Smelly and saturated with seawater, the marsh muck sucks at the waders of UC Santa Cruz graduate student Aliya Khan as she walks along a channel in Elkhorn Slough. She places a tube into the water, which will collect samples that will help uncover the salt marsh’s ability to serve as a carbon dioxide vacuum and vault.
Khan’s research is taking place at an important time.
“The year 2024 is on track to be the warmest year on record,” says the World Meteorological Organization in a press release published earlier this week. It will also be the first year with global temperatures more than 1.5 degrees Celsius above pre-industrial levels, a milestone that will intensify fires, floods, and other climate-fueled disasters.
Salt marshes, which have historically been drained and turned into farms or land ripe for real estate development, are emerging as a powerful tool in the fight against global warming.
“The vegetation that falls into standing water that has no oxygen never decomposes,” Khan says. “So that CO2 never goes back to the atmosphere.”
They differ from land-based ecosystems like forests, which release most of their stored carbon dioxide when trees die and decompose.
Scientists are measuring how much carbon dioxide Elkhorn Slough can suck from the atmosphere. Their research is funded by a $3.5 million grant awarded in 2022 by the University of California Office of the President. It is part of a broader effort to find ways to remove greenhouse gasses from the atmosphere.
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“Wetlands are one of the best natural systems to sequester CO2,” says Adina Paytan, a principal investigator at the UCSC Center for Coastal Climate Resilience. She is leading the study.
A 2023 review estimates that restoring the world’s degraded salt marshes could sequester up to 0.5% of the carbon emitted by fossil fuels annually, a big number for an ecosystem that covers less than 1% of Earth’s surface. But the authors admit massive uncertainties.
Alicia Karspeck, co-founder and chief technology officer of (C)Worthy, a nonprofit that studies ocean-based carbon dioxide removal, also emphasizes that there are many unknowns. A myriad of environmental variables can change how living systems behave.
“We don’t know exactly what is the best design to maximize the benefits,” Paytan concurs. She and colleagues are studying carbon storage in the Sacramento River Delta, South San Francisco Bay and Elkhorn Slough to find the combination of conditions that maximizes wetlands’ climate-cooling effects.
Their findings will have real-world applications. Paytan says they have a team that is studying governance to convert their findings into policy. One idea is to create a carbon market where polluters can fund the restoration of salt marshes to offset their emissions. There is also an environmental justice arm that is looking into who might benefit and who might be harmed by restoration projects.
Methane emissions could also limit their climate-cooling effects. Bacteria that live in marsh muck produce methane, a greenhouse gas that is 80 times more potent than carbon dioxide. One study found that a freshwater swamp in the Sacramento River Delta produces enough of it to offset the cooling effects of sequestering CO2. Those bacteria, however, do not like salt, so salt marshes produce less methane.
Quantifying Elkhorn Slough’s carbon storage is a complicated process.
J.J. Jabuka, a graduate student in Paytan’s lab, studies one part of the equation. She uses large towers full of sensors to measure the amount of carbon dioxide and methane that study sites in Elkhorn Slough exchange with the atmosphere. She uploads her data to Ameriflux, a database that contains similar data from over 500 ecosystems across the Americas.
Her early findings suggest that a healthy, established section of Elkhorn Slough is absorbing lots of carbon, while Hester Marsh, a site that was restored in 2018, sequesters a smaller amount.
“That information is going to be really helpful for us to understand the rate at which our restoration projects become more functional,” says Monique Fountain, director of the Tidal Wetlands Program at Elkhorn Slough.
Khan’s research focuses on the fate of the carbon that Elkhorn Slough absorbs. While some is stored in the soil, much of it is swept out to sea by tides. She analyzes what types of carbon molecules the water at the slough contains. Some forms of carbon quickly turn into CO2 and re-enter the atmosphere, while others will remain buried underwater for millennia.
Her research is ongoing. In the meantime, Khan thinks we should prioritize protecting existing salt marshes over building new ones. Restored marshes “don’t necessarily have the same ecosystem benefits as a longstanding, healthy one,” she says. “Protection over restoration, when that’s an option, 100% of the time.”
