Global kelp forest estimates doubled
First off, Oceans 2050 reassessed the size of algal forests, and came to a doubling of previous estimates. In 2016, Krause-Jensen and Duarte found that the global area covered by seaweed was holding relatively steady at about 3.5 million km² of coast (an area larger than India), with a net primary productivity of 1.52 petagram of carbon per year, sequestering about 640 million tonnes of CO2 annually.
Professor Duarte now estimates the total size of algal forests globally at 7.2 million km², with a productivity of 1.3 petagram of carbon per year, mirroring the size and productivity of the Amazon rainforest, under water.
Big variations in sequestration from farmed seaweed
Next, Oceans 2050 collected sediment cores underneath 21 seaweed farms on 5 different continents to measure the carbon sequestered in the soil. The study found carbon sequestration averaged 1.4 tons per hectare per year, but with big differences between the farms, going from 0 to 8 tons.
Seaweed farms located over rocks and rough sand or in sites with strong currents had little to no accumulated carbon, while farms with low wave energy and soft sediment had high accumulations of carbon in the soil.
Are kelp forests a carbon source or sink?
Gallagher et al caused quite a stir in the seaweed community recently, when they reassessed the carbon sequestration potential of kelp forests and came to the conclusion that they are not a carbon sink, as previously thought, but instead, a carbon emitter.
Professor Duarte rejected the conclusions of that paper. He said the paper’s dataset is incomplete. The team is looking to publish a rebuttal soon, rectifying some of the mistakes they see in the efforts of Gallagher et al.
What both Gallagher and Duarte agree on is that it is important to look what replaces an ecosystem when it disappears, and what that replacement ecosystem would offer in terms of biodiversity, nutrient recycling and greenhouse gases. In the case of kelp forests, that replacement is often an urchin barren, which has much less to offer on all three counts.
The challenge of verifying carbon removal
Oceans 2050 has submitted a concept note with Verra for verification of farmed seaweed carbon sequestration, the first formal step in the carbon-credit approval process.
Hurd et al. think it will be difficult to do so. They discuss the challenge of verifying the Monitoring Reporting Verification (MRV) of CO2 removal with seaweed farming, and suggest ‘Forensic Carbon Accounting’ as the way to approach this. You can find a (somewhat) dumbed down explanation in this Twitter thread, concluding that at the moment, techniques for reliable MRV of seaweed farming do not exist yet.
2 more preprints exploring seaweed for CDR
Wu et al. investigated open-ocean macroalgae mariculture and sinking (MOS) as an ocean-based carbon dioxide removal method with an Earth system model. Their results suggest that MOS has a considerable potential as an ocean-based CDR method. However, MOS has inherent side effects on marine ecosystems and biogeochemistry, which will require a careful evaluation beyond this first idealized modeling study.
DeAngelo et al. coupled seaweed growth and techno-economic models to estimate the costs of global seaweed production and related climate benefits. Their paper suggests that seaweed-based climate benefits may be feasible, but targeted research and demonstrations are needed to further reduce economic and biophysical uncertainties.
Under their most optimistic assumptions, sinking farmed seaweed to the deep sea to sequester a gigaton of CO2 per year costs as little as $560/tCO2 on average, while using farmed seaweed for products that avoid a gigaton of CO2-equivalent greenhouse gas emissions annually could return a profit of $30/tCO2-eq.
The carbon and macroalgae debate is not over yet.