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A year on from Hurricane Katrina, the small Louisiana town of Luling, about 25 miles west of New Orleans, embarked upon a modest experiment. Instead of discharging treated municipal wastewater through a canal and into a nearby lake, the town would pump its effluent into surrounding bayou swampland.
The change would be a small step in reversing the ecological damage of flood control along the Mississippi River, which for centuries had slowly separated America’s longest river system from its natural floodplain. Although it opened up more land for farming and habitation, flood control set the scene for the levee failures in New Orleans, where more than 1,000 people died in Katrina and its aftermath. Preventing natural seasonal flooding also deprives the Mississippi delta of sediments that renew it, contributing to the loss of 30 percent of its land in the past half-century.
A mangrove swamp might contain 25 times as much carbon as a similar patch of terrestrial forest.
In Luling, sediments that would have flowed directly into the lake would now be trapped in the town’s bayous. Nitrogen, phosphorous, and other chemicals in the cloudy wastewater would enhance the growth of native bald cypress and water tupelo trees. These in turn would soak up floodwaters, prevent erosion, and provide crucial ecosystem services. The trees would be a boon for wildlife: Bald cypresses serve as breeding grounds for amphibians, provide nesting spots for ducks and raptors, and shelter young catfish among their submerged roots.
On top of all those benefits, the revitalized swamp would store lots of carbon—at least in principle. In practice, it wouldn’t be so simple.
So-called “blue carbon” aquatic ecosystems like Luling’s bald cypresses, Australia’s seagrass meadows, and tropical tidal marshes store an estimated 300 billion tons of carbon worldwide—an amount roughly comparable, by some estimates, to the lifetime emissions of all the power plants in the world. A mangrove swamp might contain 25 times as much carbon as a similarly-sized patch of terrestrial forest.
These capacities have made wetlands appealing targets for the fast-growing, near-trillion-dollar carbon accounting industry, which uses carbon offsets and carbon credits to—hopefully—reduce global CO2 emissions. Each offset or credit is supposed to represent an actual ton of CO2 sequestered or prevented from entering the atmosphere. To ensure that happens, the science behind them, and oversight of the projects that generate them, must be rock solid. This is far from certain today, where even the largest schemes have faced accusations of shoddy measurement, weak verification, and outright fraud.
“There’s a lot of bogus carbon projects out there,” says Robert Lane, chief operating officer at Comite Resources, the coastal science consultancy behind the Luling project. “Real carbon projects take an area of land and do something to it so it sequesters more than it would without intervention. And it’s that extra sequestration that you can monetize.” But monetizing Luling’s swamps would prove tricky.
In September 2012, the 600 hectares of Luling’s wastewater wetlands became the first blue carbon project approved by the American Carbon Registry (ACR), a private non-profit greenhouse gas registry. Over the next 40 years, its credits would be sold to pay for ongoing carbon monitoring costs and to compensate the bayou’s owner, a private real estate and development company, “We calculated that the area would sequester 11,617 tons of CO2 each year,” says Lane. “If the price of carbon were $20 a ton, which is a reasonable market price, that would generate $232,340 per year.”
At first, things seemed to be going well. The wastewater diversion went smoothly, and the wetlands appeared to be flourishing. In 2014, Lane made measurements at Luling that suggested the bayou was indeed sequestering over 11,500 tons of carbon a year.
Then the science at the swamp got murky. Any natural system will have sources of carbon as well as sinks. In the case of Luling, while soils and trees sucked up carbon, microbes in the swampy soils simultaneously released greenhouse gasses like methane and nitrous oxide. And each time Lane went to the bayou to measure those gasses, he got wildly different numbers. The uncertainty was “very problematic,” Lane says. “When you’re talking about money, all the i’s need to be dotted and the t’s crossed.” Where ACR wanted uncertainty in greenhouse gas emissions to be no greater than 10 percent, some of Lane’s data had error bars of over 100 percent.
Measuring the carbon captured by the bayou was similarly frustrating. For terrestrial forests, an aircraft with a laser-imaging LIDAR system can quickly record the quantity of vegetation above ground, giving a good idea of the carbon stored in the wood. But much of the carbon in coastal ecosystems is in the soil itself, requiring expensive and time-consuming manual sampling to unearth.
In 2018, Comite at last gave up trying to get the numbers to work, and abandoned its ACR certification efforts. Although the site continues to thrive as a habitat for wildlife and a lush coastal forest, Luling has never generated a single blue carbon credit.
Blue carbon projects can also qualify through what is called avoided deforestation: protecting an intact coastal ecosystem rather than developing it to, say, farm shrimp or grow rice. In some ways that’s easier to account for than ecosystem restoration: An area can be surveyed, the carbon it contains estimated, and credits then banked for preserving it. But this approach adds another complication. “You have to prove it was going to be lost,” says Dan Friess, deputy director of the National University of Singapore’s Centre for Nature-based Climate Solutions, and a member of the International Blue Carbon Initiative. “You have to provide evidence that the ‘business as usual’ scenario would be deforestation of this area.”
In general, such evidence is not hard to come by. Researchers estimate that mangrove forests alone could release 3 billion tons of carbon by this century’s end due to ongoing clearance for agriculture, aquaculture and human settlements, and coastal erosion. But for a particular site to qualify for a carbon credit, the threat to it must be proven—which is not always easy. Critics claim that terrestrial carbon credits are too often generated for sites that are unattractive for development, and would be left untouched anyway.
Although the carbon savings of avoided deforestation projects are easier to measure, there are still only about a dozen approved blue carbon credit projects worldwide. “Generally we know enough about blue carbon to make these things work,” Friess says. “The barriers today are more about social, economic, and governance limitations.” Who owns the land? Who gets to decide how it’s used? And who has the rights to the carbon benefits it generates: the landowner or the government?
Layered upon jurisdictional and legal issues is the sheer expense of management. An experimental blue carbon mangrove project at Cispata Bay in Colombia was approved by Washington-state based Verra in 2021, and aims to capture one million tons of carbon dioxide over its 30 year span. Despite selling all of its credits at above-market rates, the income will only cover about 70 percent of the restoration and conservation costs. Mangroves are notoriously fussy about their growing conditions, and may require expensive shepherding to colonize new areas. The remainder of the Cispata Bay funds will come from governments, NGOs, and corporate partners like Apple.
Wetlands submerged by rising seas are likely to release around half of their stored carbon.
“Blue carbon isn’t cheap,” agrees Emily Landis, climate and ocean strategy lead within The Nature Conservancy’s Global Tackle Climate Change team. She hopes that if the per-ton price of carbon continues to increase, and the science of blue carbon becomes more settled, the funding gap will shrink. TNC has more blue carbon projects in the pipeline, including some that it hopes will generate carbon credits within a year.
Landis worries that as blue carbon grows in popularity, it might fall prey to the type of lower quality or even fraudulent schemes that have plagued forestry projects. “There are going to be issues because there always are where money is involved,” she says. “Investors need to be asking questions before they purchase these carbon credits.”
TNC, in partnership with Conservation International, Salesforce, and the World Economic Forum, is developing guidelines for what constitutes a high-quality blue carbon project—including, apart from all the scientific and logistical considerations, sharing revenues with local communities. Without their support, the long-term viability of projects is uncertain.
Even if the numbers add up, though, sea level change could pose an existential problem for some projects. Wetlands submerged by rising seas are likely to release around half of their stored carbon, according to Lane. “The biggest upset of the whole Luling project was trying to prove sustainability over a 50- to 100-year time span,” he says. “Proving that a wetland can survive through sea rise predictions of a meter to two meters is rather problematic, because really it can’t. If there’s a rise of two meters by the end of the century, most of the wetlands in the world are going to be submerged.” This is crucial because the highest-quality verifications, like those provided by Verra, won’t issue credits unless a project is assured of permanence for at least 100 years.
Friess is more optimistic, noting that coastal ecosystems will vary in their response to climate change. “There are some mangrove forests that will be resilient to sea level rise, because they can increase their surface elevation at the same rate,” he says. In the United States, however, a recent analysis by the U.S. Geological Survey estimates that landward migration of coastal wetlands will not counter seaward losses. Potential migration will likely occur at the expense of either coastal freshwater wetlands, or uplands such as croplands, forests, and pastures.
Whatever the eventual fate of coastal wetlands, these blue carbon ecosystems should help soften the impacts of climate change in years to come by providing fish habitats, trapping pollutants, and absorbing storm surges and other extreme weather. Mangroves alone are estimated to already provide more than $82 billion in annual storm protection globally. TNC says that it is in the final stages of getting approval and validation for monetizing that protection into something called a resilience credit, to be sold alongside the carbon credit. Resilience credits are calculated by looking at the number of people and the value of assets like farms and dwellings protected by coastal ecosystems.
“We do know that there is interest in the marketplace for resilience credits,” says Landis, “and that blue carbon credits get a higher price with resiliency on top.” The hope is that projects generating both credits would ultimately cover the entire cost of conservation or restoration.
Ultimately, blue carbon credits face the same dilemma as any nature-based climate solution. A financial framework creates incentives to support the valuable work ecosystems do in storing carbon and protecting communities. The moment those benefits are monetized, though, opportunities to game the system emerge. Yet while it is obviously absurd to reduce the glorious, multifaceted complexity of a healthy swamp to a simple financial instrument, it may give it a better chance to persist—and even help—in the climate struggles that lie ahead.
“Blue carbon is not a get-out-of-jail-free card,” warns Friess. “But if we can do it correctly, reduce the risks, and overcome some of the barriers, I think it will play a small but meaningful contribution to solving the climate crisis.”
Back in Luling, nobody is trying to measure carbon anymore. The newly-restored swamp, however, is thriving. 
Lead image: C. Ray Shea / Shutterstock
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