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Disaster and Accident

Storm Stories Told in Layers of Pond Muck

Vineyard Gazette of Edgartown, Massachusetts

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Hurricane History

With the snow still banked high and morning temperatures flirting with zero, it might not be the best week to bring up the subject of tropical cyclones.

But hurricanes may begin to strike the southern New England coastline more often and more ferociously than at any time since colonial settlement, according to Jeff Donnelly, the lead scientist in a new study that offers evidence of ancient and mighty hurricane landfalls in the northeast. The research was conducted in a coastal pond just across Vineyard Sound in Falmouth.

With unprecedented specificity, the study examines how many prehistoric hurricanes managed to overwhelm a barrier beach that faces the sound, driving gravel, sand and other grit into the northern reaches of Salt Pond, which lies between Woods Hole and Falmouth harbor. Researchers counted 32 powerful hurricane strikes in the last 2,000 years. By contrast, the number in the last 400 years is just three.

The study focuses on long-ago time periods when sea surface temperatures in the tropical Atlantic and off the East Coast warmed substantially of their own accord, and it shows how hurricanes frequently formed and came up the seaboard to assail the northeast at the same intervals.

The critical point, Mr. Donnelly said, is that with the help of man, sea surface temperatures today already exceed those of the prehistoric past, and they continue to rise at rapid rates. Taking other meteorological factors into account, this suggests that hurricanes could begin to assault southeastern New England with at least as much frequency and catastrophic power as in centuries and millennia past.

"We've already surpassed these earlier intervals of warmth," said Mr. Donnelly, who works with the geology and geophysics department of the Woods Hole Oceanographic Institution. "That's only happened in the last decade or two. The rate of change is much greater than previously, and we've blown right past these earlier intervals of warmth."

He added: "The grand experiment has begun."

The study, called Climate Forcing of Unprecedented Intense-Hurricane Activity in the Last 2,000 years, was published Monday in Earth's Future, an online journal published by the American Geophysical Union. Mr. Donnelly is a pioneer in the effort to find and evaluate layers of sediment that were washed over barrier beaches and into the bottoms of coastal wetlands and ponds by the storm surges of hurricanes in distant times.

"This particular study provided the most detailed and longest record from the northeast," the scientist said. "We had similar records from the Gulf of Mexico and some from the Caribbean and the Bahamas, but none from the Northeast until this particular study."

Up to now, core samples of sediments raised from northeastern ponds and marshes sometimes failed to delineate old-world hurricanes that made landfall decades apart. "Whereas with this record we can actually distinguish storms that happened just a year apart," Mr. Donnelly said. Hurricanes that recently read as one storm at nearby coastal sites break down into six or seven at Salt Pond.

By examining layers of organic muck, as well as lead pollution from the start of the industrial age and even evidence of nuclear weapons testing, researchers can date the strata of sediments that hurricanes deposited above and below these other layers. In the new Salt Pond study, scientists do this more specifically than ever before. For example, sediment from the New England Hurricane of1675 was spread just above rye pollen planted by the first colonists in Falmouth.

"To find the actual rye pollen that the settlers planted after they cleared landscape, and put a really precise date on that particular interval, allowed us to date that 1675 layer with much more confidence," Mr. Donnelly said. "It's pretty neat. Not only that, we also found some maize pollen from the Native Americans prior to that."

Since the middle of the 19th century, hurricanes of category two (96 to 100 m.p.h.) or greater have savaged New England about once per century. That average now appears to be deceptively low. "When you get back to the prehis toric record, its much greater. You're looking, at times, at three or even four storms per century," Mr. Donnelly said. These intervals of much greater activity include the first millennium, between the years 150 and 1150, and again between 1400 and 1675.

Both were times when sea surface temperatures warmed appreciably due to cycles in nature. The earlier warming occurred across the tropical Atlantic between Africa and the Caribbean, considered the main development region for many hurricanes. The second happened off the southeastern coastline of what is now the United States, after the tropical Atlantic had cooled. This is where Hurricane Bob, the last category two hurricane to strike southern New England, formed in August 1991.

Though the study demonstrates how precisely scientists can now count prehistoric hurricanes in the right location, and link them to intervals of sea surface temperature warming, it is harder to measure how strong each Salt Pond tempest actually was. An ancient deposit might have been left by a vast and cataclysmic hurricane striking many miles away or a smaller and slightly weaker one scoring a direct hit. But to transport gravel and sand to the most inland ends of Salt Pond, where the core samples were all taken, suggests most of these long-ago hurricanes were exceedingly strong, Mr. Donnelly said.

'At least at first blush, when we [determine] the amount of course material transported, many of these storms were able to transport much more material than [more recent] storms," he said. 'And they probably did it over a greater distance, because sea level has been rising. Currently they have to be transported 400 metres to where the core was taken. In the past, because sea level was lower and the shoreline was farther out, they probably had to be transported 600 metres."

With sea surface temperatures now warmer than any period covered by the Salt Pond study, and projected to warm further as the climate changes during this century, Mr. Donnelly said he hopes New England will prepare itself for hurricanes more frequent and more intense than anything people have experienced today, and even anything in the written record.

The nature of historic New England hurricanes is that they can form nearby, strengthen rapidly, hybridize with other weather systems and hurtle northward at highway speeds, causing immense damage to the east of the eye, where the forward motion of the storm is added to the wind speed around the core. These qualities were probably also true of coastal hurricanes hundreds and thousands of years ago, Mr. Donnelly said.

"Storms coming up the eastern seaboard at 50 or 60 miles an hour — that means we only have eight hours or so between the storm being off the Outer Banks of North Carolina and it making landfall in southern New England. We don't have a heck of a lot of time to react and make sure that people are out of harm's way," he said.

Hurricane Sandy in 2012 doesn't offer a good comparison, he added. It was large and struck New York and New Jersey at high tide. But it was weak and slow moving and still caused more than $60 billion in damages. Dr. Donnelly looked to the hear and distant past this week and offered a different analogy:

"Even if we don't see a return to these earlier intervals of activity, even if we [only] have a return to what we experienced in the middle part of the 20th century, we're in some serious trouble," he said. "We've got storms like 1938 and Hurricane Carol in 1954 impacting a region that has much more development — and the sea is another foot higher than it was when we were hit in 1938."





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Original Publication Date: February 27, 2015



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