Difference between revisions of "Ocean carbon sink"

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About 10 Pg C is released annually into the atmosphere by anthropogenic emissions of carbon dioxide (2017-2019). Part of the released CO<sub>2</sub> is transferred to the ocean by physical processes. Another part is incorporated in biomass through photosynthesis in the terrestrial and marine environment. However, the carbon stored on land and in the ocean that will not return as CO<sub>2</sub> to the atmosphere over multi-decadal periods is only a small part of the global gross primary production (~100-300 Pg C/yr <ref>Westberry, T.K., Silsbe, G.M. and Behrenfeld, M.J. 2023. Gross and net primary production in the global ocean: An ocean color remote sensing perspective. Earth-Science Reviews 237, 104322</ref><ref>Wild, B., Teubner, I., Moesinger, L., Zotta, R.-M., Forkel, M., van der Schalie, R., Sitch, S. and Dorigo, W. 2022. VODCA2GPP – a new, global, long-term (1988–2020) gross primary production dataset from microwave remote sensing. Earth Syst. Sci. Data, 14: 1063–1085</ref>). Estimates of the sequestered carbon are: about 3 Pg C/yr on land and about 2.4 Pg C/yr in the sea <ref name=LQ>Le Quere, C., Andrew, R.M., Friedlingstein, P., Sitch, S., Pongratz, J. et al. 2018. Global carbon budget 2017. Earth Syst. Sci. Data 10: 405–48</ref>.
  
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About 10 Pg C is released annually into the atmosphere by anthropogenic emissions of carbon dioxide (2017-2019). Part of the released CO<sub>2</sub> is transferred to the ocean by physical processes. Another part is incorporated in biomass through photosynthesis in the terrestrial and marine environment. However, the carbon stored on land and in the ocean that will not return as CO<sub>2</sub> to the atmosphere over multi-decadal periods is only a small part of the gross primary production. Estimates of the sequestered carbon are: about 3 Pg C/y on land and about 2.4 Pg C/y in the sea <ref name=LQ>Le Quere, C., Andrew, R.M., Friedlingstein, P., Sitch, S., Pongratz, J. et al. 2018. Global carbon budget 2017. Earth Syst. Sci. Data 10: 405–48</ref>.
 
  
  
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==Ocean CO<sub>2</sub> uptake processes==
 
==Ocean CO<sub>2</sub> uptake processes==
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The oceans’ role as a sink for CO<sub>2</sub> is driven by two processes: the solubility pump and the biological pump. The solubility pump refers to CO<sub>2</sub> transfer through the ocean-atmosphere interface and subsequent mixing in the upper ocean layer and transport to deeper ocean layers by large-scale [[ocean circulation]] currents. The name 'solubility pump' points to the strong dependency on the CO<sub>2</sub> solubility in seawater and the thermal stratification of the ocean. The biological C pump refers to the uptake of CO<sub>2</sub> by marine plankton from the surface waters through [[photosynthesis]]. Particulate material of biotic origin (POC, e.g. dead plankton cells, faecal pellets, and PIC, mainly calcium carbonate) is transferred from the ocean surface to deeper ocean layers through several processes: sinking by gravity, advection by downwelling currents and diurnal vertical migration of grazing organisms<ref>Boyd, P., Claustre, H., Levy, M., Siegel, D.A. and Weber, T. 2019. Multi-faceted particle pumps drive carbon sequestration in the ocean. Nature 568: 327–335</ref>. Cold, deep waters are generally rich in dissolved inorganic C because of the increased solubility of CO<sub>2</sub>. The carbon captured in the deep ocean has a turnover time of hundreds to thousands of years. Outgassing to the atmosphere only occurs when deep water wells up to warmer equatorial regions, where the solubility of CO<sub>2</sub> is reduced. Just a small non-mineralized fraction of the biomass produced (on the order of 0.2 Pg C/y) reaches the ocean floor and is buried in the sediment, partly as detritus and partly as calcium carbonate<ref>Cartapanis, O., Galbraith, E. D., Bianchi, D. and Jaccard, S. L. 2018. Carbon burial in deep-sea sediment and implications for oceanic inventories of carbon and alkalinity over the last glacial cycle. Clim. Past. 14: 1819–1850</ref>.  
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The oceans’ role as a sink for CO<sub>2</sub> is driven by two processes: the solubility pump and the biological pump. The solubility pump refers to CO<sub>2</sub> transfer through the ocean-atmosphere interface and subsequent mixing in the upper ocean layer and transport to deeper ocean layers by large-scale [[ocean circulation]] currents<ref>Volk, T. and Hoffert, M. 1985. Ocean carbon pumps: Analysis of relative strengths and efficiencies in ocean-driven atmospheric CO2 changes, in The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present (eds. E. T. Sundquist and W. S. Broecker). Geophys. Monogr. Ser. 32: 99–110, AGU, Washington, D. C.</ref>. The name 'solubility pump' points to the strong dependency on the CO<sub>2</sub> solubility in seawater and the thermal stratification of the ocean. Cold, deep waters are generally rich in dissolved inorganic C because of the increased solubility of CO<sub>2</sub>. Outgassing to the atmosphere occurs when deep water wells up to warmer equatorial regions along the eastern ocean boundaries, where the solubility of CO<sub>2</sub> is reduced<ref>Lefèvre, N., Veleda, D. and Hartman, S.E. 2023. Outgassing of CO2 dominates in the coastal upwelling off the northwest. African coast. Deep Sea Research Part I: Oceanographic Research Papers 200, 104130</ref>.
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The biological C pump refers to the uptake of CO<sub>2</sub> by marine plankton from the surface waters through [[photosynthesis]]. Nitrogen-fixing bacteria (so-called diazotrophs, in particular ''Trichodesmium'' and the unicellular symbiont [https://en.wikipedia.org/wiki/Candidatus_Atelocyanobacterium_thalassa UCYN-A]) feed the biological pump by enhancing primary production in the nitrogen-poor surface layer of the [[open ocean habitat|open ocean]]. Particulate organic material (POC, e.g. dead plankton cells, faecal pellets, and PIC, mainly calcium carbonate) is transferred from the ocean surface to deeper ocean layers through several processes: sinking by gravity, advection by downwelling currents and diurnal vertical migration of grazing organisms<ref>Boyd, P., Claustre, H., Levy, M., Siegel, D.A. and Weber, T. 2019. Multi-faceted particle pumps drive carbon sequestration in the ocean. Nature 568: 327–335</ref>. Sinking of small organic particles is possible through the aggregation into large-sized aggregates comprised of tens to hundreds of cells, large/dense enough to sink. Such aggregation is further promoted through association with diatoms, ingestion by grazers and incorporation into large fecal pellets<ref>Bonnet, S., Benavides, M., Le Moigne, F.A.C., Camps, M., Torremocha, A., Grosso, O., Dimier, C., Spungin, D., Berman-Frank, I., Garczarek, L. and Cornejo-Castillo, F.M. 2022. Diazotrophs are overlooked contributors to carbon and nitrogen export to the deep ocean. The ISME Journal 17: 47–58</ref>. More than 90% of the organic matter sinking below the euphotic zone is respired before it reaches a depth of 1000 m and a much smaller part (on the order of 0.2 Pg C/yr) reaches the ocean floor and is buried in the sediment, partly as detritus and partly as calcium carbonate<ref>Cartapanis, O., Galbraith, E. D., Bianchi, D. and Jaccard, S. L. 2018. Carbon burial in deep-sea sediment and implications for oceanic inventories of carbon and alkalinity over the last glacial cycle. Clim. Past. 14: 1819–1850</ref>. The carbon captured in the deep ocean (below 1000 m) has a turnover time of at least hundreds of years and can thus be considered sequestered<ref>Siegel, D.A., DeVries, T., Doney, S.C. and Bel, T. 2021. Assessing the sequestration time scales of some ocean-based carbon dioxide reduction strategies. Environ. Res. Lett. 16, 104003</ref>.  
  
 
==Ocean carbon sink estimate==
 
==Ocean carbon sink estimate==
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The average net carbon uptake from the atmosphere to the global ocean in the period 1994-2007 was estimated to be ca. 2.5 Pg C/y <ref>Gruber, N., Clement, D., Carter, B. R., Feely, R. A., van Heuven, S., Hoppema, M,. Ishii, M., Key, R.M., Kozyr, A., Lauvset, S.K., Lo Monaco, C., Mathis, J.T., Murata, A., Olsen, A., Perez, F.F., Sabine, C.L., Tanhua, T. and Wanninkhof, R. 2019. The oceanic sink for anthropogenic CO2 from 1994 to 2007. Science 363: 1193-1199</ref>, which equaled nearly 30% of the global carbon emissions during this period. This estimate was based on data-products from observations of surface ocean pCO<sub>2</sub> (partial pressure of CO<sub>2</sub>) and compared to simulation results from global ocean biogeochemical models. There is growing evidence and consistency among methods with regard to the patterns of the multi-year variability of the ocean carbon sink, with a global stagnation in the 1990s and an extra-tropical strengthening of the sink in the 2000s. Explanations for this multi-year variability range from the ocean’s response to changes in atmospheric circulation (especially the variations in the upper ocean overturning) to external forcing through surface cooling associated with volcanic eruptions and variations in atmospheric CO<sub>2</sub> growth rate<ref name=DV>DeVries, T., Le Querec, C., Andrews, O., Berthet, S., Hauck, J., Ilyina, T., Landschutzer, P., Lenton, A., Limak, I.D., Nowicki, M., Schwinger, J. and Seferian, R. 2019. Decadal trends in the ocean carbon sink. PNAS 116: 11646–11651</ref><ref name=H20>Hauck, J., Zeising, M., Le Quere, C. ,Gruber, N., Bakker, D.C.E., Bopp, L., Chau, T.T.T., Gurses, O., Ilyina,  T., Landschützer, P., Lenton, A., Resplandy, L., Rödenbeck, C., Schwinger, J. and Seferian, R. 2020. Consistency and Challenges in the Ocean Carbon Sink Estimate for the Global Carbon Budget. Front. Mar. Sci. 7: 571720</ref>. Fossil fuel CO<sub>2</sub> emissions have increased to 10 Pg C /yr in 2019 and the atmospheric CO<sub>2</sub> concentration has reached an unprecedented level of 415 parts per million in 2022. However, the fraction of emitted CO<sub>2</sub> remaining in the atmosphere has been fairly stable at about 45% on average since 1958. The ocean has sequestered about 25% of cumulative CO<sub>2</sub> emissions in the period 2010-2019. The land has sequestered 30% of cumulative emissions over the same period, but has also released a substantial amount (order 40%) of CO<sub>2</sub> by land-use change emissions<ref name=GCP>[https://globalcarbonbudget.org/ Global Carbon Project (2023)]</ref>.  
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Gruber et al. (2019<ref>Gruber, N., Clement, D., Carter, B. R., Feely, R. A., van Heuven, S., Hoppema, M,. Ishii, M., Key, R.M., Kozyr, A., Lauvset, S.K., Lo Monaco, C., Mathis, J.T., Murata, A., Olsen, A., Perez, F.F., Sabine, C.L., Tanhua, T. and Wanninkhof, R. 2019. The oceanic sink for anthropogenic CO2 from 1994 to 2007. Science 363: 1193-1199</ref>) estimated the average net carbon uptake from the atmosphere to the global ocean in the period 1994-2007 at 2.5 Pg C/yr, which equaled nearly 30% of the global carbon emissions during this period. This estimate was based on observations of surface ocean pCO<sub>2</sub> (partial pressure of CO<sub>2</sub>) and compared to simulation results from global ocean biogeochemical models. Nowicki et al. (2022<ref>Nowicki, M., DeVries, T. and Siegel, D. A. 2022. Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles 36, e2021GB007083</ref>) used computer models consistent with data from satellite-based sensors and ocean observations to estimate the carbon export from the ocean photic zone (~100 m) by the biological pump. They found an export on the order of 10 Pg C/yr with a sequestration time in the ocean on the order of 150 years.
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There is growing evidence and consistency among methods with regard to the patterns of the multi-year variability of the ocean carbon sink, with a global stagnation in the 1990s and an extra-tropical strengthening of the sink in the 2000s. Explanations for this multi-year variability range from the ocean’s response to changes in atmospheric circulation (especially the variations in the upper ocean overturning) to external forcing through surface cooling associated with volcanic eruptions and variations in atmospheric CO<sub>2</sub> growth rate<ref name=DV>DeVries, T., Le Querec, C., Andrews, O., Berthet, S., Hauck, J., Ilyina, T., Landschutzer, P., Lenton, A., Limak, I.D., Nowicki, M., Schwinger, J. and Seferian, R. 2019. Decadal trends in the ocean carbon sink. PNAS 116: 11646–11651</ref><ref name=H20>Hauck, J., Zeising, M., Le Quere, C. ,Gruber, N., Bakker, D.C.E., Bopp, L., Chau, T.T.T., Gurses, O., Ilyina,  T., Landschützer, P., Lenton, A., Resplandy, L., Rödenbeck, C., Schwinger, J. and Seferian, R. 2020. Consistency and Challenges in the Ocean Carbon Sink Estimate for the Global Carbon Budget. Front. Mar. Sci. 7: 571720</ref>. Fossil fuel CO<sub>2</sub> emissions have increased to 10 Pg C /yr in 2019 and the atmospheric CO<sub>2</sub> concentration has reached an unprecedented level of 415 parts per million in 2022. However, the fraction of emitted CO<sub>2</sub> remaining in the atmosphere has been fairly stable at about 45% on average since 1958. The ocean has sequestered about 25% of cumulative CO<sub>2</sub> emissions in the period 2010-2019. The land has sequestered 30% of cumulative emissions over the same period, but has also released a substantial amount (order 40%) of CO<sub>2</sub> by land-use change emissions<ref name=GCP>[https://globalcarbonbudget.org/ Global Carbon Project (2023)]</ref>.  
  
 
==Methods for increasing the ocean carbon sink==
 
==Methods for increasing the ocean carbon sink==
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Several methods have been proposed to increase artificially the ocean carbon sink. We shortly mention here three proposals.
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Several methods have been proposed to increase artificially the ocean carbon sink. Three proposals are briefly mentioned:
 
#Large scale cultivation of seaweed in nutrient-rich coastal upwelling zones. The biomass produced is then sunk into the deep ocean where the carbon remains trapped for hundreds to thousands of years<ref>Froehlich, H.E., Afflerbach, J.C., Frazier, M. and Halpern, B.S. 2019. Blue Growth Potential to Mitigate Climate Change through Seaweed Offsetting. Current Biology 29: 3087–3093</ref>. However, scaling up seaweed aquaculture can have negative consequences, including disruption of the natural ecosystem and the diversion of nutrients from wild food webs. See also [[Seaweed (macro-algae) ecosystem services]].
 
#Large scale cultivation of seaweed in nutrient-rich coastal upwelling zones. The biomass produced is then sunk into the deep ocean where the carbon remains trapped for hundreds to thousands of years<ref>Froehlich, H.E., Afflerbach, J.C., Frazier, M. and Halpern, B.S. 2019. Blue Growth Potential to Mitigate Climate Change through Seaweed Offsetting. Current Biology 29: 3087–3093</ref>. However, scaling up seaweed aquaculture can have negative consequences, including disruption of the natural ecosystem and the diversion of nutrients from wild food webs. See also [[Seaweed (macro-algae) ecosystem services]].
 
#Ocean iron fertilization. Soluble iron salts or ferrous dust are added to surface waters where it is currently lacking, for example in mid-ocean gyres and the Southern Ocean. This should increase primary production and boost the ocean carbon pump in these ocean regions. However, experiments with iron fertilization have been less successful than expected. Several possible causes have been suggested, such as rapid conversion of soluble ferrous sulphate to rapidly precipitating ferric hydroxide, primary production limited by other nutrients and trace metals, competition between picocyanobacteria and diatoms, where the former will not reach the deep ocean due to low sedimentation rates and grazing by microzooplankton<ref>Jiang, H-B., Hutchins, D.A., Zhang, H-R., Feng, Y-Y., Zhang, R-F., Sun, W-W., Ma, W., Bai, Y., Wells, M., He, D., Jiao, N., Wang, Y. and Chai, F.  2024. Complexities of regulating climate by promoting marine primary production with ocean iron fertilization. Earth-Science Reviews 249, 104675</ref>. Moreover, undesirable side effects on remote ecosystems cannot be excluded<ref>Aumont, O. and Bopp, L. 2006. Globalizing results from ocean in situ iron fertilization studies. Global Biogeochem. Cycles 20: GB2017</ref>. Better understanding  is needed of the biochemistry of ocean fertilization and its impact on the global carbon cycle and marine ecosystem.
 
#Ocean iron fertilization. Soluble iron salts or ferrous dust are added to surface waters where it is currently lacking, for example in mid-ocean gyres and the Southern Ocean. This should increase primary production and boost the ocean carbon pump in these ocean regions. However, experiments with iron fertilization have been less successful than expected. Several possible causes have been suggested, such as rapid conversion of soluble ferrous sulphate to rapidly precipitating ferric hydroxide, primary production limited by other nutrients and trace metals, competition between picocyanobacteria and diatoms, where the former will not reach the deep ocean due to low sedimentation rates and grazing by microzooplankton<ref>Jiang, H-B., Hutchins, D.A., Zhang, H-R., Feng, Y-Y., Zhang, R-F., Sun, W-W., Ma, W., Bai, Y., Wells, M., He, D., Jiao, N., Wang, Y. and Chai, F.  2024. Complexities of regulating climate by promoting marine primary production with ocean iron fertilization. Earth-Science Reviews 249, 104675</ref>. Moreover, undesirable side effects on remote ecosystems cannot be excluded<ref>Aumont, O. and Bopp, L. 2006. Globalizing results from ocean in situ iron fertilization studies. Global Biogeochem. Cycles 20: GB2017</ref>. Better understanding  is needed of the biochemistry of ocean fertilization and its impact on the global carbon cycle and marine ecosystem.
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#Ocean alkalinization. The concentration of carbonate or hydroxide ions in surface water is artificially raised to shift the associated chemical equilibria in seawater, thus increasing oceanic uptake of atmospheric CO<sub>2</sub> and reducing ocean acidification. The feasibility and effectiveness of adding alkalinity at the required scale are questionable and the effects are highly uncertain<ref>Gattuso, J-P., Magnan, A.K., Bopp, L., Cheung, W.W.L., Duarte, C.M., Hinkel, J., Mcleod, E., Micheli, F., Oschlies, A., Williamson, P., Billé, R., Chalastani, V.I., Gates, R.D., Irisson, J-O., Middelburg, J.J., Pörtner, H-O. and Rau, G.H. 2018. Ocean Solutions to Address Climate Change and Its Effects on Marine Ecosystems. Front. Mar. Sci. 5: 337</ref>.  
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#Ocean alkalinization. The concentration of carbonate or hydroxide ions in surface water is artificially raised to shift the associated chemical equilibria in seawater (see [[ocean acidification]]), thus increasing oceanic uptake of atmospheric CO<sub>2</sub> and reducing [[ocean acidification]]. The feasibility and effectiveness of adding alkalinity at the required scale are questionable and the effects are highly uncertain<ref>Gattuso, J-P., Magnan, A.K., Bopp, L., Cheung, W.W.L., Duarte, C.M., Hinkel, J., Mcleod, E., Micheli, F., Oschlies, A., Williamson, P., Billé, R., Chalastani, V.I., Gates, R.D., Irisson, J-O., Middelburg, J.J., Pörtner, H-O. and Rau, G.H. 2018. Ocean Solutions to Address Climate Change and Its Effects on Marine Ecosystems. Front. Mar. Sci. 5: 337</ref>.  
  
 
==Climate change impact on the carbon sink==
 
==Climate change impact on the carbon sink==
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Enhanced temperature stratification due to global warming will reduce the mixing of nutrients into the euphotic surface layer of the ocean. High temperatures and low nutrient concentrations give a competitive advantage to small phytoplankton groups that are more labile and less likely to sink than larger plankton species. The carbon sequestration by the biological pump may therefore decrease<ref>Ani, C.J. and Robson, B. 2021. Responses of marine ecosystems to climate change impacts and their treatment in biogeochemical ecosystem models. Marine Pollution Bulletin 166: 112223</ref><ref>Bindoff, N.L., Cheung, W.W.L., Kairo, J.G., Arístegui, J., Guinder, V.A., Hallberg, R., Hilmi, N,. Jiao, N., Karim, M.S., Levin, L., O’Donoghue, S., Purca Cuicapusa, S.R., Rinkevich, B., Suga, T., Tagliabue, A. and Williamson, P. 2019. Changing Ocean, Marine Ecosystems, and Dependent Communities. Ch. 5 in IPCC Special Changing Ocean, Marine Ecosystems, and Dependent Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. Cambridge University</ref>. However, other effects of climate change can lead to markedly different effects. For example, the expansion of oxygen minimum zones is likely to reduce microbial respiration associated with sinking particles and will therefore lead to remineralization of sinking particles at greater depths<ref>Stukel, M.R., Irving, J.P., Kelly, T.B., Ohman, M.D., Fender, C.F. and Yingling, N. 2023. Carbon sequestration by multiple biological pump pathways in a coastal upwelling biome. Nature Communications 14: 2024</ref>. [[Ocean acidification]] will reduce the contribution to the carbon sink by calcifying organisms, but it will also lower ocean alkalinity due to reduced calcification, allowing additional dissolution of CO<sub>2</sub> and a concomitant decrease of atmospheric CO<sub>2</sub> (as explained in the article [[Ocean acidification]]).  
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Climate change affects the ocean carbon sink in various contrasting ways. Enhanced temperature stratification due to global warming will reduce the mixing of nutrients into the euphotic surface layer of the ocean. High temperatures and low nutrient concentrations give a competitive advantage to small phytoplankton groups that are more labile and less likely to sink than larger plankton species. The carbon sequestration by the biological pump may therefore decrease<ref>Ani, C.J. and Robson, B. 2021. Responses of marine ecosystems to climate change impacts and their treatment in biogeochemical ecosystem models. Marine Pollution Bulletin 166: 112223</ref><ref>Bindoff, N.L., Cheung, W.W.L., Kairo, J.G., Arístegui, J., Guinder, V.A., Hallberg, R., Hilmi, N,. Jiao, N., Karim, M.S., Levin, L., O’Donoghue, S., Purca Cuicapusa, S.R., Rinkevich, B., Suga, T., Tagliabue, A. and Williamson, P. 2019. Changing Ocean, Marine Ecosystems, and Dependent Communities. Ch. 5 in IPCC Special Changing Ocean, Marine Ecosystems, and Dependent Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. Cambridge University</ref>. However, other climate change effects can strengthen carbon sequestration. For example, the expansion of oxygen minimum zones is likely to reduce microbial remineralization rates and will therefore allow sinking detrital particles to reach greater depth<ref>Stukel, M.R., Irving, J.P., Kelly, T.B., Ohman, M.D., Fender, C.F. and Yingling, N. 2023. Carbon sequestration by multiple biological pump pathways in a coastal upwelling biome. Nature Communications 14: 2024</ref>. [[Ocean acidification]] will reduce the contribution to the carbon sink by calcifying organisms, but it will also lower ocean alkalinity due to reduced calcification, allowing additional dissolution of CO<sub>2</sub> and a concomitant decrease of atmospheric CO<sub>2</sub> (as explained in the article [[Ocean acidification]]).  
  
  

Latest revision as of 15:58, 11 December 2024

About 10 Pg C is released annually into the atmosphere by anthropogenic emissions of carbon dioxide (2017-2019). Part of the released CO2 is transferred to the ocean by physical processes. Another part is incorporated in biomass through photosynthesis in the terrestrial and marine environment. However, the carbon stored on land and in the ocean that will not return as CO2 to the atmosphere over multi-decadal periods is only a small part of the global gross primary production (~100-300 Pg C/yr [1][2]). Estimates of the sequestered carbon are: about 3 Pg C/yr on land and about 2.4 Pg C/yr in the sea [3].


Global carbon stocks

The major global pools of potentially available C (carbon) include the atmosphere, oceans, fossil fuels, and - collectively - vegetation, soils, and detritus. The oceans are the largest C pool, encompassing an estimated 38 000 petagrams of dissolved inorganic C (1 petagram C =1 Pg C = 1 megatonne C = 1015 g C) [4]. The geological C pool, composed primarily of fossil fuels, is the next largest pool, estimated at 2000-4000 Pg C. Vegetation (mostly terrestrial, above and below ground) and detritus hold around 2000 Pg C, followed by the atmosphere, which holds about 800 Pg C - which is comparable to the carbon stock in the top 1000 m of the oceans[5]. The ocean carbon pool consists mainly of dissolved CO2 , bicarbonate (HCO3-), carbonate (CO32-) and carbonic acid (H2CO3).

Ocean CO2 uptake processes

The oceans’ role as a sink for CO2 is driven by two processes: the solubility pump and the biological pump. The solubility pump refers to CO2 transfer through the ocean-atmosphere interface and subsequent mixing in the upper ocean layer and transport to deeper ocean layers by large-scale ocean circulation currents[6]. The name 'solubility pump' points to the strong dependency on the CO2 solubility in seawater and the thermal stratification of the ocean. Cold, deep waters are generally rich in dissolved inorganic C because of the increased solubility of CO2. Outgassing to the atmosphere occurs when deep water wells up to warmer equatorial regions along the eastern ocean boundaries, where the solubility of CO2 is reduced[7].

The biological C pump refers to the uptake of CO2 by marine plankton from the surface waters through photosynthesis. Nitrogen-fixing bacteria (so-called diazotrophs, in particular Trichodesmium and the unicellular symbiont UCYN-A) feed the biological pump by enhancing primary production in the nitrogen-poor surface layer of the open ocean. Particulate organic material (POC, e.g. dead plankton cells, faecal pellets, and PIC, mainly calcium carbonate) is transferred from the ocean surface to deeper ocean layers through several processes: sinking by gravity, advection by downwelling currents and diurnal vertical migration of grazing organisms[8]. Sinking of small organic particles is possible through the aggregation into large-sized aggregates comprised of tens to hundreds of cells, large/dense enough to sink. Such aggregation is further promoted through association with diatoms, ingestion by grazers and incorporation into large fecal pellets[9]. More than 90% of the organic matter sinking below the euphotic zone is respired before it reaches a depth of 1000 m and a much smaller part (on the order of 0.2 Pg C/yr) reaches the ocean floor and is buried in the sediment, partly as detritus and partly as calcium carbonate[10]. The carbon captured in the deep ocean (below 1000 m) has a turnover time of at least hundreds of years and can thus be considered sequestered[11].

Ocean carbon sink estimate

Gruber et al. (2019[12]) estimated the average net carbon uptake from the atmosphere to the global ocean in the period 1994-2007 at 2.5 Pg C/yr, which equaled nearly 30% of the global carbon emissions during this period. This estimate was based on observations of surface ocean pCO2 (partial pressure of CO2) and compared to simulation results from global ocean biogeochemical models. Nowicki et al. (2022[13]) used computer models consistent with data from satellite-based sensors and ocean observations to estimate the carbon export from the ocean photic zone (~100 m) by the biological pump. They found an export on the order of 10 Pg C/yr with a sequestration time in the ocean on the order of 150 years.

There is growing evidence and consistency among methods with regard to the patterns of the multi-year variability of the ocean carbon sink, with a global stagnation in the 1990s and an extra-tropical strengthening of the sink in the 2000s. Explanations for this multi-year variability range from the ocean’s response to changes in atmospheric circulation (especially the variations in the upper ocean overturning) to external forcing through surface cooling associated with volcanic eruptions and variations in atmospheric CO2 growth rate[14][15]. Fossil fuel CO2 emissions have increased to 10 Pg C /yr in 2019 and the atmospheric CO2 concentration has reached an unprecedented level of 415 parts per million in 2022. However, the fraction of emitted CO2 remaining in the atmosphere has been fairly stable at about 45% on average since 1958. The ocean has sequestered about 25% of cumulative CO2 emissions in the period 2010-2019. The land has sequestered 30% of cumulative emissions over the same period, but has also released a substantial amount (order 40%) of CO2 by land-use change emissions[16].

Methods for increasing the ocean carbon sink

Several methods have been proposed to increase artificially the ocean carbon sink. Three proposals are briefly mentioned:

  1. Large scale cultivation of seaweed in nutrient-rich coastal upwelling zones. The biomass produced is then sunk into the deep ocean where the carbon remains trapped for hundreds to thousands of years[17]. However, scaling up seaweed aquaculture can have negative consequences, including disruption of the natural ecosystem and the diversion of nutrients from wild food webs. See also Seaweed (macro-algae) ecosystem services.
  2. Ocean iron fertilization. Soluble iron salts or ferrous dust are added to surface waters where it is currently lacking, for example in mid-ocean gyres and the Southern Ocean. This should increase primary production and boost the ocean carbon pump in these ocean regions. However, experiments with iron fertilization have been less successful than expected. Several possible causes have been suggested, such as rapid conversion of soluble ferrous sulphate to rapidly precipitating ferric hydroxide, primary production limited by other nutrients and trace metals, competition between picocyanobacteria and diatoms, where the former will not reach the deep ocean due to low sedimentation rates and grazing by microzooplankton[18]. Moreover, undesirable side effects on remote ecosystems cannot be excluded[19]. Better understanding is needed of the biochemistry of ocean fertilization and its impact on the global carbon cycle and marine ecosystem.
  3. Ocean alkalinization. The concentration of carbonate or hydroxide ions in surface water is artificially raised to shift the associated chemical equilibria in seawater (see ocean acidification), thus increasing oceanic uptake of atmospheric CO2 and reducing ocean acidification. The feasibility and effectiveness of adding alkalinity at the required scale are questionable and the effects are highly uncertain[20].

Climate change impact on the carbon sink

Climate change affects the ocean carbon sink in various contrasting ways. Enhanced temperature stratification due to global warming will reduce the mixing of nutrients into the euphotic surface layer of the ocean. High temperatures and low nutrient concentrations give a competitive advantage to small phytoplankton groups that are more labile and less likely to sink than larger plankton species. The carbon sequestration by the biological pump may therefore decrease[21][22]. However, other climate change effects can strengthen carbon sequestration. For example, the expansion of oxygen minimum zones is likely to reduce microbial remineralization rates and will therefore allow sinking detrital particles to reach greater depth[23]. Ocean acidification will reduce the contribution to the carbon sink by calcifying organisms, but it will also lower ocean alkalinity due to reduced calcification, allowing additional dissolution of CO2 and a concomitant decrease of atmospheric CO2 (as explained in the article Ocean acidification).


Related articles

Blue carbon sequestration
Ocean acidification
Ecosystem services
Governance policies for a blue bio-economy
Greenhouse gas regulation


References

  1. ↑ Westberry, T.K., Silsbe, G.M. and Behrenfeld, M.J. 2023. Gross and net primary production in the global ocean: An ocean color remote sensing perspective. Earth-Science Reviews 237, 104322
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  6. ↑ Volk, T. and Hoffert, M. 1985. Ocean carbon pumps: Analysis of relative strengths and efficiencies in ocean-driven atmospheric CO2 changes, in The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present (eds. E. T. Sundquist and W. S. Broecker). Geophys. Monogr. Ser. 32: 99–110, AGU, Washington, D. C.
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  11. ↑ Siegel, D.A., DeVries, T., Doney, S.C. and Bel, T. 2021. Assessing the sequestration time scales of some ocean-based carbon dioxide reduction strategies. Environ. Res. Lett. 16, 104003
  12. ↑ Gruber, N., Clement, D., Carter, B. R., Feely, R. A., van Heuven, S., Hoppema, M,. Ishii, M., Key, R.M., Kozyr, A., Lauvset, S.K., Lo Monaco, C., Mathis, J.T., Murata, A., Olsen, A., Perez, F.F., Sabine, C.L., Tanhua, T. and Wanninkhof, R. 2019. The oceanic sink for anthropogenic CO2 from 1994 to 2007. Science 363: 1193-1199
  13. ↑ Nowicki, M., DeVries, T. and Siegel, D. A. 2022. Quantifying the carbon export and sequestration pathways of the ocean's biological carbon pump. Global Biogeochemical Cycles 36, e2021GB007083
  14. ↑ DeVries, T., Le Querec, C., Andrews, O., Berthet, S., Hauck, J., Ilyina, T., Landschutzer, P., Lenton, A., Limak, I.D., Nowicki, M., Schwinger, J. and Seferian, R. 2019. Decadal trends in the ocean carbon sink. PNAS 116: 11646–11651
  15. ↑ Hauck, J., Zeising, M., Le Quere, C. ,Gruber, N., Bakker, D.C.E., Bopp, L., Chau, T.T.T., Gurses, O., Ilyina, T., Landschützer, P., Lenton, A., Resplandy, L., Rödenbeck, C., Schwinger, J. and Seferian, R. 2020. Consistency and Challenges in the Ocean Carbon Sink Estimate for the Global Carbon Budget. Front. Mar. Sci. 7: 571720
  16. ↑ Global Carbon Project (2023)
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  21. ↑ Ani, C.J. and Robson, B. 2021. Responses of marine ecosystems to climate change impacts and their treatment in biogeochemical ecosystem models. Marine Pollution Bulletin 166: 112223
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  23. ↑ Stukel, M.R., Irving, J.P., Kelly, T.B., Ohman, M.D., Fender, C.F. and Yingling, N. 2023. Carbon sequestration by multiple biological pump pathways in a coastal upwelling biome. Nature Communications 14: 2024


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