Millennium Ark: Hot News

by David Terraso
Georgia Tech Univ.

THE NUMBER OF CATEGORY 4 AND 5 hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes has dropped since the 1990s, new research shows.

Chart: This chart shows the increase in Category 4 and 5 hurricanes worldwide during the past 35 years.

The shift occurred as global sea-surface temperatures have increased over the same period. The research by the Georgia Institute of Technology and the National Center for Atmospheric Research (NCAR) appeared in the Sept. 16, 2005, issue of the journal Science.

Peter Webster, a professor at Georgia Tech’s School of Earth and Atmospheric Sciences, along with NCAR’s Greg Holland and Georgia Tech’s Professor Judith Curry and senior research scientist Hai-Ru Chang, studied the number, duration and intensity of hurricanes (also known as typhoons or tropical cyclones) that occurred worldwide from 1970 to 2004. The National Science Foundation (NSF) sponsored the research.

“What we found was rather astonishing,” Webster says. “In the 1970s, there was an average of about 10 Category 4 and 5 hurricanes per year globally. Since 1990, the number of Category 4 and 5 hurricanes has almost doubled, averaging 18 per year globally.”

Category 4 hurricanes have sustained winds from 131 to 155 mph; Category 5 systems, such as Hurricane Katrina at its peak over the Gulf of Mexico, feature winds of 156 mph or more.

“Category 4 and 5 storms are also making up a larger share of the total number of hurricanes,” says Curry, chair of the Georgia Tech School of Earth and Atmospheric Sciences. “Category 4 and 5 hurricanes made up about 20 percent of all hurricanes in the 1970s, but over the last decade, they account for about 35 percent of these storms.”

The largest increases in the number of intense hurricanes occurred in the North Pacific, Southwest Pacific, and the North and South Indian Oceans, with slightly smaller increases in the North Atlantic Ocean.

All this happened as sea-surface temperatures rose across the globe – ranging from one-half to one degree Fahrenheit, depending on the region – from 1970 to 2004.

“Our work is consistent with the concept that there is a relationship between increasing sea-surface temperature and hurricane intensity,” Webster says. “However, it’s not a simple relationship. In fact, it’s difficult to explain why the total number of hurricanes and their longevity has decreased during the last decade when sea-surface temperatures have risen the most.”

The only region that is experiencing more hurricanes overall is the North Atlantic, where the storms have become more numerous and longer-lasting, especially since 1995. The North Atlantic has averaged eight to nine hurricanes per year in the past decade, compared to the six to seven per year before the increase. Category 4 and 5 hurricanes in the North Atlantic have increased at an even faster clip – from 16 during the 1975 to 1989 timeframe to 25 from 1990 to 2004, a rise of 56 percent.

“This long period of sustained intensity change provides an excellent basis for further work to understand and predict the potential responses of tropical cyclones to changing environmental conditions,” says NCAR's Holland.

A study published in July 2005 in the journal Nature yielded conclusions similar to the Georgia Tech and NCAR study. Focusing on North Atlantic and North Pacific hurricanes, Professor Kerry Emanuel of the Massachusetts Institute of Technology found an increase in their duration and power, although it used a different measurement to determine a storm’s power.

But whether this trend stems from human-induced global warming is still uncertain, Webster says. “We need a longer data record of hurricane statistics, and we need to understand more about the role hurricanes play in regulating the heat balance and circulation in the atmosphere and oceans.”

Jay Fein, director of NSF’s climate and large-scale dynamics program, which funded the Georgia Tech and NCAR research, says: “Basic physical reasoning and climate model simulations and projections motivated this study. These results will stimulate further research into the complex natural and anthropogenic processes influencing these tropical cyclone trends and characteristics.”

At NCAR, Holland and his colleagues are conducting a series of computer experiments capable of resolving thunderstorms and the details of tropical cyclones. “The results will help explain the observed intensity changes and extend them to realistic climate change scenarios,” Holland explains.

In ongoing research at Georgia Tech, Webster is trying to determine the basic role of hurricanes in the climate of the planet.

“The thing they do more than anything is cool the oceans by evaporating the water and then redistributing the oceans’ tropical heat to higher latitudes,” he says. “But we don’t know a lot about how evaporation from the oceans’ surface works when the winds get up to around 100 mph, as they do in hurricanes.”

This physical understanding will be crucial to connecting trends in hurricane intensity to overall climate change, Webster explains.

“If we can understand why the world sees about 85 named storms a year and not, for example, 200 or 25, then we might be able to say that what we’re seeing is consistent with what we'd expect in a global warming scenario,” he says. “Without this understanding, a forecast of the number and intensity of tropical storms in a future, warmer world would be merely statistical extrapolation.”

CONTACTS:
Peter Webster at 404-894-1748 or pw83@mail.gatech.edu
Judith Curry at 404-894-3948 or judith.curry@eas.gatech.edu

http://gtresearchnews.gatech.edu/reshor/rh-f05/k-brew.html