The Centrality of Salt Marshes: NE CASC Researchers Quantify Role of Coastal Wetlands in Decarbonizing the Atmosphere

In the accelerating quest to protect natural resources from the most damaging impacts of a changing climate, much attention has been given to natural “carbon sinks:” those primarily terrestrial areas of the globe that absorb and sequester more carbon than they release. While scientists have long known that coastal salt marshes are just such a sink for “blue carbon,” or carbon stored in the ocean and coastal ecosystems, they have struggled to accurately estimate the amount of blue carbon sequestered in these ecologically vital wetlands. As a result, research on carbon storage has disproportionately focused on terrestrial sinks such as forests and grasslands. A new study published by a team of NE CASC researchers promises to alter that focus. The group, which includes former NE CASC fellow Wenxiu Teng as well as NE CASC investigators Brian Yellen, Qian Yu, and Jon Woodruff, has developed a novel and highly accurate method for quantifying carbon capture in the Northeast’s salt marshes. And it’s a lot.

The ocean stores nearly a third of industrial carbon dioxide emissions, and there is a growing global appreciation for the role that coastal ecosystems like salt marshes play as carbon sinks. NE CASC research shows that salt marshes store approximately 10 million cars’ worth of carbon in their top meter of soil and suggests that salt marshes add approximately 15,000 additional cars’ worth every year. The results, published in the Journal of Geophysical Research: Biogeosciences, are an important step toward meeting the challenges of a warming world.

“The amazing thing about tidal marshes, from a climate perspective,” says Wenxiu Teng, lead author of the paper and a Ph.D. candidate at UMass Amherst, “is that they can continuously increase their carbon storage. They don’t fill up.”

This is because wave-by-wave, tide-by-tide, storm-by-storm, new layers of carbon-trapping sediment are continually stored in the thick salt marsh grasses. Furthermore, as glaciers melt, salt marshes grow vertically in order to keep up with rising sea levels, thus storing even more carbon. 

“Salt marshes are far more persistent carbon sinks than forests or other terrestrial sites,” says Yellen, who is also the Massachusetts State Geologist and a UMass faculty member. “There are many people who are excited about technological solutions to scrub carbon from the atmosphere, but here we have a natural mechanism that works, and works very well, right now. Our work helps to clarify the size of this natural carbon sink and provides a method that is scalable to other regions of the world.”

The team’s research also holds a warning — that 10 million cars’ worth of carbon is also a potential “carbon bomb”. If salt marshes are disturbed or their natural processes altered, they could release all those greenhouse gasses, exacerbating climate change, rather than helping to naturally mitigate it. “If salt marshes were to degrade due to the combined threats of local environmental stressors and global climate change,” says Yellen, “they would become huge sources of carbon emissions.” 

To determine how much blue carbon salt marshes can hold, scientists need both a baseline for how much has already been stored and an accurate way to measure the rate at which the marshes can sequester carbon. Both have been very difficult to pinpoint, in part because marshes themselves are highly variable ecosystems with diverse storage rates. The ideal would be to take a soil sample from every meter of every marsh and measure the carbon stored in it — a prohibitively expensive and time-consuming process.

Another option would be to turn to satellite images, but satellites, powerful as they are, can’t detect the carbon stored in the salt marsh’s sediment itself. However, satellites can see variable water depth and vegetation across the marsh, the two main factors that drive marsh soil formation and carbon storage. Consequently, the team used a common tool from the satellite remote sensing world called the Normalized Difference Water Index (NDWI) to look at spatial patterns of water depth and vegetative vigor to map out soil differences across marshes. Yet, the NDWI constantly fluctuates with seasonal vegetation growth and tidal changes, further complicating the team’s efforts.

Seeking to overcome this problem, the researchers eventually realized they could compare satellite NDWI data from multiple seasons and different tidal levels against robust samples of salt marsh sediment they had collected at 19 sites extending from the Gulf of Maine to Long Island Sound. 

“We started looking at the satellite data plotted against the field samples, and we had this ‘a-ha!’ moment,” says Yellen. The team could clearly see that there were particular tidal conditions and times of year where the satellite data closely tracked the data they had gathered in the field.

“It’s really all about inundation at high tide — that’s when you want the satellite to capture the picture,” says Yellen.

Once the team knew what type of satellite images were the most reliable, they could find those focused on the Northeast and use them to generate the most accurate estimate yet of just how much blue carbon these marshes have stored and are continuing to store. “Salt marshes alone can’t account for all the carbon that we’re currently releasing into the atmosphere,” says Yellen, “but if we are going to achieve carbon neutrality in the future, salt marshes can help offset the hardest parts of the economy to decarbonize. We just need to be sure that we protect them in the meantime.”

“These salt marshes are crucially important ecosystems for all sorts of reasons,” says Teng. “Now we know that they’re rich not only in terms of biodiversity, but also in terms of helping the planet to weather the worst of climate change.”

This article was adapted from a UMass Amherst press release.