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How to optimise CO2 capture by the ocean

Laurent Bopp
Laurent Bopp
CNRS Research Director at Laboratoire de Météorologue Dynamique de l’Institut Pierre-Simon Laplace
T. Alan Hatton
Ralph Landau Professor of Chemical Engineering Practice at MIT
Key takeaways
  • The oceans could help reduce greenhouse gas emissions: CO2 is captured at the surface by natural physico-chemical processes.
  • But the increase in atmospheric CO2 concentration linked to human activities means that today the ocean only absorbs 25% of emissions.
  • Alkalising the water to raise its pH would improve the ocean's capacity to absorb carbon dioxide and counteract its acidification.
  • According to one model, it would be possible to double the capture potential of the Mediterranean after 30 years of alkalinisation.
  • However, scientists still have little experience of these processes, which have only been studied for a few decades, and the ocean itself is a poorly understood system.

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.

Ocean and CO2, a hidden potential

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”.

“Alkalising water to raise the pH level”

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.

Alkalinisation: time to take the plunge

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
1IPCC, 2022: Cli­mate Change 2022: Mit­i­ga­tion of Cli­mate Change. Con­tri­bu­tion of Work­ing Group III to the Sixth Assess­ment Report of the Inter­gov­ern­men­tal Pan­el on Cli­mate Change [P.R. Shuk­la, J. Skea, R. Slade, A. Al Khour­da­jie, R. van Diemen, D. McCol­lum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belka­ce­mi, A. Hasi­ja, G. Lis­boa, S. Luz, J. Mal­ley, (eds.)]. Cam­bridge Uni­ver­si­ty Press, Cam­bridge, UK and New York, NY, USA. doi: 10.1017/9781009157926
2Web­site con­sult­ed 7 June 2023: https://​www​.tech​nol​o​gyre​view​.com/​2​0​2​3​/​0​6​/​0​6​/​1​0​7​4​1​2​4​/​m​e​t​a​s​-​f​o​r​m​e​r​-​c​t​o​-​h​a​s​-​a​-​n​e​w​-​5​0​-​m​i​l​l​i​o​n​-​p​r​o​j​e​c​t​-​o​c​e​a​n​-​b​a​s​e​d​-​c​a​r​b​o​n​-​r​e​m​oval/
3Web­site con­sult­ed 7 June 2023: https://​samueli​.ucla​.edu/​u​c​l​a​-​i​n​s​t​i​t​u​t​e​-​f​o​r​-​c​a​r​b​o​n​-​m​a​n​a​g​e​m​e​n​t​-​t​o​-​u​n​v​e​i​l​-​s​e​a​w​a​t​e​r​-​b​a​s​e​d​-​c​a​r​b​o​n​-​r​e​m​o​v​a​l​-​p​i​l​o​t​-​s​y​s​t​e​m​s​-​i​n​-​l​o​s​-​a​n​g​e​l​e​s​-​a​n​d​-​s​i​n​g​a​pore/
4] Web­site con­sult­ed 7 June 2023: https://​icm​.ucla​.edu/work
5Kim, S., et al. (2023), Asym­met­ric chlo­ride-medi­at­ed elec­tro­chem­i­cal process for CO2 removal from ocean­wa­ter, Ener­gy Env­i­ron. Sci, 16, 2030–2044.
6IEA (2021), Net Zero by 2050, IEA, Paris https://​www​.iea​.org/​r​e​p​o​r​t​s​/​n​e​t​-​z​e​r​o​-​b​y​-2050, License: CC BY 4.0
7Nation­al Acad­e­mies of Sci­ences, Engi­neer­ing, and Med­i­cine. 2019. Neg­a­tive Emis­sions Tech­nolo­gies and Reli­able Seques­tra­tion: A Research Agen­da. Wash­ing­ton, DC: The Nation­al Acad­e­mies Press. doi: 10.17226/25259.
8Buten­schön, M, et al. (2021), Alka­lin­iza­tion sce­nar­ios in the Mediter­ranean Sea for effi­cient removal of atmos­pher­ic CO2 and the mit­i­ga­tion of ocean acid­i­fi­ca­tion, Front. Clim., Sec. neg­a­tive emis­sion tech­nolo­gies, vol­ume 3.

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