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Deep Seagrass Bed Could Stall Climate Change, If Climate Change Doesn't Kill It First

Researchers studied the carbon storage of deep-water seagrasses living at Lizard Island, Australia.
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Researchers studied the carbon storage of deep-water seagrasses living at Lizard Island, Australia.

Amid a sea of dire climate change news, researchers say they've found a rare bright spot.

A meadow of seagrass among Australia's Great Barrier Reef — estimated to be twice the size of New Jersey — is soaking up and storing carbon that would otherwise contribute to global warming.

Scientists call this carbon-removal powerhouse a "blue carbon sink." The term refers to an ocean or coastal ecosystem — including seagrasses, salt marshes and mangrove forests — that captures carbon compounds from the atmosphere, effectively removing carbon dioxide, a known greenhouse gas that contributes to climate change.

"These coastal Blue Carbon ecosystems can sequester or remove carbon from the atmosphere about four times the rate of terrestrial forests on land, and they store about 10 times more carbon in the system itself compared to forest on land," says Jennifer Howard, director of marine climate change at Conservation International, in an interview with NPR's Michel Martin.

A new study published in Biology Letters suggests that these deep-water seagrass meadows play a more central role in the carbon cycle than previously thought. Authors Peter Macreadie of Deakin University in Australia, and Paul York and Michael Rasheed, both from James Cook University, compared carbon stocks from deep-water, mid-water and shallow-water seagrass living at Lizard Island in the Great Barrier Reef. The researchers found that the seagrass in deeper regions contained similar carbon levels as seagrass in shallower waters.

Previously, data has been very sparse on deep-water seagrass as Blue Carbon sinks. They're hard to get to, buried deep beneath water and invisible even from satellite.

"You usually have to throw somebody in the water with a scuba mask to go actually find them," Howard says. "And because of that we just don't know how many of these large patches of sea grasses there actually are out there."

Howard says her organization has mapped nearly 109,000 square miles, "but that's probably less than half of what's actually out there."

What's more, that number only reflects the more detectable, shallower seagrasses. As for the deeper-water seagrasses analyzed in the study, the Australian researchers figured that if the deep-water seagrass stores a comparable amount of carbon as other deep-water meadows in the region, the area around the Great Barrier Reef may be sequestering tens of millions of tons of carbon.

Worldwide, Howard says, "We think that there's probably about several billion tons of carbon locked away in these seagrass meadows" — ecosystems, she says, that exist on every continent except Antarctica.

These new findings are valuable to policymakers working to curb climate change, according to Howard. But the value of these ecosystems, she says, disappears if they're not protected.

"When you destroy those ecosystems, all that carbon can be re-released back into the atmosphere. So, through poor land use management or through degradation, the significant carbon sink can actually become a global carbon source."

Seagrasses are vanishing globally at a rate of 1.5 percent per year, Reuters reports, a decline that's comparable to coral reefs and tropical rainforests. Scientists point to coastal development as the culprit behind water pollution that perpetuates erosion. And, while seagrasses can mitigate climate change, climate change can also destroy the grasses.

"When you have pollutants and too much sediment running down the river, it blocks out the light, it buries the seagrass and they start to die," Howard says.

She says the solution to alleviating seagrass loss has to be "land-based."

"You're going to have to address the pollution component first. Remove the threat, and then planting [seagrass] could be a very viable option to increase carbon stock."

NPR's Chad Campbell and Martha Wexler produced and edited this story for broadcast.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Michel Martin is the weekend host of All Things Considered, where she draws on her deep reporting and interviewing experience to dig in to the week's news. Outside the studio, she has also hosted "Michel Martin: Going There," an ambitious live event series in collaboration with Member Stations.