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Coastal wetlands have long been seen as one of the casualties of climate change, doomed by the rising seas that are steadily swallowing their ecosystems. New research offers a surprising twist, suggesting that marshes aren’t just adapting to the tides—they may even be helping to cool the planet as sea levels rise.
New research by Virginia Institute of Marine Science coastal geomorphologist Matthew Kirwan has revealed that some marshes, migrating as they adapt to changing conditions, may release carbon (primarily as carbon dioxide) but gain an enhanced capacity to store methane. Methane is a more potent greenhouse gas than carbon dioxide, and sequestering it may have an atmospheric cooling effect.
“Even degrading marshes can still sequester carbon effectively.”
As sea level rises, freshwater marshes get saltier and turn into salt marshes. Conventional wisdom has long held that as freshwater marshes shrink, they release carbon stored in their soil and biomass. But Kirwan pointed out that as freshwater marshes degrade and salinize, their microbial populations are affected in a way that causes the marshes to emit less methane.
“Even degrading marshes can still sequester carbon effectively,” Kirwan said. He will present this research on 10 December at AGU’s Annual Meeting 2024 in Washington, D.C.
A Nuanced Transition
Earlier research used data from estuaries across the United States to examine the potential for coastal wetlands to adapt to sea level rise through inland migration. Patrick Megonigal, an ecosystem ecologist at the Smithsonian Environmental Research Center who collaborated with Kirwan on the new research, built on this work by computing for how these adaptations affected the release and sequestration of different greenhouse gases.
“This switch [from freshwater marshes to salt marshes] is helping to balance out the carbon loss we might see from erosion and changes in marsh landscapes,” Megonigal said.
He emphasized that their projection is an optimistic view based on a hypothetical and instantaneous change from the state of coastal wetlands in 2024 to projected sea level in 2100.
In reality, the shift will be more nuanced. For instance, landscape transitions that result in greater carbon emissions, such as when an upland forest evolves into a salt marsh, can happen faster than landscape transitions that result in carbon sinks.
“I think the bigger point here is that these transitions will be complex. They’ll influence [and] simultaneously be influenced by how much carbon is being sequestered in wood and soil” and by the emission and absorption of gases, said Megonigal.
Risks for Biodiversity
The nuanced transition is not only about climate. Deborah Landau, director of ecological management for the Maryland/D.C. chapter of The Nature Conservancy, has witnessed firsthand the gradual yet accelerating changes happening in the marshes she has studied for more than 2 decades.
Landau described watching a bench overlooking a marsh become submerged, over the course of years, by rising water. What started as a gauge of sea level rise came to represent a symbolic disappearance of the coastline.
“It’s happening faster now than ever before,” she said, noting that it is sometimes hard to distinguish between the effects of rising seas and saltwater intrusion.
Erin Peck, a geomorphologist at the University of Rhode Island specializing in salt marshes, said the findings “challenge our gut reaction” that rising seas will hinder the capacity of marshes to absorb carbon.
Peck is cautious about drawing too many conclusions from the new research, however. While freshwater marshes are slowly giving way to their salinized counterparts, Peck stressed that salt marshes, too, have been in decline for decades. Coastal development has contributed to eradicating both salt marshes and the lands they could potentially migrate to as the sea rises.
“We still need to do all we can to protect salt marshes, because they provide more than just carbon burial—they offer flood protection, habitat for species, and much more.”
“We still need to do all we can to protect salt marshes,” she said, “because they provide more than just carbon burial—they offer flood protection, habitat for species, and much more.”
Landau pointed out that The Nature Conservancy is buying land adjacent to existing marshes specifically to allow these ecosystems to migrate inland as sea levels rise, but said policymakers need to do more.
“If sea level rise led to the complete demise of marsh carbon cycling, restoration efforts would be fleeting,” Kirwan said. “But instead, the resilience of these ecosystems makes investing in them worthwhile.”
—Rambo Talabong (@ramboreports), Science Writer
Citation: Talabong, R. (2024), As seas rise, marshes may still trap carbon—and cool the planet, Eos, 105,https://doi.org/10.1029/2024EO240544. Published on 10 December 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
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