Hanover — After researching Tropical Storm Irene and other damaging rainfall events in the Northeast, a team of Dartmouth College scientists have gained fresh insights about why New England has become more flood-prone over the past 20 years.

“The dice are loaded for more precipitation,” said Jonathan Winter, an assistant professor of geography at Dartmouth whose findings were recently published in the Journal of Geophysical Research. The study could help guide municipal planners throughout the Northeast in how to protect their most vulnerable residents from flooding.

A year ago, Winter and his colleagues — Huanping Huang and Erich Osterberg — found that, in 1996, the number of “extreme precipitation events” (2 inches or more of rainfall in a single day) in the Northeast spiked, and has remained elevated ever since.

The study was remarkable, not only for identifying such a clear dividing line between the two different rainfall eras, but also because it identified a geographic boundary to the change — there has been no corresponding rainfall trend in the American Midwest or the South.

“This is unique,” Winter said. “No other places in the U.S. have seen this kind of increase.”

After analyzing weather data from hundreds of stations throughout the Northeast, including one in Hanover, Winter and his team are ready to say why the Northeast’s extreme rainfall events have increased by 53 percent.

Nearly half of the bump is due to a fourfold increase in so-called tropical cyclones, the term weather experts use to refer to hurricanes and tropical storms, such as 2011’s Irene.

And those storm systems are caused by a warming Atlantic Ocean, which feeds enormous amounts of water into the atmosphere that the systems load up before drifting over the Eastern Seaboard to release their payload with sometimes-violent results.

Frequent Flood Preparations

When Winter and his team first identified that an extreme rainfall era had debuted 20 years ago, many in the region took note, including the Two Rivers-Ottauquechee Regional Commission in Woodstock. Planners there advised municipalities and contractors to use the new data to revise their expectations of a 100-year flood, and to scale up their culverts and stormwater systems accordingly.

That advice already was being heeded by the Community Resilience Organization of Hartford, which was formed three years ago to buttress the railroad town’s resilience — its ability to withstand and bounce back from disasters ranging from exploding oil tankers to natural disasters.

“I think, with the aftermath of the Irene flood, people recognize that the climate is changing,” said Matt Olson, Hartford’s town planner and liaison to the resilience organization, “and we’re going to be dealing with more significant weather events going into the future.”

Len Brown, executive director of the Bugbee Senior Center, said seniors are particularly vulnerable to flooding, power outages and other consequences of extreme rainfall.

“A lot of our seniors in this community are homebound,” Brown said. “They have limitations on their mobility. That makes it hard to get away from the floodwater when there’s 40 days and nights of rain, you know what I mean? There’s a lot of people who are off the beaten path who are still in their homes, and they’re often isolated.”

And simply hunkering down to wait out a rescue effort often is not practical for seniors who may need access to medicine, medical devices, food delivery or home care services.

“It’s a tough nut to crack,” Brown said.

Brown is working with the resilience organization and an Antioch University researcher to get a better handle on how seniors might withstand extreme weather events, with a survey of the community’s elders.

Survey-takers are asked about various disaster scenarios.

“How concerned are you that (extreme weather) warnings will be hard to understand?” asks one question. Others ask for levels of concern that “my house may be unsafe in a severe storm and I don’t have money or ability to make needed changes,” or “roads needed to access or leave my house can be blocked by flooding.”

Brown said he is distributing the survey through the Bugbee Senior Center, area churches and to some of the center’s Meals on Wheels clients.

Human Contribution

Winter said that, while his research has pinned the blame for the Northeast’s frequent soakings on a warming Atlantic Ocean, it’s going to take a little more work to answer the next question.

“What we’re not able to do is attribute it to climate change,” he said.

Instead, Winter said, the extreme precipitation seems to be the result of a double whammy: human-caused climate change that is coinciding with a natural warming cycle in the Atlantic.

The Earth’s ocean waters are part of a very complex system that’s influenced by ocean currents, Arctic ice, greenhouse gases and cloud cover, among other things.

“It’s just chaotic energy pushing around the Earth,” Winter said.

The last time the Atlantic Ocean warmed up due to natural climate variability was in 1928, and it stayed heated until 1963.

This time, said Winter, the ocean heated up in 1996, and that natural warming period could end next year, or it could hang around for decades.

However, said Winter, this time there’s an overlay on the natural heat, caused by the well-documented phenomenon of global warming caused by human activity, including the burning of fossil fuels.

“On top of that, humans have been increasing the temperatures,” he said. “It’s safe to say we’re causing part of it.”

Winter said the next research question he hopes to tackle is exactly what the divide is between what’s natural, and what’s unnatural, about the warmth of the ocean.

That, he said, will let planners know whether the heightened risk posed by extreme weather events will end in a relatively short period of time — or extend for the foreseeable future.

Matt Hongoltz-Hetling can be reached at mhonghet@vnews.com or 603-727-3211.