Climate scientists are largely in agreement that the Earth is getting warmer. But for years, they've shied away from pointing to a seeming uptick in natural disasters, namely hurricanes, as evidence. But a new report out of the University of Copenhagen's Centre for Ice and Climate suggests what many people already believed: Rising sea temperatures may be causing more frequent and more severe hurricanes.
According to the study, hurricanes in the southeast Atlantic have been increasing over the past 90 years, and there seems to be more hurricanes in "warm years," where water surface temperatures are higher than normal.
Sea surface temperatures have risen between half a degree and a degree Fahrenheit over the past 100 years, according to the Scripps Institution of Oceanography.
"We show that there is a difference in frequency of cyclones between cold and warm years, and that the effect is strongest for the larger cyclones and hurricanes," according to the Copenhagen study, published in Proceedings of the National Academy of Sciences.
Catastrophic hurricanes, such as 2005's Katrina, are twice as likely to occur during warm years than they are in cold years, according to Aslack Grinsted, one of the report's authors.
"You can't say [global warming] caused any single event, but when we start to see a trend like this, I think it shows that there's a good chance these hurricanes wouldn't be happening without warming," Grinsted says. "What I show is only correlation, but it's purely consistent with the hypothesis that warming goes along with more frequent, large hurricanes."
Climate scientists have suggested that rising water temperatures are associated with bigger and more frequent hurricanes, but haven't had solid proof because hurricane monitoring has been inconsistent over the years — with better detection devices such as satellites and airplanes, an increase in hurricanes could be easily explained by better detection technology.
But Grinsted decided to gather records from several stations in the southeast United States, which are "simple devices that have been used for hundreds of years to measure sea level." According to Grinsted, large hurricanes cause sea levels to rise for several days over many hundreds of miles, so a relatively small number of tide monitoring stations can be used to detect a large number of hurricanes.
According to his research, there was an average of 5.5 land-falling storms annually in the southeast Atlantic during the mid 1920s — by 2100, he predicts there will be close to 10 land-falling storms per year.
Though Grinsted is reasonably sure that warming sea temperatures are associated with more hurricanes in the Atlantic, he's not sure if the effect would carry over to the Pacific Ocean and other areas where tropical cyclones are common.
"You can't necessarily extrapolate this to another region," he says. "But if the dominant mechanism causing this is sea surface temperature, you'd expect it to hold in other regions. But right now, those regions don't have the same quality data, so it's hard to test."
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Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org.