Warming Oceans Are Fueling Stronger Hurricanes, Study Finds
for National Geographic News
March 16, 2006
Rising ocean surface temperatures are the primary factor fueling a 35- year trend of stronger, more intense hurricanes, scientists report in a new study.
The finding backs up the results of two controversial papers published last year that linked increasing hurricane intensity to rising sea-surface temperatures, said Judith Curry, an atmospheric scientist at the Georgia Institute of Technology in Atlanta.
"Global warming is sending sea-surface temperatures up, so we're looking at an increase in hurricane intensity globally," Curry said.
She added that in the North Atlantic Ocean basin—where hurricanes that affect the U.S. form—the number of hurricanes may also increase.
"Other ocean basins don't show an increase in [the] number [of hurricanes], but the North Atlantic does," she said.
Curry is a co-author of the new study, which appears in tomorrow's issue of the journal Science.
She also co-authored a study published last September in Science, that found the yearly number of hurricanes that reach Category Four and Five—the strongest storms on the hurricane intensity scale—has doubled since 1970.
This finding coincides with a 1°F (0.5°C) rise in global sea-surface temperature over the same time period.
Not everyone is convinced by the new study.
For example, Christopher Landsea, a researcher with the National Hurricane Center in Miami, Florida, is uncertain whether the global trend toward stronger hurricanes is real.
"We look at hurricanes a lot differently today than we did in the early 1970s," he said.
Storms that were recorded 30 years ago as weak may actually have been much stronger, according to Landsea.
The ocean basin with the best historical record, the North Atlantic, shows the smallest increase in stronger storms. And that increase, Landsea said, can be attributed to natural variability.
The Atlantic was stormy in the 1940s, '50s, and '60s; quieted down through the mid-1990s; and is now active again. The current study only looks at hurricane activity since 1970—a relatively quiet period.
Landsea also noted that an uptick in hurricane intensity due to warmer sea-surface temperatures does not match the predictions of scientific models.
According to those models, by the end of the 21st century sea-surface temperatures are expected to rise 3 to 4°F (1.7 to 2.2°C), corresponding with a 5 percent increase in hurricane intensity.
Today's 1°F (0.5°C) change in ocean temperatures should correspond to about a one percent increase in hurricane strength, which is too small for modern instruments to detect, according to Landsea.
"So either the theory is wrong, which is possible, or the data is poor or inaccurate, or some combination of the two," he said.
Curry said that although concerns about the data are valid, "in order for our conclusions to be wrong, 50 percent of the Category One and Two storms in the 1970s would have been misclassified and [actually] been Category Four. Nobody thinks the data is that bad."
According to Curry, the link between sea-surface temperature and increasing hurricane intensity was apparent in data from earlier studies.
But "we didn't do the nitty-gritty statistical and data analysis to really nail down the link," she said.
Some hurricane forecasters, including Landsea, questioned whether other factors, such as wind shear, might be driving the trend.
Decreased wind shear—upper-level winds that can disrupt hurricane formation—would lead to more and stronger hurricanes.
Using statistical analysis, the new study shows that even in ocean basins where reduced wind shear plays a role, warming sea-surface temperature is the dominant driver.
"This new study really nails down that link," Curry said.
But the lack of wind shear influence still troubles Landsea.
"That doesn't fit my physical concept of how these things work," he said. "If you really get that many more Category Four and Five [storms], wind shear would have to go down."
Roger Pielke, Jr., is the director of the Center for Science and Technology Policy Research at the University of Colorado at Boulder.
He says the new study adds weight to findings of a link between warmer oceans and hurricane intensity.
He cautioned, however, that burning oil and coal to drive our cars and heat our homes—which releases greenhouse gases into the atmosphere—is not to blame for an increase in the damage done by hurricanes.
"It's important to recognize that [storm] damage is going to increase going forward, no matter what happens to sea-surface temperatures or hurricanes, as more people move to vulnerable locations on the coastline," he said.
"If hurricanes do become more intense than they have in the past, then that would be an additional factor," he added.