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.

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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.

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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