Remember last year's hurricane season? Expect more of the same, scientists say.
After the massive damage wrought by Hurricane Katrina, many began to ask whether humans had any hand in the intensity of the storm through global warming. That debate is ongoing, but new research in the journal Science adds weight to the idea that we may have set ourselves up for more monster storms.
"Last fall you heard a lot about other factors, like wind shear, that are also at play," said Judy Curry, a climate scientist and chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. "But this study nails down the connection between sea surface temperature and the trend toward hurricane intensity."
Curry, along with graduate student Carlos Hoyos and others, used statistical analysis and theory models to isolate the cause of hurricane strength from 1970 to 2004 in six ocean basins, including the North Atlantic, Pacific and Indian oceans. While they considered other variables such as vertical wind shear, higher humidity in the lower atmosphere, and air circulation patterns, they found that only warmer sea surface temperatures had a statistically significant link to the greater occurrence of bigger storms.
What's making sea surfaces in the Tropics warmer? Some say the answer is obvious.
"Call it what you like," said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge, "but it is distinctly man-made in nature. It's hard not to point the finger at global warming."
Emanuel published a study last fall in the journal Nature that attributed hurricane intensity to sea surface warming. He sees Curry's new work as an answer to critics who claim he overlooked other factors.
"I got in trouble with the forecasters who said I didn't take into account wind shear or humidity," he said. "I think this paper reconciles at least one bone of contention."
Doubting the Data
So is the debate now settled? Hardly, says Chris Landsea, a meteorologist with the National Hurricane Center in Miami.
"There's no doubt that warming is going to have an impact," Landsea said. "The question is how much."
Landsea is skeptical of the notion that Category 4 and 5 storms have in fact doubled in the last 30 years, as Emanuel and the Science authors suggest. The data just isn't good enough to know that's true, he says. He points out that researchers have only been getting solid data on hurricanes for the Atlantic over the last half decade. Look at storm records in other parts of the world, and the trend isn't so clear.
For Landsea, this suggests the big storms in recent seasons may be the result of a natural cycle, rather than any man-made cause. Warming is likely having an effect, he says, but it's nearly negligible.
"The studies that have examined this in the past say that any change to hurricane winds due to sea surface temperature is likely to be only a 5 percent increase near the end of [the] 21st century," he said. "That means it amounts to a 1 percent increase today."
Scientists may debate the reliability of hurricane data for decades to come. But Curry and others say this new work still establishes a clear and significant link between bigger storms and warming.
Tilting the Odds
So what does that mean for future hurricane seasons? Even if storms are becoming more intense, Emanuel points out that more intense storms alone don't necessarily spell big trouble. Devastating hurricanes like Katrina are rare events, he says, because where and when a big storm hits makes all the difference between a devastating big storm and a forgettable one.
For example, everyone will remember Hurricane Katrina for years to come. But hardly anyone probably recalls another Category 4 storm -- Hurricane Brett -- which struck a remote part of Texas in 1999. That storm killed a few cows and quickly left public memory.
"It's like playing the lottery," Emanuel said. "Buying two tickets may double your probability of winning, but you're still talking about a tiny probability that you'll hit all the numbers. It comes down to a roll of the dice."
Give it another 50 years to 100 years, however, and there may be more cause for concern.
"It may be a roll of the dice," Curry said, "but the dice are loaded. And as the oceans keep warming, that's only going to get worse."