News•October 15, 2012

New Study Ties Hurricane Strength To Global Warming

One of the major unanswered questions about climate change is whether hurricanes have become more frequent and stronger as the world has warmed. Until now, there hasn’t been enough evidence to settle the question, but a report published Monday in Proceedings of the National Academy of Sciences may have changed all that. Using an entirely new method of tallying hurricane power and frequency, a team of scientists say that hurricanes are, indeed, more of a danger when ocean temperatures are higher. “In particular, we estimate that Katrina-magnitude events have been twice as frequent in warm years compared with cold years,” the report says.

Until now, the problem with such calculations is that until satellites came along in the 1970’s, nobody knew for sure how many hurricanes formed during a given year. That’s because some hurricanes never strike land, and unless a ship or a plane happened upon one of these storms, nobody might even know it had ever existed, and certainly not how strong it was.

Credit: flickr/NASA Goddard Space Flight Center

The record from the '70's onward is much more complete — but since hurricane numbers wax and wane based on a natural cycle, that’s not long enough to see if there’s a warming-related pattern on top of ordinary fluctuations. Ocean temperatures fluctuate according to natural cycles as well, although studies have shown an overall increase in North Atlantic sea surface temperatures, a trend that has been linked to manmade global warming.

But Alex Grinsted of the University of Copenhagen and his colleagues came at the problem in an entirely different way. They looked not at hurricanes themselves, but at the storm surges tropical storms drive before them as they come ashore, and surges have been reliably measured by devices known as tide gauges all the way back to the 1920's.

“Using surges as an indicator,” Grinsted said in an interview, “we see an increase in all magnitudes of storms when ocean temperatures are warmer.” As ocean temperatures have risen inexorably higher in the general warming of the planet due to human greenhouse-gas emissions, the scientists concluded, hurricane numbers have moved upward as well. The implication: they’ll keep increasing along with global temperatures unless emissions are cut significantly.

There’s one obvious caveat about the new results: not every hurricane creates a storm surge, since they don’t always hit land. And not every storm surge is caused by a hurricane. “The storm surge index,” Grinsted said, “is sensitive to strong winter storms as well.” And it’s quite possible, he said, that the intensity of a given storm surge could be made greater or less by the angle at which a hurricane hits land.

Surges aren’t, in short, a perfect stand-in for hurricanes, but Grinsted said that they’re pretty good. In cases where they could do so, the team has lined up hurricane data with surge data, and, he said, “there are clear correlations. So while our paper might not explain everything, it is still useful.”

Credit: flickr/Ana Rodríguez Carrington

In fact, Grinsted said, storm surges are more relevant to peoples’ lives than hurricanes. “Surges are one of the most damaging aspects of hurricanes,” and that’s going to become increasingly true as sea level continues to rise over the rest of this century. “If we want to talk about threat and risk, then this could be a more important measure,” Grinsted said.

Since it measures hurricanes indirectly, the paper is likely to prove controversial, said Kerry Emanuel, an MIT climate scientist who has done extensive research on the hurricane-climate connection (he also served as editor for the PNAS paper). “It’s a legitimate piece of research,” he said in an interview, “but it won’t convince everyone.”

That much is clear from comments other scientists have made after reading Grinstead's paper. “Although they have shown a correlation between their surge index and measures of hurricane activity,” said Gabriel Vecchi, of NOAA's Geophysical Fluid Dynamics Laboratory in New Jersey, in an email, “it is far from a 1:1 relationship.”

That, plus the fact that surges depend on angle and other factors, leaves Vecchi underwhelmed. Tom Knutson, another GFDL scientist, has a second worry. “The start date in 1923,” he said, also by email, “is a relatively low period in landfalling and basin-wide hurricane/or tropical storm time series with relatively higher activity in the late 1800s.” His point: if you start during unusual lull, it's not surprising that you get what looks like a significant increase afterward.

But Emanuel is still impressed, in part because he thinks the technique Grinsted and his collaborators used is so enormously clever. “I kick myself,” he said, “for not having thought of it myself.”