Global sea levels have risen about nine inches since we started keeping track in 1880. If they were to keep rising at this rate, by century’s end they would be roughly five inches above where they are today. But most scientists expect to see anywhere between an additional twenty-four to eighty-four inches of sea level rise by 2100, and every year the estimates creep higher still. Between the turn of the last century and 1990, sea levels rose, on average, 1 to 1.2 millimeters per year. Then the rate of the rise itself started to increase, rapidly. In the intervening quarter century, the per-year increase has risen to 4 millimeters, and, like so many other climate change signals, it shows little sign of slowing down.

      As the rate of the rise continues to accelerate, tidal marshes are becoming inundated and, as here, they are starting to rot. If you drill into a healthy marsh you quickly encounter a network of rhizomes and black, iron-rich sediment. This sediment, which cements most marshes together, is so dense it doesn’t contain any oxygen. And this anoxic environment is, in part, what makes marshes such good carbon sinks—whatever organic matter is stored there decomposes extremely slowly because it is never touched by air. But when salt water sits on a marsh and cannot drain, as is happening in tidal wetlands the world over, the marsh grass rhizomes either retreat or rot.

      Some places, like the southern edge of Louisiana and the Isle de Jean Charles, have already passed through this transitional process. Others, like vast swaths of the Everglades, are just beginning to show signs of collapse. As these marshes become flush with salt water, they are contributing to atmospheric warming—but just how much, and at what rate, remains unclear. That’s partly because each location is unique, with different kinds of flora respiring at different rates, and also more generally because throughout Western history tidal wetlands were thought to be the homes of swamp serpents and marsh monsters, the boggy, slimy sources of malaria, disease, and death. As such, they have long gone overlooked, which is why the research taking place out here in the Gulf of Maine is so important.

      The US Fish and Wildlife Service didn’t understand the connection between marsh rot and climate when it decided to “plug” a ditch in the Sprague likely dug by the Civilian Conservation Corps in the early 1930s. The Sprague River Marsh is not unique in this way. By the end of the decade following the Depression, over 90 percent of New England’s saltwater marshes were grid-ditched, mostly in attempts to reduce mosquito populations in coastal communities. All along the Eastern Seaboard, workers took shovels to swampy land, hoping to drain the sections prone to retaining water.

      The Civilian Conservation Corps didn’t care that ditching would transform the hydrology of the entire ecosystem. The standing water in which mosquito larvae hatched was greatly reduced—and with it went hundreds of other species. Dragonflies and water beetles. Mummichogs and silversides. The seaside sparrow. The great egrets and white ibis. So, over a decade ago, the US Fish and Wildlife Service started plugging the ditches. They thought intervening in an already altered hydrological system might be able to return the marsh to a state of equilibrium. They thought they might be able to bring back the water beetles and wading birds. But, it turned out, layering one kind of human intervention on top of another only dragged the Sprague further from its starting point.

      Not much more than a four-foot-by-eight-foot piece of plywood, a ditch plug is a simple-enough idea: it is meant to stop tidal flow through man-made channels, reintroducing an element of standing water into the marsh. But ditch plugs are too effective at restricting flow. Fresh water from the upland side filters into the marsh and does not continue toward the sea. And whenever an exceptionally high tide or storm surge arrives, breaching the barrier, salt water gets stuck in place there too. As a result everything above the plug is permanently inundated with saline-rich water, and as the water starts to evaporate, the saline concentrations shoot even higher. The rhizomes in the marsh grasses, unused to these conditions, begin to decompose; the ground around them collapses; and the greenhouse gases long stored in the sediment are released into the air. At least that is what these scientists suspect is happening.

      “The Fish and Wildlife Service really screwed this up,” says Beverly, straddling the channel behind the plug, bloated with brackish water. “Though they know this now.” The edges of the plywood in front of her are egg-yolk yellow and dusty green, the center buckled.

      Later, when I type “what rots” into Google, the search engine tries to finish my question, suggesting What rots teeth? What rots first when you die? What rots quickly? I discover that acid rots teeth. Cell membranes in the liver are the first thing in the human body to rot. When improperly stored, potatoes rot quickly, and I don’t need Google to tell me that they smell bad when they do.

      Google does not suggest making my sentence What rots marshes? It is not the first time the search engine—thanks in part to its millions of users, whose habits dictate the autocomplete option—has been, in my humble opinion, misleading. Because if marshes are among the largest carbon sinks in the world, and if rot transforms them into huge carbon sources, then we surely do want to know what rots marshes and, perhaps more importantly, if there is anything we can do to better prepare them for the future that is already here.

      When I look out across the white slime that coats the once-loamy ground above the ditch plug, I know that what is happening in the Sprague is, in a very basic sense, what will happen to many of the world’s marshes as the height of our oceans continues to climb. My fever dreams of tidal wetlands—and all the species endemic to them—drowning, of our coastlines contracting, and of mass migrations inland return with prehensile force. They drag me deeper into the marsh, out into the rotting cordgrass where the ground quakes like chocolate pudding. There, at the decomposing center of the Sprague, I stand dumbstruck by our planet’s transformations.

      I am starting to be able to see not just the dead trees sprinkled along the shore like so much confetti, or the fistful of decaying grass I hold in my hand. I am beginning to make out the rough outline of our future coastline. Everywhere that once was a tidal marsh will likely be open water. The words of Ben Strauss echo again in my mind: “It is not a question of if but when.”