Is the Time Right to Grow Carbon-sucking Kelp?

James Temple writes in MIT’s technologyreview.com that more and more corporations are scouring the market for carbon credits that allow them to offset their CO₂ emissions and claim progress toward the goal of carbon neutrality. That has spurred a growing number of companies, investors, and research groups to explore carbon removal approaches that range from planting trees to grinding up minerals to building giant C0₂-sucking factories.

Kelp has become an especially active area of inquiry and investment because there’s already an industry that cultivates it on a large scale — and the theoretical carbon removal potential is significant. An expert panel assembled by the Energy Futures Initiative estimated that kelp has the capacity to pull down about 1 billion to 10 billion tons of carbon dioxide per year.

But scientists are still grappling with fundamental questions about this approach. How much kelp can we grow? What will it take to ensure that most of the seaweed sinks to the bottom of the ocean? And how much of the carbon will stay there long enough to really help the climate?

Impact on the sea floor?

In addition, no one knows what the ecological impact of depositing billions of tons of dead biomass on the sea floor would be.

Certain species of seaweed, like giant bladder kelp, have tiny gas bladders on their blades, enabling the macroalgae to collect more of the sunlight necessary to drive photosynthesis. The bladders can also keep the remnants or rafts afloat for days or longer depending on the species, helping currents carry dislodged kelp to distant shores.

When the carbon in kelp decomposes on land — or turns into dissolved inorganic carbon dioxide in shallow seawater — it can return to the atmosphere, says David Koweek, science director at the research organization Ocean Visions. The carbon may also be released if marine creatures digest the kelp in the upper oceans.

But some kelp sinks into the deep ocean as well. Bladders degrade. Storms push the seaweed down so deep that they deflate. Certain species are naturally nonbuoyant. “We just have zero experience with perturbing the bottom of the ocean with that amount of carbon,” says Steven Davis, an associate professor at the University of California, Irvine, who is analyzing the economics of various uses of kelp. “I don’t think anybody has a great idea what it will mean to actively intervene in the system at that scale.”

The scientific unknowns, however, haven’t prevented some ventures from rushing ahead, making bold promises and aiming to sell carbon credits. If the practice doesn’t sequester as much carbon as claimed it could slow or overstate progress on climate change, as the companies buying those credits carry on emitting on the false promise that the oceans are balancing out that pollution, ton for ton.

“For the field as a whole, I think, having this research done by universities in partnership with government scientists and national labs would go a long way toward establishing a basic level of trust before we’re commercializing some of this stuff,” says Holly Buck, an assistant professor at the University at Buffalo, who is studying the social implications of ocean-based carbon removal.

Are Carbon Credits Premature?

Some carbon removal experts fear that market forces could propel kelp-sinking efforts forward, whatever the research finds about its effectiveness or risks. The companies or nonprofits doing it will have financial incentives to sell credits. Investors will want to earn their money back. Corporate demand for sources of carbon credits is skyrocketing. And offset registries, which earn money by providing a stamp of approval for carbon credit programs, have a clear stake in adding a new category to the carbon marketplace.

Sinking seaweed to the bottom of the ocean creates especially tricky challenges in verifying that the carbon removal is really happening. After all, it’s far easier to measure trees than it will be to track the flow of carbon dissolved in the deep ocean. That means any carbon accounting system for kelp will rely heavily on models that determine how much carbon should stay under the surface for how long in certain parts of the ocean, under certain circumstances. Getting the assumptions right will be critical to the integrity of any eventual offset program — and any corporate carbon math that relies on them.