With the oceans already taking up almost a third of man-made carbon emissions and containing 40 times the carbon as the atmosphere, there is understandable excitement around the potential for us to tweak the existing sequestration mechanisms to help to deliver GT / year of carbon dioxide removal (CDR). As with everything in CDR, this will almost certainly be a portfolio approach as any given mechanism, pushed to extreme scales, starts to incur negative externalities, or at least unacceptable risks. (You can get a back-of-the-envelope sense of it with the Road to 10 GT game.) With the ocean, the risks of CDR at scale are even more present because we understand so damn little about it and, of course, being a huge interconnected system, many risks are difficult to localise.
The area of ocean CDR is nascent and complex, but there are now great resources from some initiatives focussed specifically on the space. The below video from OceanCDR.net is a great intro to the processes, and I’ve used some of their illustrations below. Another great resource is the CDR page of Ocean Visions. For those of you with more time / interest, this webinar also offers a good introduction. This Climate Now episode with Dr Wil Burns gives an indication of the complexity of scaling any of these. Finally, it’s worth saying explicitly that the challenges and opportunities of the ocean extend well beyond carbon management - conservation / food production is a whole other topic that merits its own coverage, but I won't cover here.
Two broad buckets of ocean CDR relating to the two different carbon cycles - biological, which has to do with drawdown of carbon via photosynthesis, and abiotic (or non-biological), which has to do with the ocean chemistry.
Biological CDR pathways look to enhance or expand the process of sequestration through phytoplankton or algal growth or through coastal or near-shore ecosystems (blue carbon):
Phytoplankton fertilisation - there are a couple of different approaches to this, either adding nutrients directly to the water, such as iron, or by “artificial upwelling”, bringing up nutrient rich water from the deep ocean in the same way that happens naturally and creates biodiversity hot spots like off California [hence Monterey as whale watching spot]
Blue Carbon - sequestration in marine or coastal ecosystems, salt marshes, mangroves, sea grass meadows. Existing ecosystems need to urgently be preserved and expanded, which has the advantage of having significant co-benefits in terms of biodiversity and natural infrastructure like flood defences.
Macroalgae cultivation (a.k.a. seaweed farming) - farming of kelp or sargasso, which is then either sunk to the deep ocean or can be locked up in bioproducts. [The most prominent company pursuing kelp-as-sequestration is Running Tide , from whom Stripe and Shopify have bought carbon removal credits.]
Abiotic CDR pathways look to accelerate the ‘slow carbon cycle’ of the oceans by which alkaline fresh water flows into the ocean and allows carbon to be dissolved in the ocean in the form of bicarbonate and carbonate ions. The ocean also absorbs carbon from the atmosphere at the surface when CO2 concentrations are higher in the atmosphere, which is then brought down to the deep through the ocean conveyor belt.
Ocean alkalinity enhancement - putting carbonate or silicate rocks in the ocean (‘ocean liming’) or other minerals such as olivine or basalt on beaches (‘coastal enhanced weathering’). [Some companies pursuing this latter approach are Project Vesta and 44.01, both of which have also sold carbon removal credits to Stripe.]
Electrochemical Direct Capture - using electricity to rearrange the water and salt into hydrochloric acid and sodium hydroxide (H2O + NaCl → HCl + NaOH), the reverse of the titration experiment that everyone will have done in secondary school. The acid can either be reacted with seawater to create gaseous CO2, which can then be captured and stored, or it can be used for other applications, including hydrogen production. The base is added back into the ocean to, yes, enhance the alkalinity and allow it to take up more CO2 from the atmosphere. A couple of interesting points on this: firstly, where the alkaline is added back into the sea it could contribute locally to reversing ocean acidification and improve conditions for sensitive ecosystems; secondly, oceanCDR.net suggests that removing CO2 using offshore wind installations might be more cost effective at abatement than using the same electricity for replacing that from coal-fired power stations. [I find this assertion pretty amazing - one of the chief and, I think, valid challenges to CDR is that it soaks up clean electricity capacity that might more efficiently be used to replace fossil generation. It could be that it rests on high current costs for a nascent floating wind industry.]
Artificial downwelling - accelerating the rate at which the CO2-saturated surface waters are moved down to the depths. This would be done either by increasing salinity or cooling (cold, salty water, being denser). The cooling approach is basically a non-starter though due to the energy requirements.
Marine Geoengineering - further out on the science and risk curve, but I’ll just note for completeness sake:
Maritime cloud brightening - spraying particles of sea water to increase the number of droplets in clouds and therefore make them more reflective. Main risks are around inadvertently changing weather patterns. [One interesting discussion on geoengineering is about creating ‘winners’ and ‘losers’, for example if one region would benefit from greater rainfall at the expense of another. Of course there is already vast disparities in the effects of climate change, where it is going to hit those who did the least to create it hardest. Hard to see the effects of geoengineering getting more unfair than that, but this is where our bias towards overweighting sins of commission vs sins of omission come back to bite us.]
Micro bubbles in foam – makes surface of ocean brighter, reflect more heat away. This is probably even more of a non-starter than cloud brightening because bubbles dissipate quickly without the use of chemicals.