This ecologist thinks coastal wetlands can outrun rising seas. Not everyone’s convinced

By digging deep into coastal marshes around the Chesapeake Bay, Matt Kirwan has unearthed a story of surprising resilience.

Michael O. Snyder is a Bertha Climate Journalism Fellow based in Charlottesville, VA

BLACKWATER NATIONAL WILDLIFE REFUGE IN MARYLAND—Coastal scientist Matt Kirwan has a sense of what it’s like to flee from rising seas. More than 100 years ago, Kirwan’s great-great-grandfather owned a farm close to this sprawling wetland refuge near the Chesapeake Bay, a key annual migration stop for hundreds of thousands of geese, ducks, and other waterbirds. Joseph Josiah Robbins sold his homestead in 1909, family members say, after salty, brackish waters invaded his fields, stunting and killing his crops.

Kirwan’s ancestors were early climate refugees, he says, even if nobody called them that. “There are large tracts of farmland that were usable a generation ago but no longer are.”

Seas are rising especially fast near the Chesapeake, where geologists say Earth’s crust is tilting into the ocean, amplifying the effects of a warming climate. But many more people around the world could soon share his ancestors’ experience, says Kirwan, who works at the Virginia Institute of Marine Sciences (VIMS). The rate of global sea level rise is increasing as ice from Greenland and Antarctica melts and warmer seawater expands. By 2050, sea levels could surge by 10 to 25 millimeters per year, according to climate modelers, up from just 5 to 6 millimeters per year here at the refuge and about 3 millimeters per year globally.

Saltwater is forecast to slosh into places it hasn’t been since long before humans arrived. In the continental United States alone, just 1 meter of relative sea level rise could allow high tides to submerge as much as 49,000 square kilometers of now dry land—an area the size of Vermont and New Hampshire combined. Other countries have it worse. Egypt and Bangladesh could lose almost one-fifth of their habitable land. More than 200 million people could face flooding risk by 2050, studies suggest.

Humans aren’t the only species at risk. Rising seas could also drown huge swaths of coastal wetlands, destroying habitats that are among the most ecologically valuable on Earth. Besides providing homes for plants, fish, and many other organisms, wetlands trap vast amounts of carbon that might otherwise escape into the atmosphere and contribute to planetary warming. They also buffer some of the world’s largest cities from violent, damaging storm surges. Some 20% to 90% of today’s tidal wetlands could be lost by century’s end, depending on how fast oceans rise, studies have suggested.

Kirwan, however, has argued that such forecasts are needlessly bleak. Studies that he and colleagues have conducted—many around the Chesapeake Bay—have shown that in the right conditions, tidal marshes can build themselves up to keep pace with rising seas, while also migrating inland as water creeps up coastlines. Some coastal wetlands could even expand as seas rise, the studies suggest—if people don’t block their paths with seawalls, levees, and other infrastructure designed to hold back the flood.

“Marsh vulnerability tends to be overstated,” Kirwan and colleagues wrote in a 2016 paper in Nature Climate Change, concluding that sea level rise does not pose “an immediate, catastrophic threat to many marshes.”

A road in Maryland’s Blackwater National Wildlife Refuge cuts through a marsh that, because of sea level rise, has mostly converted to open water.

Michael O. Snyder

Kirwan’s findings are “very compelling,” says ecologist Keryn Gedan of George Washington University, and have “given the marsh community a ray of hope.”

But many researchers are skeptical, and some are challenging Kirwan’s results. Models suggesting wetlands can keep pace with high rates of sea level rise are “seriously flawed,” says ecologist Neil Saintilan of Macquarie University.  “This idea of wetlands expanding under accelerated sea level rise, it basically violates some of the most basic geologic theory,” says Torbjörn Törnqvist, a geologist at Tulane University. “It’s just not going to happen.”

The debate isn’t just academic. Forecasts of how coastal wetlands will respond to rising water will help determine how aggressively the United States and other nations work to save these critical ecosystems, and which policies they adopt. “We should set a goal as ambitious as possible, given the facts,” says Jeffrey Peterson, a retired policy adviser at the Environmental Protection Agency. “But we don’t know what the facts are.”

Early one morning this spring, Kirwan and two other VIMS researchers—then-undergraduate Emily Hall and lab manager Tyler Messerschmidt—mustered on a spit of land along the Severn River in southern Virginia. It’s a soggy landscape where roads have names such as Ditch Bank and Low Ground. Hall and Messerschmidt loaded imposing data-collection gear onto their backs and trekked into what may be one of the world’s youngest marshes.

Less than 100 years ago, farmers grew crops here. Those farmers, like most people through history, saw wetlands not as a boon, but as an impediment. In the United States, more than half of the nation’s original wetlands were drained for agriculture or development. At this site, earthen levees 1 meter or so high—built by enslaved people, local lore has it—were erected to keep fields dry. “This is how people fought against flooding in the past,” Kirwan said as he stood atop one of the still-intact levees.

In a low-lying pool, Hall and Messerschmidt erected a tripod topped by a sensor that collected hyperaccurate measurements of the marsh’s topography. As the unit’s beeps and boops mixed with bird song, the pair walked a transect through the marsh, battling sharp, stiff grasses. Every 5 meters, Hall jammed a metal shank called a peat corer into the muck and gave it a firm twist to cut out a plug of soil. The top few centimeters were full of marsh grass roots that hadn’t decomposed. Below that, however, was mineral soil, revealing that the site was dry land until recently.

A core from Virginia’s Goodwin Island reveals when dry land became marsh. Carbon absorbed by marsh plants can be stored in soil for centuries.

Michael O. Snyder is a Bertha Climate Journalism Fellow based in Charlottesville, VA

Kirwan’s disarmingly informal manner and often-battered field clothes belie his status as one of the world’s most respected coastal experts. He got his start as an undergraduate at the College of William & Mary, just down the road in Williamsburg, Virginia. After a doctorate at Duke University and a stint at the U.S. Geological Survey, he returned to William & Mary and VIMS in 2011 as a professor, and the Chesapeake Bay’s wetlands became his laboratory. Thanks to childhood visits to family in the area, Kirwan is as comfortable bantering with hunters who flock to places like the Blackwater refuge as with fellow scientists.

Through his research, Kirwan has developed a view of coastal wetlands as dynamic, resilient ecosystems that have danced an eonslong tango with the rising and falling ocean. And some of his findings have challenged conventional thinking about how future sea level rise could change the planet’s coastlines.

Kirwan’s first big splash was a 2010 study in Geophysical Research Letters showing that just two factors—the tidal range (the elevation difference between low and high tide) and the amount of sediment in the water—determined how much sea level rise salt marshes around the world could withstand. In places with large tidal ranges, plants near the high tide line will remain partly above water even as seas rise, whereas in places with less tidal variation, such as the Blackwater reserve, even a little bit of additional water can fully drown marshes. Meanwhile, in places where plentiful sediment settles out of water, marshes can build new ground at rates up to an astounding 10 centimeters per year, modeling by Kirwan and colleagues suggested, enabling them to outrun even high rates of sea level rise.

A few years later, in an influential 2013 paper published in Nature, Kirwan and Patrick Megonigal of the Smithsonian Environmental Research Center expanded on those findings to argue that most wetlands globally were keeping up with sea level rise just fine. One key, they wrote, is a biophysical feedback loop: Wetland plants slow incoming ocean water, causing fine sediment it carries to drop out and accumulate. As sea level rises and wetlands flood more often, the plants trap more sediment, building up the soil surface. Meanwhile, marsh plant roots die, but mostly don’t decompose, further raising the ground.

Kirwan also found that wetlands’ ability to migrate landward had been dramatically underappreciated. Many scientists, he and Gedan say, assumed most migrating wetlands would encounter either a natural barrier such as a steep slope or a bluff, or a human barrier such as a seawall. But Kirwan found many areas lacked such impediments and could accommodate enormous marsh migrations. In 2017, for example, his research team reported that since 1850, the flat, fast-sinking Chesapeake region has gained as much marsh as it has lost: more than 40,000 hectares, or more than twice the area of Washington, D.C.

One of Kirwan’s research sites, Goodwin Island at the mouth of the York River, provides a powerful illustration. There, researchers documented a marsh that has been creeping uphill for more than 600 years. And Kirwan’s team has found that even the newest marshes provide important ecosystem services, such as absorbing and storing carbon.

The encroachment of salty water killed trees near the Blackwater National Wildlife Refuge in Maryland, creating a so-called ghost forest. Such dead forests can be an ecological win, says coastal researcher Matt Kirwan, because marsh grasses that take over the site build soil carbon and provide other benefits.

Michael O. Snyder

On Goodwin Island and elsewhere, those new marshes often occupy land that was forest until saltwater poisoned the trees. Such ghost forests have been expanding along the U.S. east coast for 100 years, Kirwan and Gedan pointed out in a 2019 paper in Nature Climate Change. Although these dead forests can look apocalyptic, the marshes that replace them actually provide more benefits for nature and people, and should be welcomed, Kirwan says. “From a pure ecological perspective, ghost forests are a win.”

What has stirred the most debate, however, are Kirwan’s forays beyond quantifying past change to forecasting the future, when sea level rise is likely to accelerate rapidly. In 2016, Kirwan and colleagues critiqued forecasts of massive coastal wetland loss in a paper published in Nature Climate Change. The study, titled “Overestimation of marsh vulnerability to sea level rise,” argued that some widely used models downplayed marshes’ ability to build up soil as oceans rise. Models that account for those soil-building processes, the researchers argued, suggest marshes could withstand ocean rise of up to a half-meter per decade—among the direst scenarios for the coming century.

Three years ago, Kirwan and Mark Schuerch, a geographer at the University of Lincoln, went further. Using models, they predicted that, through 2100, coastal wetlands globally could suffer little or no net loss at all, and under some assumptions could even increase their area by up to 60%.

Wetland or water?

If tidal wetlands are able to build up and migrate inland as seas rise, they could take over coastal areas that are currently well above sea level.Potential migration zonesMDVANJDEAtlanticOceanDelawareBayD.C.RichmondBaltimoreChesapeakeBaySea level rise of 1 meter could drown many wetlands according to widely used models that assume they have limited ability to build soils or migrate horizontally.If wetlands can’t keep upMDVANJDEAtlanticOceanDelawareBayD.C.RichmondBaltimoreChesapeakeBayThe bay region, including the area around the Blackwater National Wildlife Refuge (upper center in box), currently supports extensive tidal marshes.A marshy coastMDVANJDEAtlanticOceanDelawareBayD.C.RichmondBaltimoreChesapeakeBay050km0–11–22–3Elevation (m)MarshBeachSwampThe fast-sinking Chesapeake Bay region offers a window into future scenarios for tidal wetlands. Some computer models suggest vast swaths of wetlands could disappear in coming decades as oceans surge. But other models forecast that coastal wetlands could maintain or even expand their areas by building soils vertically and migrating to higher ground that is currently dry.

(Data) National Wildlife Federation; Warren Pinnacle Consulting, Inc.; Matt Kirwan; (Graphic) K. Franklin/Science

Such rosy forecasts offered a reason for optimism in a field where nearly every forecast is bleaker than the last. One group of researchers, for example, used data from Kirwan and others to predict that thanks to their ability to build vertically, tidal wetlands in the United States will pull carbon dioxide from the air at faster rates as oceans rise.

But Kirwan’s positive outlook didn’t sit well with some experts. Randall Parkinson, a coastal geologist at Florida International University, argued that Kirwan’s 2016 analysis failed to account for the fact that coastal wetlands sink over time as plant roots partially decompose and soils compact under the weight of newly deposited sediment. Those processes ultimately reduce wetlands’ surface elevation, Parkinson says, making them more vulnerable to sea level rise than Kirwan’s team had estimated. Many of today’s wetlands are already on their way to drowning as sea level rise accelerates, he says.

To bolster his argument, Parkinson points to the most recent period when oceans rose quickly: the end of the last ice age. About 20,000 years ago, as massive glaciers began to melt, seas surged; they ultimately rose some 120 meters in 13,000 years. Parkinson collected sediment profiles that were deposited off the coast of Florida during that surge. They showed that mangrove forests—the tropical analog of salt marshes—survived only along narrow coastal strips and were constantly on the move. In contrast, oceans have inched upward far more gently over the past 7 millennia, allowing mangroves and coastal marshes to expand dramatically.

Other recent studies have added weight to Parkinson’s challenge. Last summer, for example, Saintilan and colleagues published an analysis in Science of sediment cores from 78 coastal sites around the globe. It found that mangroves virtually disappeared everywhere whenever sea level rise exceeded about 7 millimeters per year—a rate many regions could see again in just a few decades.

A study of tidal marshes around the United Kingdom that Benjamin Horton, director of the Earth Observatory of Singapore, helped conduct found a similar survival threshold, as did a 2020 study of historical wetlands in Louisiana led by Törnqvist. “The future of the wetlands in this region is not looking very good,” Törnqvist says.

In April, Törnqvist and others took their most direct aim at Kirwan’s work, detailing in AGU Advances their case for why many of today’s wetlands will soon drown. One big problem, they wrote, is that many coastal wetlands aren’t receiving nearly enough sediment to build soil at the rate needed. Although marshes now at the high tide line might be able to hang on for a few decades, they will collapse once they have burned through this “elevation capital,” Törnqvist argues.

Kirwan has welcomed the critique. The AGU Advances study is “fantastic,” he says. “It’s probably the best challenge to some of my own work that I’ve read. … I sent it around to everybody in our lab group.” But he is not convinced that the fate of ancient wetlands foreshadows what will happen in coming decades. “We don’t know a lot about what a marsh [represented by] a sediment core [from] 8500 years ago … actually looked like in nature,” he says. Lower temperatures and atmospheric carbon dioxide levels in the past, for example, meant plants probably grew more slowly, potentially making it harder for marshes to keep their heads above water.

Researchers Emily Hall and Matt Kirwan analyze a soil core from a recently formed marsh near Virginia’s Severn River while Tyler Messerschmidt takes a GPS reading.

Michael O. Snyder is a Bertha Climate Journalism Fellow based in Charlottesville, VA

Horton, who has co-authored papers with both Kirwan and his critics, says more data from more places are urgently needed. “There’s a lack of empirical data to predict the resilience or vulnerability of an increasingly valuable ecosystem.”

Still, Kirwan acknowledges that if waters rise too fast relative to land, as is happening in Louisiana, for example, wetlands will no longer be able to keep up by building vertically. In those cases, they will have to migrate horizontally, colonizing new areas now subject to daily tides.

Whether such inland migrations will happen, however, is largely in the hands of people. In and around cities, many marshes and mangrove forests are, or soon will be, squeezed between rising seas and human infrastructure, such as seawalls, levees, roads, and houses, that block the path to higher ground.

That reality has led some wetland advocates to call for restrictions on development in current uplands that could become wetlands. In Norfolk, Virginia, a nonprofit group called Wetlands Watch is even urging city officials to create incentives for landowners to abandon and demolish structures once migrating wetlands approach within a certain distance.

On a recent rainy spring day, as the Lafayette River spilled over a battered seawall onto streets in Norfolk’s low-lying Larchmont neighborhood, Skip Stiles and Mary-Carson Stiff, two leaders of Wetlands Watch, visited a property they see as a likely future wetland. Already, marsh grasses had encroached into the neat lawn behind the three-story house, whose balcony affords a fine view of the river. Stiff, a lawyer, had started discussions with the property owner. But so far, she says, neither that property owner nor any other has committed to forfeit their house to a moving marsh.

A historical map shows land that was once farmed near the Severn River in Virginia. Many of those fields are now salt marsh thanks to rising sea levels.

Michael O. Snyder is a Bertha Climate Journalism Fellow based in Charlottesville, VA

The idea may be an easier sell in rural places where land is cheaper and less extensive infrastructure blocks wetlands’ path inland. At the Blackwater refuge, for example, federal officials have already bought dry land likely to be overtaken by marsh. The effort was catalyzed by an analysis showing that although some 2000 hectares of refuge marshes have drowned and been replaced by open water since the preserve opened in the 1930s, new marshes have taken over more than 1300 hectares of dry land in and around the refuge.

Those invasions impressed refuge biologist Matt Whitbeck, who began to identify areas where the Fish and Wildlife Service could purchase uplands to create wetland retreat corridors and maintain critical bird habitat. The realization that marshes were readily occupying higher ground and could continue to provide ecosystem services there, Whitbeck says, “was a huge shift in our thinking.”

Humans may even be able to play a positive role in marsh migration. Gedan, for example, noticed that the first salt-tolerant plants to invade previously dry land are often weedy shrubs with few ecological or human benefits. To build a better marsh, she and colleagues have begun a restoration experiment in former farm fields on a fast-sinking peninsula south of the Blackwater refuge. Earlier this year, marsh grasses and other vegetation the researchers had planted were just starting to green up, giving Gedan hope. Farmland like this, she explained, “for a long time has been considered impermeable to marsh migration,” because farmers often used levees and other techniques to keep them from becoming waterlogged.

If such efforts to turn farmland into high-value marsh succeed, experts say paying farmers to give up soggy fields could help wetlands migrate and enhance habitat for species such as black ducks. In Maryland, one state-funded pilot project along these lines is already underway. But some farmers there are still building berms and installing tide gates, sometimes illegally, to keep marshes out. And countries such as Germany and the Netherlands, which heavily armored their coasts long ago, have yet to offer much land back to the sea. “It is a very difficult discussion to have, to give this land up,” Schuerch says.

Ultimately, if Kirwan is right, many wetlands may overcome both rising seas and humanmade barriers. Along the Severn River, for example, Kirwan and his colleagues are finding that the old levees, durable though they were, ultimately failed. Inside and outside the walls, marsh grasses have colonized the former fields with nearly equal vigor. Young crabs skitter about. The marsh, though just a few decades old, seems to be doing its job—a good thing for nature, and for the people whose houses dot the marsh’s edge.

“There’s always been this assumption that people wouldn’t let marshes move; they wouldn’t give up their land,” Kirwan says. “What I’ve tried to show is, not only would we let it happen, we already have.”