Along Earth’s coastlines, grassy wetlands flooded by seawater, called salt marshes, trap and store carbon at rates roughly 40 times higher than forests on land. As salt marshes have expanded in some regions, scientists were hopeful their carbon stores might have largely recovered as well, but a new study found that’s not the case.
Researchers measuring carbon storage in salt marsh soil found that destruction of the world’s salt marshes resulted in a net loss of roughly half a million metric tons of surface soil organic carbon (SOC) between 2002 and 2019 — the equivalent of the emissions from 6,600 passenger cars over the same period. Most of that was from mature salt marshes that stored much more carbon than newly established marshes.
“The most surprising finding … is the paradox that salt marsh area is recovering globally, yet soil organic carbon is undergoing a net loss,” study co-author Xinxin Wang, a wetland ecologist at Fudan University in China, told Mongabay.
The southern U.S. is a global hotspot for SOC loss from marshes, the study notes, with Louisiana’s Gulf Coast ground zero. Hurricanes Katrina and Rita hit the region in 2005, causing immediate damage to rich marshlands. Louisiana’s salt marshes have suffered from decades of industrialization, including more than 75,000 oil and gas recovery wells and nutrient runoff from agriculture.
The weakened salt marshes were torn apart by the storms and largely transformed into open mudflats. Nearly 200,000 barrels of oil and other petrochemicals were spilled during Hurricane Katrina. The same pattern was observed again with the 2017 hurricane season, worsened by rising sea levels. The U.S. made up around 60% of global salt marsh losses from 2002-2019, according to the study.
By contrast, Asia has added almost a million metric tons of surface SOC to the global tally since 2002, the study found; more than three-quarters of it from China, which expanded and restored existing salt marsh areas without suffering any major losses.
“China has not simply restored a few isolated wetlands but built a complete framework from top-level design to on-the-ground implementation,” Wang said.
“This model, which relies on human activity retreatment plus natural recovery, is low-cost, fast-acting, and suitable for most coastal nations worldwide,” he added.
Salt marsh ecosystems “cover less than 10% of the global land area,” the study said, but store one-third of global soil carbon. When these ecosystems are degraded, a significant amount of climate-warming carbon is released.
“Salt marshes are the most capable and silent ecological guardians and carbon storage warehouses along coastlines,” Wang said. “They can lock large amounts of carbon dioxide in soils for centuries or even millennia.”
They’re also crucial habitat for wildlife like fish, crabs and migratory birds, and protect humans from natural disasters and coastal erosion, the study says.
Banner image: Nansha Wetlands Park, China. Image by Sam May via Flickr (CC BY 2.0).
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