How to optimise CO2 capture by the ocean

What if the oceans could help lim­it glob­al warm­ing? Of course, redu­cing green­house gas (GHG) emis­sions is cru­cial. But bal­an­cing the quant­it­ies of CO₂ emit­ted with those nat­ur­ally absorbed – by plants, oceans, and soils – is a chal­lenge at the cur­rent rate of human activ­ity. Until this bal­ance is achieved, the con­cen­tra­tion of CO₂ in the atmo­sphere will con­tin­ue to rise. Anthro­po­gen­ic cap­ture of atmo­spher­ic CO₂ is con­sidered neces­sary by the Inter­gov­ern­ment­al Pan­el on Cli­mate Change (IPCC) to lim­it glob­al warm­ing to 2°C¹. Exist­ing solu­tions include refor­est­a­tion, bioen­ergy with cap­ture and stor­age, biochar spread­ing and ocean-based methods.

The ocean is a major nat­ur­al car­bon sink. In its latest report, the IPCC explains: “The oceans con­tain 45 times more car­bon than the atmo­sphere, and ocean absorp­tion has already con­sumed nearly 30–40% of anthro­po­gen­ic car­bon emis­sions”. CO₂ is cap­tured at the sur­face of the oceans by nat­ur­al phys­ic­al and chem­ic­al pro­cesses. Once dis­solved, the car­bon is trans­por­ted by ocean cur­rents to the deep oceans. Unfor­tu­nately, this phe­nomen­on is not enough to com­pensate for the rap­id increase in atmo­spher­ic CO₂ con­cen­tra­tion linked to human activ­it­ies. “With the absorp­tion of anthro­po­gen­ic car­bon, the sur­face ocean is rap­idly becom­ing sat­ur­ated and the pro­cesses that trans­port car­bon to the deep ocean are not fast enough to com­pensate for the sharp rise in atmo­spher­ic con­cen­tra­tions,” explains Laurent Bopp. “As a res­ult, the ocean now absorbs only 25% of our emis­sions”. Then there are the reper­cus­sions of cli­mate change. “Cer­tain effects are redu­cing the effi­ciency of the oceans: rising sur­face water tem­per­at­ures, changes in ocean cur­rents and a drop in phyto­plank­ton pro­duc­tion,” con­tin­ues Laurent Bopp.

Today, the ocean only absorbs 25% of our emissions.

This led to the idea of “boost­ing” the ocean­ic pump. Since the late 1980s, the idea of fer­til­ising phyto­plank­ton with iron has been gain­ing ground. By increas­ing the pro­ductiv­ity of these plants, car­bon trans­port to the seabed is enhanced and the oceans can cap­ture more atmo­spher­ic CO₂. “Since the 2000s, the poten­tial of this tech­nique has been explored through mod­el­ling,” says Laurent Bopp. “Fer­til­ising the oceans as a whole would be very inef­fi­cient in the face of anthro­po­gen­ic emissions”.

Anoth­er major solu­tion is the arti­fi­cial alka­lin­isa­tion of the oceans. “We are explor­ing the idea of re-alkalising the water to raise the pH, which would allow more atmo­spher­ic CO₂ to be cap­tured,” explains T. Alan Hat­ton. Alkaline min­er­al powders can be added to the ocean or elec­tro­chem­ic­al pro­cesses can be used. These tech­niques have the advant­age of increas­ing the cap­ture capa­city of the oceans but also off­set­ting their ongo­ing acid­i­fic­a­tion, which is harm­ful to eco­sys­tems. “We are at an early stage of devel­op­ment, with research mainly in labor­at­or­ies, although there are a few pilot-scale demon­stra­tions”, sum­mar­ises T. Alan Hat­ton. In early June, the MIT Tech­no­logy Review² revealed that Mike Schroep­fer, former CTO of Meta­Plat­forms, had just set up an organ­isa­tion (Car­bon to Sea) ded­ic­ated to arti­fi­cial alka­lin­isa­tion. “We have less exper­i­ence with alka­lin­isa­tion than with fer­til­isa­tion, and only a few exper­i­ments have been car­ried out near the coast – not in the open sea,” explains Laurent Bopp. At the Uni­ver­sity of Cali­for­nia (UCLA), an insti­tute ded­ic­ated to CO₂ cap­ture announced at the end of 2022³ that it would be set­ting up two pilot sys­tems in Los Angeles and Singa­pore via its start-up SeaChange. The pro­cess is based on the alka­lin­isa­tion of water by elec­tro­lys­is: the CO₂ dis­solved in the water is trans­formed into sol­id car­bon­ate and/or aqueous bicar­bon­ate⁴.

Mean­while, a research team at the Mas­sachu­setts Insti­tute of Tech­no­logy (USA) has just developed a new alka­lin­isa­tion pro­cess⁵ that it believes is effect­ive and inex­pens­ive. Elec­tro­lys­is is also used, but the pro­cess does not use mem­branes or chem­ic­als, which add to the cost and com­plex­ity of oth­er elec­tro­lys­is pro­cesses. In prac­tice, the sys­tem is sim­il­ar to a bat­tery: an elec­tric cur­rent flows between two elec­trodes. The elec­trodes are immersed in sea­wa­ter, where they gen­er­ate chem­ic­al reac­tions. The CO₂ dis­solved in the water is extrac­ted in gaseous form and con­fined. The water is then alka­lin­ised before being dis­charged. “The mod­ules could be installed on sta­tion­ary plat­forms at off­shore wind or sol­ar farms, or on cargo ships ply­ing the seas, or integ­rated into onshore desal­in­a­tion pro­cesses […],” write the authors.

Once optim­ised, the sys­tem could cap­ture a tonne of CO₂ for $56. “We believe that it is pos­sible to indus­tri­al­ise the pro­cess, even if a cer­tain num­ber of improve­ments are required before­hand”, explains T. Alan Hat­ton, co-author of the study. It should be noted that once the CO₂ gas has been con­fined by the sys­tem, it still has to be “recovered”. It is pos­sible to trans­form it into a syn­thet­ic fuel, or to store it long-term in geo­lo­gic­al reser­voirs, pro­cesses that have not yet been imple­men­ted on a large scale.

Is arti­fi­cial alka­lin­isa­tion the solu­tion for cap­tur­ing resid­ual anthro­po­gen­ic emis­sions? The Inter­na­tion­al Energy Agency estim­ates that it will be neces­sary to cap­ture and store 7 giga­tonnes of CO₂ per year by 2050 in order to achieve car­bon neut­ral­ity⁶. The US Nation­al Academy of Sci­ences puts the fig­ure at 10 Gt per year⁷. Accord­ing to the IPCC, the ocean is the­or­et­ic­ally cap­able of stor­ing thou­sands of giga­tonnes of CO₂ without exceed­ing pre-indus­tri­al levels of car­bon­ate sat­ur­a­tion if the fal­lout is dis­trib­uted evenly over the ocean sur­face. Sev­er­al stud­ies estim­ate the stor­age poten­tial of the oceans at a few Gt of CO₂ per year, and mod­el­ling shows that it is pos­sible to double the cap­ture poten­tial of the Medi­ter­ranean after 30 years of alka­lin­isa­tion⁸. “It is vital to estim­ate and mon­it­or the addi­tion­al atmo­spher­ic CO₂ absorbed by these tech­niques,” com­ments Laurent Bopp. But exist­ing stud­ies still con­tain a lot of uncer­tain­ties, and there is still very little known about the potential.

It is pos­sible to double the cap­ture poten­tial of the Medi­ter­ranean after 30 years of alkalinisation.

Because of the high­er con­cen­tra­tion of CO₂ in the oceans than in the atmo­sphere, the pro­cess is of major interest. “Unlike phyto­plank­ton fer­til­isa­tion, alka­lin­isa­tion is based on physico-chem­ic­al pro­cesses, which are much bet­ter known than bio­lo­gic­al pro­cesses,” adds Laurent Bopp. Anoth­er advant­age is that the pro­cess has no the­or­et­ic­al stor­age lim­its. “It is one of the key pro­cesses that reg­u­lates the cli­mate on long time scales,” says Laurent Bopp. How­ever, sci­ent­ists still have little exper­i­ence of these pro­cesses, which have been stud­ied for sev­er­al dec­ades, and the ocean itself is a poorly under­stood sys­tem. The rein­jec­tion of alkaline water could coun­ter­act the harm­ful effects of ocean acid­i­fic­a­tion, but the effects on eco­sys­tems have been little stud­ied. “It is import­ant to ensure that alka­lin­ised water is dis­persed so as not to dis­rupt biod­iversity,” con­cludes T. Alan Hat­ton. And we need to be aware of the impact of ocean water fil­tra­tion on the loc­al envir­on­ment: reten­tion of nutri­ents, loc­al hab­it­ats, etc.”.

Interview by Anaïs Marechal