SCIENCE:

Study finds big storms on a 1,000-year rise

This story was updated at 2:15 p.m. EDT.

The North Atlantic Ocean has spawned more hurricanes and tropical storms over the last decade than it has since a similarly stormy period 1,000 years ago, according to a new study.

The research, published yesterday in the journal Nature, tries to trace the pattern of storms along North America's Atlantic and Gulf coasts back to A.D. 500, well before humans were recording weather observations.

The study's lead author, climate scientist Michael Mann of Pennsylvania State University, said finding a reliable way to reconstruct centuries of past hurricane activity could help scientists tease out whether future climate change will alter storm patterns.

"One of the driving motivations for this research is to place in a longer-term context ... Atlantic tropical cyclone behavior, and the extent to which it may be anomalous," he said.

That meant trying to divine information about the past beyond historical records collected by ships and observers on shore, and later by storm-tracking aircraft and satellites.

The scientists relied on two different methods to reconstruct the past -- a foray into the emerging field of "paleotempestology," or the study of ancient storms.

First, they examined layers of sediment collected from coastal ponds and salt marshes that tend to flood when hurricanes make landfall nearby. Each flood deposits a layer of coarse barrier beach sand on the muddy pond floor, creating a record scientists can examine by collecting sediment cores. For the new study, Mann and his co-authors at the University of Massachusetts and the Woods Hole Oceanographic Institution collected cores at eight sites along the U.S. Atlantic and Gulf coasts, ranging from southern Massachusetts to Vieques Island, Puerto Rico.

The researchers also used a computer model to simulate 1,500 years of Atlantic storms, feeding in information collected between 1851 and 2006 about factors known to influence hurricane activity, including sea surface temperatures in the tropical Atlantic Ocean, the occurrence of El Niño weather patterns, and fluctuations in the jet stream.

The two methods produced similar overall results, Mann said, such as showing a major peak in storm activity about 1,000 years ago.

But 'paleotempestology' doesn't end the debate

Taken together, he said, they suggest that warmer temperatures produce more storm activity -- meaning that coming climate change could increase the frequency of hurricane activity.

"The paleoclimate evidence seems to reinforce the notion that, all other things being equal, when you have warm sea surface temperatures in the tropical Atlantic, you see more activity," he said.

But Mann said there's also a possibility that climate change could alter the frequency of El Niño, which blunts hurricane activity, and counteract the effects of future ocean warming.

"Current state-of-the-art climate model projections are more or less split between whether there will be more El Niño conditions or more La Niña-like conditions," he said.

"The jury is still out. But this study is an independent data point from the paleo record that gives more weight to the proposition that warming the tropical Atlantic will continue to give us a higher level of hurricane and tropical cyclone activity."

Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology who has published studies linking climate change to stronger hurricanes, called the new research "an impressive piece of work, melding two completely independent approaches to estimating past hurricane activity."

Emanuel said the Nature study "shows that hurricane activity is indeed quite sensitive to climate, and although we are still not completely sure about global warming effects, the paper raises again the flag that potentially they could be large."

But James Elsner, a climate scientist at Florida State University, said the "rather large levels of uncertainty" in the new study's results gave him pause.

"I don't see it as settling the debate on climate change and hurricane activity," said Elsner, who helped develop the statistical methods employed in the new study. "I think it does provide evidence that warmth is important."

At the same time, he explained, "the lack of a real tight physical theory between ocean warmth and frequency indicates this is not the smoking gun that would allow us to confidently project what might happen as oceans warm in the future."

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