What did they consider? Here I'll list their starting points, together with my haven't-read-the-report reactions.
Nutrient fertilization (Chapter 3): Addition of micronutrients (e.g., iron) and/or macronutrients (e.g., phosphorus or nitrogen) to the surface ocean may in some settings increase photosynthesis by marine phytoplankton and can thus enhance uptake of CO2 and transfer of organic carbon to the deep sea where it can be sequestered for timescales of a century or longer. As such, nutrient fertilization essentially locally enhances the natural ocean biological carbon pump using energy from the sun, and in the case of iron, relatively small amounts are needed.
WMC: seems sane.
Artificial upwelling and downwelling (Chapter 4): Artificial upwelling is a process whereby water from depths that are generally cooler and more nutrient and carbon dioxide rich than surface waters is pumped into the surface ocean. Artificial upwelling has been suggested as a means to generate increased localized primary production and ultimately export production and net CO2 removal. Artificial downwelling is the downward transport of surface water; this activity has been suggested as a mechanism to counteract eutrophication and hypoxia in coastal regions by increasing ventilation below the pycnocline and as a means to carry carbon into the deep ocean.
WMC: sounds a bit mad to me.
Seaweed cultivation (Chapter 5): The process of producing macrophyte organic carbon biomass via photosynthesis and transporting that carbon into a carbon reservoir removes CO2 from the upper ocean. Large-scale farming of macrophytes (seaweed) can act as a CDR approach by transporting organic carbon to the deep sea or into sediments.
WMC: also a bit wild-eyed but perhaps possible.
Recovery of ocean and coastal ecosystems (Chapter 6): Carbon dioxide removal and sequestration through protection and restoration of coastal ecosystems, such as kelp forests and free-floating Sargassum, and the recovery of fishes, whales, and other animals in the oceans.
WMC: sounds rather limited.
Ocean alkalinity enhancement (Chapter 7): Chemical alteration of seawater chemistry via addition of alkalinity through various mechanisms including enhanced mineral weathering and electrochemical or thermal reactions releasing alkalinity to the ocean, with the ultimate aim of removing CO2 from the atmosphere.
Electrochemical approaches (Chapter 8): Removal of CO2 or enhancement of the storage capacity of CO2 in seawater (e.g., in the form of ions, or mineral carbonates) by enhancing its acidity, or alkalinity, respectively. These approaches exploit the pH-dependent solubility of CO2 by passage of an electric current through water, which by inducing water splitting (“electrolysis”), changes its pH in a confined reaction environment. As one example, ocean alkalinity enhancement may be accomplished by electrochemical approaches.
WMC: sounds expensive.
Immeadiately after this is their key take-home message in the form of table S1 on pages 18-21. At least I hope it is, then I can stop reading early. Unfortunately, despite the need they assess for ~20 Gt/y, their "scalability high" is only "more than 1 Gt/y". But happily, that knocks out half of the ideas, leaving only fertilisation, alkalinity, and electrochemical. Coming to cost, of those three only fertilisation comes at under $100, so we have a winner. Well, that was quick.
What do you mean we're not finished? Well, of course not: these are people looking for research funds. Even hopeless ideas will still get something, of course. And at this stage that isn't too silly. Although why they want to shovel the most money at the weird electrochem stuff I don't know.
Although the fertilisation stuff is, as you'd expect, mostly about iron the PR puff says Nutrient Fertilization — This approach adds nutrients such as phosphorus or nitrogen to the ocean surface and mentions iron nowhere, which makes me doubt their good faith. The PR fluff is also careful to avoid any mention of scalability or relative cost of the different approaches.
The recommendations tip-toe around, both to avoid treading on anyone's toes by closing off people's pet ideas, and desperately (and pointlessly) trying to stop the zealots from screaming at them. Recommendation 3, includes "Research agenda that emphasizes advancing understanding of ocean fertilization, seaweed cultivation, and ocean alkalinity enhancement" which is a bit odd; it isn't clear to me why they picked those three.
So, in the end, meh: yes, you shoud do some reseach.
1. Pic by Nordin Catic at the Christmas Head. We're a scratch mixed VIII.