In the race to combat global climate change, 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, it has been difficult to get an accurate estimate of just how much they store, and so most of the focus has been on terrestrial sinks such as forests and grasslands.

Now, a team of scientists at the University of Massachusetts Amherst has debuted a new, highly accurate method for quantifying carbon capture in the Northeast’s salt marshes—and it’s a lot.

Their work 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 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.

“The amazing thing about tidal marshes, from a climate perspective,” says Wenxiu Teng, lead author of the paper and a Ph.D. candidate in the Department of Earth, Geographic and Climate Sciences (EGCS) 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 continue to 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 Brian Yellen, Massachusetts’s state geologist, research assistant professor at UMass Amherst and one of the paper’s co-authors. “There are many people who are excited about technological solutions to scrub carbon from the atmosphere, but here we have a natural one 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 work also holds a warning—that 10 million cars’ worth of carbon is also a potential carbon bomb. If the salt marshes are disturbed or their natural processes altered, they could release all those greenhouse gases, 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 figure out 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, notes Qian Yu, EGCS associate professor and one of the paper’s co-authors, satellites, as powerful as they are, can’t see 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. 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.

What the team realized it needed to do was to compare satellite NDWI data from multiple seasons and different tidal levels against a robust sample of salt marsh sediment gathered in the field—which they had already done, traveling to 19 sites from Long Island Sound to the Gulf of Maine and collecting 410 samples representing multiple locations in each salt marsh.

“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 the specific ones focused on the Northeastern coast 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.”

EGCS professor Jon Woodruff and Bonnie Turek, who contributed to this research as part of her graduate work at UMass Amherst, are also co-authors.