Climate scientists know with impressive accuracy how fast carbon dioxide is increasing in the atmosphere. A decade ago, there were about 370 parts per million of the heat-trapping gas floating around; today, the number is up to 393 or so. Much of that increase comes from the burning of fossil fuels like coal, gas and oil — but not all, and putting numbers on other sources is crucial to figuring out how to keep carbon emissions under control.
Now a team of researchers has put at least a rough number on one of those sources. According to a new paper in the journal PLOS ONE, so-called blue carbon – carbon that is pulled from the atmosphere by coastal vegetation including mangroves, sea grasses and salt marshes and stored away in sediments at the edge of the sea—is now being released at the prodigious rate of between 150 million and 1.02 billion tons every year.
That’s between 3 and 19 percent of the carbon released by deforestation, say the authors, and results in economic damage of between $6 billion and $42 billion annually.
The reason carbon is escaping after being stored for many tens of thousands of years, co-author Daniel Donato of the University of Wisconsin, Madison, said in an interview, is “disturbances” from human activity.
“There’s a whole range of them,” Donato said, including the conversion of coastal mangrove swamps to shrimp aquaculture, conversion of marshes to rice paddies, creation of river dams that starve downstream areas of sediments, offshore dredging, and urban development, to name just a few.
Natural forces, such as hurricanes, disturb coastal ecosystems, too. “Those have been happening for millennia, and the ecosystems have evolved to handle them,” Donato said. It's far more difficult to recover from coastal development and construction, which tend damage ecosystems far more severely, and far more permanently.
Until recently, few thought much about blue carbon. “It really started getting attention only in the past year,” said co-author Linwood Pendleton, a resource economist at Duke University’s Duke’s Nicholas Institute for Environmental Policy Solutions and the acting chief economist at NOAA as well. “At first, people didn’t know whether to take it seriously or not.”
Late last year, Pendleton organized a meeting to talk about the issue and assess the state of the science; the new paper, written jointly by participants in that meeting, is the result.
Once the scientists began looking into it, Donato said, “we realized that organic matter can accumulate into these huge, deep layers that can be as much as 25 feet thick. The plants grow and die, but once they’re buried under sediments, the lack of oxygen means they don’t decompose.”
The amount of buried carbon per acre is significantly more than the amount stored in a tropical rainforest, he said, which most people think of as a major carbon storehouse. And that's true even if you only assume the buried layer of blue carbon is only 3 feet deep, which Pendleton and his colleagues did for their calculations so they'd be sure to err on the side of being conservative.
Even so, the emissions and cost figures the authors came up with span a wide range. The reason, Pendleton said, is that while ecologists have a good idea of how fast salt marshes and sea grass habitats are being disturbed, they’re more uncertain about what’s happening to mangroves.
Nevertheless, the study is a crucial first step in understanding an important source of atmospheric carbon, and in figuring out how to limit emissions.
“We’re going to submit this to the IPCC for their next major report,” Pendleton said, “and say ‘this seems to be a substantial source of carbon. Policymakers should figure out what to do about it.”