False Alarm: Hurricanes Are Getting Stronger
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, according to a study by researchers at the Georgia Institute of Technology and the National Center for Atmospheric Research (NCAR). The shift occurred as global sea surface temperatures have increased over the same period. The research appears in the September 16 issue of Science.
Peter Webster, professor at Georgia Tech’s School of Earth and Atmospheric Sciences, along with NCAR’s Greg Holland and Georgia Tech’s Judith Curry and Hai-Ru Chang, studied the number, duration, and intensity of hurricanes (also known as typhoons or tropical cyclones) that have occurred worldwide from 1970 to 2004. The study was supported by the National Science Foundation (NSF), NCAR’s primary sponsor.
“What we found was rather astonishing,” said Webster. “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 miles per hour; Category 5 systems, such as Hurricane Katrina at its peak over the Gulf of Mexico, feature winds of 156 mph or more.
“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”, said NCAR’s Holland.
“Category 4 and 5 storms are also making up a larger share of the total number of hurricanes,” said Curry, chair of the School of Earth and Atmospheric Sciences at Georgia Tech and coauthor of the study. “Category 4 and 5 hurricanes made up about 20% of all hurricanes in the 1970s, but over the last decade they accounted for about 35% 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 is happening as sea surface temperatures have risen across the globe between one-half and one degree Fahrenheit, depending on the region, for hurricane seasons since the 1970s.
“Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity,” said Webster. “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.”
“NCAR is now embarking on a focused series of computer experiments capable of resolving thunderstorms and the details of tropical cyclones,” said Holland. “The results will help explain the observed intensity changes and extend them to realistic climate change scenarios.”
The only region that is experiencing more hurricanes and tropical cyclones overall is the North Atlantic, where they have become more numerous and longer-lasting, especially since 1995. The North Atlantic has averaged eight to nine hurricanes per year in the last decade, compared to 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 in the period of 1975-89 to 25 in the period of 1990-2004, a rise of 56%.
A study published in July in the journal Nature came to a similar conclusion. Focusing on North Atlantic and North Pacific hurricanes, Kerry Emanuel (Massachusetts Institute of Technology) found an increase in their duration and power, although his study used a different measurement to determine a storm’s power. […]
Reality Check: Global warming reducing hurricanes in U.S., report says
Intensifying one of the hottest debates in science, a new report concludes that global warming is diminishing the number of hurricanes that strike Florida and the rest of the United States.
The study found that the planet’s oceans have been warming for more than a century. No surprise there, but this may be: Those warmer oceans are producing stronger crosswinds that tend to suppress the development and growth of hurricanes, the scientists said.
“We found a gentle decrease in the trend of U.S. landfalling hurricanes as global oceans warmed up,” said Chunzai Wang, an oceanographer and climate scientist at the National Oceanic and Atmospheric Administration.
Some previous studies found that global warming was increasing the number and intensity of hurricanes, a conclusion that supported the conventional wisdom that warmer seas automatically turbocharge hurricane development.
The new study, by Wang and Sang-Ki Lee of the University of Miami, will be published today in the journal Geophysical Research Letters. […]
30-year low for N. Hemisphere tropical cyclone activity
The past two years have seen a “remarkable” downturn in hurricane activity, contradicting predictions of more storms, researchers at Florida State University say.
The 2007 and 2008 hurricane seasons had the least tropical activity in the Northern Hemisphere in 30 years, according to Ryan Maue, co-author of a report on Global Tropical Cyclone Activity.
“Even though North Atlantic hurricane activity was expectedly above normal, the Western and Eastern Pacific basins have produced considerably fewer than normal typhoons and hurricanes,” he said.
Maue’s results dovetail with other research suggesting hurricanes are variable and unconnected to global warming predictions, said Stan Goldenberg, a hurricane researcher with the National Oceanic and Atmospheric Administration.
“The simplistic notion that warmer oceans from global warming automatically lead to more frequent and or stronger hurricanes has not been verified,” said Goldenberg, whose research points to periods of high and low hurricane activity that last several decades each. […]