WASHINGTON — Along the three coasts of the contiguous United States, a silent race unfolds. Fueled by climate change, sea levels rise faster than at any time in recent geologic history. Meanwhile, coastal wetlands cling to a slim lead, lifted on newly accumulated sediments or moving inland as encroaching tides nip at their heels.
Now, the first national-level study of this competition reveals that coastal wetlands — among the country's most valuable ecosystems, protecting infrastructure and providing homes for wildlife — are losing ground: 84% are gradually inundating, with many on track to transform into open water. Wetlands along the Gulf Coast are generally faring the worst while certain stretches of the Atlantic Coast appear more resilient. Some wetlands may be able to migrate inland or build up sediment fast enough to keep pace with sea level rise, perhaps with the right help from land managers.
"Our study provides wetland managers with new information to help them make better-informed decisions and identify meaningful solutions to issues they may be encountering, including to facilitate wetland migration upslope," said Glenn Guntenspergen, a coastal wetland ecologist with the U.S. Geological Survey and senior author on the study.
The study will appear Monday, 3 August in Earth's Future, AGU's journal for research on the state of the planet and its inhabitants and their future resilience in the Anthropocene.
Water, water everywhere
Globally, sea level rise averaged 1.7 millimeters per year during the 20th century but has sped up in recent decades, hitting 3.8 millimeters per year from 2006 to 2018. Up to a point, higher water levels can stimulate the growth of wetland plants, a phenomenon that has helped many wetlands persist up to now. But even the most water-loving plants can only take so much.
"All coastal plants have optimum ranges," Guntenspergen explained. "But if they get flooded too much, then the system shuts down. The plants decrease production and eventually die."
Justine Neville, a wetland ecologist at Duke University and lead author on the study, noted that many endangered and endemic species and migratory birds in the United States depend on these ecosystems for habitat. Wetlands dissipate the impact of storm surges, protecting buildings and transportation networks in coastal towns and cities. People visit them to go birdwatching or, in some places, hunting. All told, Neville said, coastal wetlands in the United States provide trillions of dollars' worth of services every year.
"There's a whole host of benefits to culture, to flora and fauna, and economically speaking, so it is really important that we understand how [wetlands] are changing," she said.
To get a nationwide view of that change, the team drew on existing data from nearly 450 monitoring stations across the contiguous U.S. coasts tracking wetlands' elevations for up to 20 years. Comparing this with tide gauge data allowed them to see which wetlands were — and weren't — gaining elevation fast enough to stay above water.
Previous studies had mostly looked at just a handful of wetlands at a time, partly due to the laborious nature of the fieldwork. It takes hours to gather data from the analog devices that track sediment buildup and land subsidence in wetlands, Neville said, and to even reach the site, researchers must cross marshy ground carrying heavy equipment. Access sometimes requires a boat; Guntenspergen recalls one site so remote it demanded a helicopter.
"The logistics are quite daunting," he said.
A fighting chance remains
The team found that 89% of coastal wetlands are gaining elevation through sediment buildup, but only 16% are doing so fast enough to outpace sea level rise, while the rest are submerging. "I was not very surprised, to be honest," Neville said. "A lot of the literature that we've read from all the different regions is showing similar results."
However, regional differences still emerged. Overall, the Atlantic Coast appeared least vulnerable: submergence is slow enough at about 75% of Atlantic monitoring stations to rule out any critical threat of submergence, partly thanks to higher rates of marine sediment deposit from ocean waves.
In contrast, submergence is fastest on the Gulf Coast, where wetlands sink under the weight of numerous human pressures in addition to sea level rise. Neville cited the historical impact of shipping canals altering the Gulf's waterways, which has lessened the flow of river sediments that sustain wetlands, and of oil and gas extraction, which reduces pressure underground enough that wetlands above literally subside.
"What is really important about what we found, though, is how many of these wetlands have the opportunity to migrate," Neville said. Seventy-three percent of wetlands in the study still have a chance to expand inland rather than drown in place, provided the land slope isn't too steep and no human structures block the way. Gulf wetlands may have the most available space, while hills and cliffs often hem in their Pacific counterparts.
Having all this information can help reveal where land management solutions — including removing dikes, artificially adding sediment to wetlands and protecting inland areas for wetlands to migrate into — will have the most impact.
"It's not all doom and gloom," Neville said. "Yes, there is an urgency to understand what's happening with our wetlands, but one thing this paper can demonstrate is where we might want to focus our [conservation] efforts."
