Hurricanes as powerful as Katrina have become nearly twice as common in the last 35 years as ocean surface temperatures around the world have warmed, according to a team of researchers from Colorado and Georgia.
Their new report is likely to fuel the debate about whether human-caused climate changes are affecting the behavior of powerful tropical storms.
“You can’t point at any one storm and say, ‘Here’s an example global warming has arrived,”‘ said Greg Holland, a climatologist with the National Center for Atmospheric Research in Boulder.
“But we have had quite a major shift toward the more intense cyclones,” said Holland, an author of the study published Thursday in the journal Science.
The trend toward more powerful hurricanes is “consistent” with the theory that human- caused climate change is affecting the violent storms, said Peter Webster, a Georgia Institute of Technology climatologist and lead author.
Webster said he can’t come up with a natural climate system that would account for the global trend his team uncovered. Natural climate swings that affect hurricanes typically occur in just one region at a time, Webster said.
Warm ocean surfaces fuel hurricanes. Since 1970, the surface of almost every ocean on the planet has warmed by about 1 degree Fahrenheit.
“That’s a substantial increase as far as storms are concerned,” Webster said.
The study is the second this year to link ocean temperatures to increased storm fury, measured by wind speeds and duration. In it, scientists categorized every hurricane from tropical oceans since 1970. Category 4 storms have sustained winds of 131 to 155 miles per hour; category 5 storms, like Katrina before landfall, have winds of 156 mph or more.
In the 1970s, about one in five tropical cyclones hit category 4 or 5, Holland said. “Nowadays, one in three becomes a category 4 or 5.”
Roger Pielke Jr., a political scientist at the University of Colorado at Boulder, said despite the new paper, it will take years of additional data to elucidate the connection between global warming and hurricanes.
Staff writer Katy Human can be reached at 303-820-1910 or at khuman@denverpost.com.
