turn provides added fuel for storms as they travel over open oceans.

The researchers used statistical models and techniques from a field of mathematics called information theory to determine factors contributing to hurricane strength from 1970 to 2004 in six of the world's ocean basins, including the North Atlantic, Pacific and Indian oceans.

They looked at four factors that are known to affect hurricane intensity:

• Humidity in the troposphere — the part of the atmosphere stretching from surface of the Earth to about 6 miles up
• Wind shear that can throttle storm formation
• Rising sea-surface temperatures
• Large-scale air circulation patterns known as "zonal stretching deformations"

Of these factors, only rising sea surface temperatures was found to influence hurricane intensity in a statistically significant way over a long-term basis. The other factors affected hurricane activity on short time scales only.

"We found no long-term trend in things like wind shear," said study team member Judith Curry of the Georgia Institute of Technology. "There's a lot of year to year variability but there's no global trend. In any given year, it's different for each ocean."

Answering critics
The new study potentially addresses one major criticism leveled by scientists skeptical of any strong link between sea surface temperatures and hurricane strength, said Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology who was not involved in the study.

Last year, Emanuel published a study correlating the documented increase in hurricane duration and intensity in the Atlantic and Pacific oceans since the 1970s to rises in sea surface temperatures over the same time period.

"We were criticized by the seasonal forecasters for not including the other environmental factors, like wind shear, in our analysis," Emanuel said in an email. "[We didn't do so] because on time scales longer than 2-3 years, these do not seem to matter very much. This paper more or less proves this point."

Kevin Trenberth, the head of climate analysis at the National Center for Atmospheric Research (NCAR), believes the new study's main finding is accurate but thinks the effects of some of the environmental factors on hurricane intensity might have been underestimated.

"The reason is they're covering a period from 1970 to 2004. 1979 is the year when satellites were introduced into the [NCEP/NCAR] Reanalysis. The quality of the analysis prior to 1979 is simply nowhere near as good," said Trenberth, who also was not involved in the study.

The NCEP/NCAR Reanalysis is the database the researchers drew upon for information about the effects of troposphere humidity, wind shear and zonal stretching deformation on hurricane intensity; sea surface temperature data came from a different database.

Curry acknowledged that reanalysis data prior to 1979 is of slightly lower quality than more recent data but believes this doesn't substantially change the study's main finding. Trenberth agreed: "I suspect they may well have gotten the right answer anyway," he told LiveScience.

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