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Millions of homes face substantial flood risk — far more than previously predicted, study finds

A man watched waves roll onto Fan Pier in Boston's Seaport district in 2018.
A man watched waves roll onto Fan Pier in Boston's Seaport district in 2018.Keith Bedford

Nearly 200,000 properties in Massachusetts face a substantial risk of flooding, with thousands more in jeopardy of being inundated as the global climate warms in the coming decades — far more than existing flood maps show, according to a nationwide study.

The analysis, by a nonprofit group, found that the federal government’s models for where flooding may occur in New England and beyond are woefully out of date, underestimating the risk to nearly six million homes and businesses.

Using data that account for sea-level rise, greater bursts of precipitation, and other effects of climate change, the report estimates that 14.6 million properties face a substantial risk of flooding at least once over the next century — nearly 70 percent more than indicated on the flood maps produced by the Federal Emergency Management Agency.

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FEMA’s current method of determining flood risk leaves millions of Americans unaware of that risk from increasingly heavy rainfall events and sea level rise,” said Ed Kearns, chief data officer of First Street Foundation, a New York-based group of academics and experts who released the report along with a website that allows property owners to see the risks in their neighborhoods.

The states with the largest proportion of properties at substantial risk include West Virginia, where nearly a quarter of homes and businesses are in jeopardy; Louisiana and Florida, where more than a fifth face the same threats; and Idaho and Montana, where about 15 percent of properties are at risk, according to the report, which was produced with the help of researchers from the Massachusetts Institute of Technology, Columbia University, and the University of California Berkeley.

In Massachusetts, more than 336,000 properties are at risk of flooding, with 193,000 facing substantial risks — 65 percent more than existing flood maps show, according to the report. Another 22,000 properties are likely to experience similar risks by 2050.

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In all, nearly 9 percent of properties in Massachusetts face substantial risks of flooding, 1 percent less than the national average, according to the report. By 2050, the authors predict that 10 percent of the state’s properties will have a substantial risk of flooding.

“This report quantifies what we’ve long known: that climate change is increasing flooding here in Massachusetts,” said Julia Blatt, executive director of the Massachusetts Rivers Alliance.

She urged state officials to do more to ensure that fewer properties are built near floodplains.

“New development should be carefully sited, so as not to make flooding worse,” she said. “Floods happen when water has nowhere to go, so one of the simplest and best strategies to address this problem is to protect open space in river floodplains and along the coast.”

State environmental officials said they understand the threat and have responded by committing $1 billion through 2022 to programs that seek to mitigate the dangers from flooding and other consequences of a warming planet. Climate models suggest that seas could rise as much as 10 feet by the end of the century, with heavy storms bringing substantially more precipitation and flooding in Massachusetts.

Moreover, state transportation officials have been developing a new model to better predict coastal flooding, they said.

“The administration is currently reviewing this report, and remains committed to addressing the urgent issue of climate change adaptation,” said Katie Gronendyke, a spokeswoman for the Executive Office of Energy and Environmental Affairs.

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The report said Boston has more than 19,000 homes and businesses at substantial risk of flooding, or 19 percent of existing properties. By 2050, an additional 8,600 in the city will be threatened.

The cities and towns with the greatest proportion of properties at risk include Hull, where 65 percent face substantial risks; Adams, where nearly half do; and Salisbury, where 44 percent are in danger.

In the coming decades, some towns on Cape Cod face what are among the highest risks, with four times more properties at substantial threat of flooding in Dennis Port, nearly three times in Falmouth, and more than double in Harwich Port, according to the report.

Climate change will also disproportionately endanger properties in Chelsea, Cambridge, and Everett, each of which is likely to see the risks increase roughly twofold.

“We must begin to develop in ways that allow us to be more resilient to flooding, and to adapt our existing landscape to accommodate what we know is coming,” said Julie Wood, deputy director of the Charles River Watershed Association.

For example, she and others said, municipalities need to invest more in rebuilding aging dams and stormwater drainage systems.

“These systems can actually cause flooding in areas well outside mapped flood zones, and in areas that may have never experienced flooding before,” Wood said.

At the Trustees of Reservations, which owns more coastal land than any other property owner in the state, officials have been seeking solutions to protect the shoreline.

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They have lobbied for bills to allow the state to buy land from the owners of the most vulnerable properties. They have also worked on projects to fortify tidal wetlands, salt marshes, and barrier beaches.

“Coastal landowners, municipalities, and business owners will have some tough decisions to make in the near future,” said Tom O’Shea, director of coast and natural resources at the Trustees, which owns 120 miles of coast in 25 communities. “The numbers in this new report are striking, but also potentially conservative for what we may face just 30 years from now — the life of a mortgage.”


David Abel can be reached at david.abel@globe.com. Follow him on Twitter @davabel.