Standing in a cove off Massachusett’s North Shore, Juliet Simpson holds a tube filled with some of the most precious mud in the world, mud that could have significant impact in the fight against climate change. But first, that mud needs to revolutionize how we think of sea grass.

Simpson, a coastal ecologist at MIT’s Sea Grant program, is on the search for carbon, and in this particular mud sample, which came from a sea grass bed about eight feet below the water’s surface in Nahant Harbor, chances are she’ll find quite a bit. Sea grass, also called eelgrass, photosynthesizes carbon out of the water column and then stores, concentrates, and locks it into the soils beneath it. There, because there is little to no oxygen, bacteria can take centuries to millennia to break it down and to re-release it back into the water and atmosphere.

Two years ago, in a first-ever global assessment, scientists calculated that the soils in sea grass meadows — despite being less than 0.2 percent of the world’s oceans — captured at least 10 percent of the ocean’s carbon. Since then the estimate has increased. Fred Short, a University of New Hampshire marine ecologist, puts the latest range between 12 and 20 percent. When combined with marshes and tropical mangroves, sea grasses are part of ecosystems comprising only 2 percent of ocean area — but accounting for a whopping 50 percent of ocean carbon storage.

Those discoveries are quickly elevating the concept of “blue carbon” among scientists and rapidly suggesting that these marine forests are as critical to controlling climate change as the emerald green Amazonian jungles and North American boreal expanses.

For some perspective, known terrestial forests take up at least 40 times more area than sea grasses, marshes, and mangroves. Yet the carbon storage capability of just 1 acre of sea grass is equal to 40 acres of terrestrial forest, Australian researcher Peter Ralph said in an interview last year with Voice of America. Marine plants, at the most minimal estimates, can bury carbon at a rate at least half that of all land forests combined. More optimistic estimates suggest their total ability at least equals and probably surpasses that of forests.

In the bays of Virginia, Karen McGlathery, an environmental scientist at the University of Virginia, is studying the world’s largest restoration of sea grass beds, a 5,000-acre effort led by the College of William & Mary. “We easily think of forests as important to climate change because they are tall and visible,” McGlathery said. “We see this little strip of grass along the ocean, and we think they can’t be as important as forests — but they are.”

But even as the importance of sea grass is being recognized, many coastal environments are literally slipping away. Short says the world has lost 29 percent of its known seabeds since 1879, and they continue to disappear at an annual rate of 7 percent — a loss that leads to the re-release of carbon back into the atmosphere at a volume equivalent to the emissions of 350 Bostons.

“[More sea grass] is disappearing than coral reefs, and that doesn’t account for meadows we don’t know about,” Short said. “What we do know is that we’re losing something like 20 football fields a day.”

New England is part of that global decline. Charles Costello, who has mapped sea grass beds for the last two decades for the Massachusetts Department of Environmental Protection, said the state has lost half its sea grass beds since colonial times. Massachusetts continues to lose up to 3 percent a year due to development, boating, and excess nitrogen and other nutrients in coastal waters from human waste as well as fertilizer and pollution runoffs from farms, lawns, and streets. Too many nutrients promote the growth of algae and seaweed, which block light from reaching sea grass beds. As the Smithsonian Environmenal Research Center says, “darkness can kill.”

While there have been small sea grass recoveries in waters off Gloucester, New Bedford, and Wareham, there have been significant losses off Salem and Martha’s Vineyard. Despite its globally praised cleanup, which has aided new pockets of sea grass growth, even Boston Harbor has overall annually lost four percent of its sea grass since 1994. In a parallel story, the harbor has lost 81 percent of its pre-colonial salt marsh and much of today’s remaining marsh is “degraded,” according to the Massachusetts Bays Program.

“Our lifestyle is a big problem,” Costello said. “There is so much activity on the coastline. I live on the coast, everybody wants to boat, everyone wants access to a marina.”

Attention should quickly turn toward coming up with policies to save the beds left and create programs to promote new ones. Cities, states, and the federal government should take a fresh look at water-quality efforts near our shorelines, from human waste to agricultural runoff to riverside industrial pollution. The clearer the water, the deeper and farther out sea grass can grow.

Though New England contributes to part of the decline, it also hold the hope for repair. Simpson — with the help of MIT Sea Grant Program engineer Michael Sarcany, Suffolk University student Briana McDowell, and Environmental Protection Agency research diver Phil Colarusso, among others — is working to map out the first estimates of carbon in Boston-area sea grass beds. The first measurements this summer off Nahant and Gloucester indicate that our local sea grass beds may contain as much or more carbon by area as what is being found in the rest of the world. Simpson believes the region’s salt marshes probably hold similar possibilities as constantly growing and shedding plant material decomposes into the soil. “In our more undisturbed areas,” she said, “you could go out to a salt marsh, pick up the soil and half of what you’re holding in your hand is probably straight carbon.”

Colarusso is also an adviser on the Commission for Environmental Cooperation, created by the United States, Canada, and Mexico as part of the North American Free Trade Agreement. The CEC is making a continental assessment of sea grass, marsh, and mangrove carbon. While Virginia’s sea grass renaissance is inspiring, Colarusso cautions that a large-scale restoration up and down New England’s coastline with its countless coves and crannies would be very expensive.

“The best thing we should be doing is conserving what we have in water quality,” Colarusso said. “We should be doing everything we can to keep nitrogen out of the water in drainage and runoffs.”

He adds, “With the cleanup we’ve had, we’ve seen some small patches of eelgrass appear by Logan Airport, the Outer Brewsters, by Hull, and off Winthrop. If we clean up the water further, we might see even more.”

More of a little-seen, but much-needed, powerful weapon in the fight against climate change.