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New England marshes and seagrasses have huge potential to combat climate change

Scientists are just beginning to understand how much carbon is sequestered by salt marshes and seagrasses— and how much could be released into the atmosphere if those ecosystems collapse.

The Rev. Mariama White-Hammond, chief of Environment, Energy, and Open Space for the city of Boston, swam in the giant ocean tank at the New England Aquarium to highlight work done by the EPA on blue carbon habitats.Danielle Parhizkaran/Globe Staff

As states across New England rush to meet greenhouse gas emission reduction targets, a massive carbon reservoir sits just below the tide: reed marshes and seagrass beds.

Scientists call these coastal habitats “blue carbon,” because the plants, and importantly, the sediment underneath them, can sequester a lot of carbon — several times more than a terrestrial forest, for example.

At least 7.5 million metric tons of carbon are held beneath New England’s salt marshes and eelgrass meadows, tall grasses that grow beneath the water, according to a first-of-its-kind report published by the US Environmental Protection Agency last year.

The EPA’s findings were highlighted at an event celebrating a new temporary exhibit at the New England Aquarium on Wednesday. Massachusetts has by far the most blue carbon habitats in New England: more than 112,000 acres, or about half of the region’s total salt marshes and seagrass beds.

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The research was jump-started in 2017 after a conference of governors across New England identified blue carbon as a possible strategy for New England’s climate action plan.

“The more that we understand how these ecosystems work, the better we’ll be able to protect them so that they can protect the coastal communities that people live in,” said David Cash, EPA administrator for Region 1, in an interview at the event.

The Rev. Mariama White-Hammond, chief of Environment, Energy, and Open Space for the city of Boston, spoke at the giant ocean tank at the New England Aquarium.Danielle Parhizkaran/Globe Staff

In climate terms, New England’s salt marshes and eelgrasses hold back approximately the same amount of carbon dioxide that is polluted by roughly 6 million gasoline-fueled vehicles for a year. Put another way, the marshes store the equivalent of not powering 3.5 million homes for a year.

But many of the region’s coastal habitats are under stress, said Phil Colarusso, a marine biologist for the EPA and lead author of the report.

Freshwater runoff carrying lawn fertilizers drown the marshes and grasses in nitrogen; sewage contamination degrades the water; rising seas upset low-lying ecosystems; and even the sound of nearby construction can rupture plant cells.

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That’s a huge problem, not only because the habitats are absorbing carbon dioxide, but if they die, their roots will no longer be able to hold the sediment in place. Instead, the sediment — and all the carbon trapped in it — can float away, causing something akin to a mini underwater Dust Bowl the next time a big storm happens.

Scientists are just beginning to understand how vital these coastal habitats are, and suspect that the area’s marshes and seagrasses probably contain many more times the amount of carbon than the EPA was able to calculate.

To gather the data, teams of divers extracted tubes of mud in eelgrass and salt marsh habitats between Long Island, N.Y., and Maine. Those samples were analyzed and mapped for their carbon sequestration value. But only the first 30 to 50 centimeters of sediment beneath the plants was able to be gathered, meaning much more carbon probably sits far below where the EPA’s instruments could reach: perhaps even 10 times more, Colarusso said.

“It’s a significant underestimate of what’s actually there,” Colarusso said.

Myrtle, a green sea turtle believed to be between 75 and 95 years old, swam in the 200,000-gallon salt-water tank at the New England Aquarium. Danielle Parhizkaran/Globe Staff

As states and nations seek to reduce economic reliance on fossil fuels to hit key targets for reducing emissions, the blue carbon sinks could play a key role, said Cash, the EPA regional administrator. The whole point of the EPA’s report, he said, was to create a foundation of science “that can help policymakers make better decisions.”

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Cash said that he hopes the research will help cities and towns across New England create policies that better protect against harmful human activities such as boating in sensitive areas. He said that the EPA is also working to prevent sewage pollution and nutrient runoff.


Erin Douglas can be reached at erin.douglas@globe.com. Follow her @erinmdouglas23.