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