Metro

Study finds stormy centuries linked to warm seas

Over the last two millennia, the Northeast has weathered unprecedented periods of intense and frequent hurricanes unlike what anyone living today has ever seen, according to a study published Wednesday.

The study in the journal Earth’s Future shows that looking back farther than scientists have been previously able to, the region has experienced storm activity more powerful than our current planning scenarios. Both periods coincided with warmer temperatures in the Atlantic Ocean, suggesting that the same pattern might occur now as sea surface temperatures rise because of climate change.

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“Our sense of risk is probably skewed. Even collectively, over generations, we haven’t seen hurricane activity like the sediment record suggests happens,” said Jeffrey Donnelly, a paleoclimatologist at Woods Hole Oceanographic Institution, who led the work. “We have a lot of focus on human-induced climate change. Here’s an instance where, without human intervention, prior to the industrial age, we had hurricanes like we’ve never experienced. The risk is potentially much greater than we appreciated.”

The new finding emerges from the nascent study of paleotempestology — a field that attempts to extend understanding of storms beyond the roughly 150 years that people have been keeping detailed scientific observations of the weather. To detect those ancient storms, scientists probe traces that the most extreme storms have left in sediment.

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In the Massachusetts study, the researchers floated out to the deep part of the Salt Pond in Falmouth in an improvised pontoon made up of two canoes, plywood, and a tripod. They extracted a 30-foot-long core of the sediment beneath the water. That core is a record of big storms, preserving the times when the ocean surges over the beach and washes sand into the bottom of the pond.

They found that during a 1,000-year span beginning in 150 AD, 23 powerful hurricanes slammed into New England, on average, once every 40 years. From 1400 to 1675, there was another period of intense hurricane activity with 10 events.

To put that in perspective, recent storms that most people think of as major, destructive events weren’t even big enough to register in the pond’s sediments: hurricanes Irene and Sandy didn’t leave a trace.

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The only hurricane in the past hundred years to leave a layer of coarse-grained in the sediment was a category 2 storm, Hurricane Bob, which struck in 1991. Before that, the other recent storms recorded in the sediment were in 1675 and 1635.

Those bouts of increased hurricane activity correlated with warmer seas, with a slightly different scenario each time, Donnelly said. During the first period, ocean temperatures were warm in the tropics. In the second, surface sea temperatures along the Eastern Seaboard were warm.

Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology who was not involved in the work, said that other research already indicates that a warmer climate will increase the frequency of very powerful, high category hurricanes.

But its real importance may simply be in showing what occurred even without human-induced climate change, since history is used as an indicator of what could happen in the future. “In the distant past, hurricanes have been more numerous or intense in New England than in the recent past,” Emanuel said. “Which means it can happen again, regardless of global warming.”

Andrew Kemp, a geologist from Tufts University who uses a similar technique to study ancient sea level, said that the approach focuses on prehistory, but could have relevance for understanding storms today.

“You try and look back in time for a period that might have some resemblance to what we expect today on the horizon and in the future,” Kemp said. “So if you go back to a warmer period, a period where the ocean had a trend of rising sea surface temperatures and you see a change in the hurricane frequency, then maybe that gives you some way to calibrate your thinking for the future.”

Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.
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