Hakai Magazine

Seagrass Meadows, Cenderawasih Bay, West Papua, Indonesia
Seagrass grows in thick mats on the seafloor. The flowering plants trap carbon dioxide, but exactly how much is up for debate. Photo by Reinhard Dirscherl/Alamy Stock Photo

Slinging Sediment

Scientists may have been systematically overestimating seagrass as a carbon sink wonder.

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by Evan Lubofsky

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Seagrass meadows take up less than 0.1 percent of the world’s oceans; nevertheless, they are considered a huge carbon sink. Seagrass draws carbon dioxide out of the atmosphere, using it to fuel its own growth through photosynthesis. When the seagrass dies, much of this carbon is locked away in the sediment. Estimates from more than a decade of what’s called “blue carbon” research suggest seagrass beds store as much as 83,000 metric tonnes of carbon per square kilometer—three times as much as forests—and lock it away for millennia.

These dramatic rates of carbon storage have caused scientists, conservationists, and others to champion seagrass beds as a way to mitigate climate change. But there’s one problem: the numbers may be wrong.

“Researchers in the blue carbon community have overestimated how much carbon stays buried in seagrass beds,” says Sophia Johannessen, a geochemical oceanographer with Fisheries and Oceans Canada (DFO). “They certainly don’t store as much carbon as is currently claimed in the big international studies.”

Johannessen’s conclusions, which were published last year in the scientific journal Environmental Research Letters, have wide-ranging implications for scientists’ understanding of the planet’s carbon balance. Typically, scientists think seagrass meadows are responsible for 18 percent of the ocean’s carbon storage. Johannessen says those estimates could be 10, or even as much as 3,000 times, too high.

Johannessen’s findings have been met with resistance from the blue carbon research community. Yet if she’s right, they could cause an important recalculation of marine carbon cycling.

More than that, Johannessen’s findings could shake up the burgeoning carbon offset industry. Greenhouse gas markets base the sales of carbon credits from seagrass bed restoration projects on estimates of how much carbon they store. If seagrass stores less carbon than thought, the value of these seagrass beds plummets. Similarly, if carbon credits are awarded on the basis of overblown estimates, the net effect could be an unintended increase in the amount of carbon dioxide being released into the atmosphere.

So how, according to Johannessen, did blue carbon researchers get their estimates so wrong?

Johannessen says it all comes down to bad math. Blue carbon researchers, she says, misunderstand how marine sediments receive, process, and store organic carbon.

“The authors of all the previous blue carbon papers have expertise in seagrass biology or terrestrial carbon burial, but they do not understand how marine sediments work,” she says.

In particular, she says the rate at which sediment builds up in seagrass beds has been overestimated, as have the concentrations of carbon within the sediments. And carbon storage estimates are made by multiplying those two factors.

Some blue carbon researchers, however, believe Johannessen is wrong. Yesterday, a group of scientists from Deakin University in Australia fired back, publishing a response in Environmental Research Letters criticizing her methods and conclusions. They insist that previously published carbon storage rates are indeed correct.

“Johannessen’s paper misreported, miscalculated, and cherry-picked what they reported,” says Peter Macreadie, head of Deakin’s Blue Carbon Lab and lead author of the response.

“The blue carbon world is aware of the limitations and scientific challenges ahead when it comes to measuring carbon dioxide abatement, but it’s unhelpful to have tire kickers who aren’t prepared to be part of the solution,” Macreadie says.

Johannessen says she’s only trying to help. In the past, when she has come across inaccurate storage estimates in international protocol documents, she made a point of reaching out to the authors, offering to explain how they could improve their methods and calculations. No one, she says, has taken her up on the offer.

Johannessen has ruffled some feathers with her bold claim, but she’s not alone in her assertion that blue carbon researchers have got their math wrong. Thomas Bianchi, a professor of geology at the University of Florida, says he understands her stance. Without an accurate representation of how carbon moves through marine sediments over time, he says, scientists have only snapshots of the current carbon storage.

The two camps are arguing over more than mud. The stakes—an accurate assessment of global carbon stores—are high. But Johannessen says there’s still time to make things right.

“There is nothing to be done about the scientific papers that are already published,” she says. “But it’s not the end of the world for the field.”

“This is how science advances,” she adds.