Ocean-based techniques and their carbon capture potential

The business of carbon capture can be controversial among those who place priority on ending carbon emissions rather than relying on technologies that seem to prolong fossil fuel use. But it promises to be a necessary US$259 billion industry by 2050, and a new report from the National Oceanic and Atmospheric Administration details the ocean-based carbon capture techniques that could be deployed to achieve climate goals.

Those ideas extend beyond direct air capture and other more familiar geoengineering strategies. Of the 11 ocean-based carbon removal strategies presented in the “Strategy for NOAA Carbon Dioxide Removal Research,” a white paper released last week, four were rated most highly by authors from the Pacific Marine Environmental Laboratory (PMEL) for their potential.

Coastal blue carbon is one solution. It reflects the carbon that’s stored in natural salt marshes, mangroves, and other coastal wetlands. “Coastal wetlands form deep, carbon-rich soils, and store carbon at a much greater rate per unit area than terrestrial habitats, which store carbon primarily in aboveground biomass,” explain the team from NOAA’s Coastal Blue Carbon Working Group, along with marine habitat resource specialist Janine Harris.

The potential impacts are high, the existing know-how is high, and the costs are comparatively low. “These coastal blue carbon habitats provide additional benefits, including fishery nursery habitat, improved water quality, recreation, tourism, and flood and erosion mitigation,” the authors add.

Ocean alkalinity enhancement is another example. Authors Richard Feely and Brendan Carter explain that oceans hold 45 times more carbon than the atmosphere does because they are more alkaline due to minerals that have accumulated over thousands of years. As a result, seawater is less acidic and more basic. This allows it to naturally remove carbon in the air and store it as bicarbonate (HCO3).

“Overall, some estimates suggest that the timescale of carbon sequestration by alkalinity enhancement could be 100,000 years,” said the authors. But more research is needed on the cost and the impacts to marine ecosystems.

Colleagues Jordan Hollarsmith of NOAA Fisheries and PMEL oceanographer Simone Alin present how rapidly growing microalgae can absorb and store carbon in the marine environment, and do so now without intentional cultivation. It’s a low-tech but high-impact option through aquaculture and restored marine habitats. The costs are low, but the microalgae solution might be difficult to scale. It also might meet with social and cultural resistance where algae is a food source that doesn’t deliver the same carbon removal benefits.

“Macroalgae harvested for consumption or fertilizer represents sequestration on the order of months to a few years (while also potentially displacing food and fertilizer derived from more carbon-intensive means),” the authors note. “Deep ocean sequestration may be stable for timescales on the order of hundreds of years.”

Marine Ecosystem Recovery may offer benefits too. “The role of animals in biogeochemical cycles and ecosystem structure has been understudied, although recent work indicates that living biomass may be a larger opportunity to aid in ocean carbon removal than previously thought,” say climate coordinators Zachary J. Cannizzo and Sara Hutto. That said, benefits may be limited if boosting marine populations merely results in more resources used for food. Additional research is needed on the carbon removal potential.