Seaweed and Kelp as Blue Carbon

Blue carbon is gaining attention as an effective nature-based solution for carbon abatement – and its potential is veering from optimism to full-on hype. As the global climate and ocean crises become more pronounced, firms and policymakers are turning to blue carbon solutions as an innovative way to offset their greenhouse gas (GHG) emissions and put the global economy on a net-zero trajectory.

A recent report from McKinsey does a deep dive into the promise of blue carbon – citing the ability of mangroves, saltmarshes and seagrasses to sequester more carbon per hectare than terrestrial forests. The report also investigates the feasibility of seaweed aquaculture as a blue carbon offset strategy. Its case studies find that despite the potential of macroalgae carbon offsets, seaweed projects can’t be readily monetised on existing carbon markets. The underlying baselines and rich co-benefits are difficult to illustrate in current offset propositions.

McKinsey also found that the carbon credits produced by seaweed ventures are comparatively expensive, making it difficult for macroalgae to compete with more established forest-based carbon credits. Though it might be possible – and potentially critical – for kelp and seaweed projects to overcome their challenges to become viable carbon offsets, the way offsets are designed may keep macroalgae as an emerging solution instead of an established one.

The carbon offset triangle

Carbon offsets need to address three core elements to meet their climate goals: permanence, leakage and additionality. In blue carbon offsets, permanence means that the marine ecosystem being preserved or restored must remain intact for enough time to meet its carbon sequestration goals. Practitioners need to know how long the mangroves and macroalgae will remain in situ without experiencing illegal harvesting or habitat degradation.

Blue carbon projects also need to make sure that the carbon they’re sequestering won’t just lead to unintended and increased emissions outside the project boundary – the carbon can’t “leak” into other areas of the biosphere. A potential example of carbon leakage would be restoring a patch of mangroves and having a shrimp farmer remove them as his farm footprint expands. This would negate any potential carbon sequestered by the project – the leakage means that no emissions were offset.

The final element of the offset triangle is additionality. Blue carbon offsets need to demonstrate that the coastal area they are preserving would not have been protected otherwise; and that the climate benefit they render is “additional” to the status quo. Additionality is an – admittedly – opaque way of describing “what would have happened anyway” in a marine ecosystem. If a coastal habitat was never going to be destroyed, then being paid to maintain it is a false investment. Blue carbon offsets need to protect threatened ecosystems to meet their goals.

Demonstrating these three elements allows carbon offsets to establish a verifiable baseline – or the amount of carbon sequestered. In addition to this baseline, carbon offsets need to prove that they aren’t “double counting” the carbon they store. If a nature-based offset solution can meet these benchmarks, it lends itself easily to carbon markets and stakeholder investment. But – as McKinsey notes – few blue projects meet this standard. Blue carbon isn’t necessarily an offsetting slam-dunk.

The three types of blue carbon solution

McKinsey put the various blue carbon initiatives into three broad categories: established, emerging and nascent. “Established” solutions meet the minimum standards of scientific understanding and implementation potential – practitioners can fulfil the criteria of the offset triangle and can quantify exactly how much carbon they are abating with their efforts. These projects tend to focus on protecting and restoring mangroves, saltmarshes and seagrass meadows. Established blue carbon solutions tend to gain more traction because they can be funded and monetised through existing carbon markets. The two most notable examples are the Mikoko Pamoja and Cispatá Bay mangrove projects in Kenya and Colombia respectively.

“Emerging” blue carbon solutions are those that have a body of peer-reviewed research verifying their carbon-abatement potential, but don’t meet the calibre of established projects. These projects may fall short on the scientific side – when researchers can’t draw concrete conclusions about their impacts. McKinsey put seaweed farming and kelp forest restoration in this category. Emerging blue carbon initiatives may struggle to align with current funding frameworks like the Core Carbon Principles – making it hard to put these projects on voluntary carbon markets.

Though preserving marine fauna would certainly yield an ecosystem and biodiversity benefit, fish themselves aren’t considered a form of carbon sequestration. These solutions also face a key challenge on the financial side. Researchers and investors can’t easily understand or quantify the impacts – whether positive or negative – of nascent solutions. It’s difficult to establish the permanence, leakage and additionality in these systems. Since we can’t quantify how these solutions and organisms trap and sequester carbon, the category isn’t financeable through carbon markets.

Quantifying the co-benefits

Carbon offsets are not created equal – and translating a forest-based carbon offset into a seaweed blue carbon project illustrates this point. Seaweed growers rightly point out that their work generates multiple co-benefits that aren’t accounted for in blue carbon financing or the offset triangle.

In addition to storing carbon, macroalgae can store nitrogen, making it useful in ameliorating pollution and agricultural run-off. Seaweed populations can also help reduce ocean acidification and clean up excessive nutrient concentrations along the coast. One case study found that seaweed farms helped restore oxygen-depleted “dead zones” in the Gulf of Mexico. The biodiversity benefits of restoring macroalgae could also jumpstart ecotourism and additional income opportunities for coastal residents.

Seaweed’s potential for nutraceuticals and feed additives might make the seed and sink carbon offset less appealing. Studies on the red seaweed Asparagopsis taxiformis found that including it in cattle feed rations can reduce enteric methane emissions by as much as 98 percent. This potential benefit would be lost if the seaweed remained in the water – illustrating that some downstream boons aren’t captured when institutions pitch seaweed blue carbon projects. The fundamental assumptions of carbon offsets may need to be re-imagined to make them applicable for marine environments.

Conclusion

There are also multiple unknowns with macroalgae that might be risky. Hypothetically, the nutrients absorbed by a large-scale seaweed farm might deprive phytoplankton populations of the nutrients they need to grow. In another example, growing seaweed on large, floating artificial structures could impact marine life in unknown ways. Unless these factors can be accounted for, macroalgae blue carbon may remain a theoretical solution.

McKinsey’s analysis found that if all three categories of blue carbon offset successfully came online and met their sequestration goals, they would abate between four and five gigatonnes of CO₂ annually. This is equal to offsetting between 7 and 10 percent of the 40 gigatonnes of carbon we emit each year. Even in blue carbon’s most optimistic implementation and impact scenario, it isn’t a get-out-of-jail-free card. We still need to do the hard work of decarbonising our economies.