Dissolved Inorganic Carbon Storage in Oceans

The durability of a Carbon Dioxide Removal (CDR) process refers to the length of time for which CO₂ is removed from the Earth’s atmosphere and cannot contribute to further climate change. This Module details the durability, reversal risks and requirements for storage of removed atmospheric CO₂ as dissolved inorganic carbon (DIC) in the ocean by CDR approaches. This Module is intended for use in conjunction with other Isometric Protocols and Modules, and assumes the following:

• The full quantification of the net tonnes of CO₂e removal for Crediting has already occurred following an Isometric Protocol.
• All environmental and social safeguards have been followed prior to storage in the ocean.

The information and requirements outlined within this Module are based on the best known science at the time of writing. This Module will be updated in future version in line with changes in scientific consensus.

Dissolved Inorganic Carbon (DIC) is the largest reservoir of carbon in the combined ocean-atmosphere-biosphere system, containing approximately 50 times more carbon than the atmosphere and over 10 times more carbon than in all the plants and soils on land.¹ The ocean already plays a significant role in storing carbon derived from anthropogenic emissions; more than 25% of anthropogenic emissions are currently taken up by the ocean and stored as DIC.² Thus, the vast ocean carbon reservoir, with existing Gt scale fluxes and capacity for carbon storage, is an important component in meeting the projected IPCC CDR storage needs of several Gt CO₂/yr.

In the ocean, inorganic carbon exists in different dissolved forms, which are collectively referred to as DIC: dissolved CO₂, carbonic acid (H₂CO₃), bicarbonate (HCO₃^-) and carbonate (CO₃^2-). Atmospheric CO₂ enters the ocean at the sea surface, reacts with water to form carbonic acid (H₂CO₃) and further dissociates into bicarbonate and carbonate (Equation 1).

$$CO_2(aq) + H_2O ⇔ H_2CO_3 ⇔ HCO_3^- + H^+ ⇔ CO_3^{2-}+ 2H^+$$

Equation 1

Carbonic acid is highly unstable, and thus, most of the DIC in the ocean consists of CO₂, bicarbonate and carbonate. The relative equilibrium concentrations of dissolved CO₂, bicarbonate and carbonate are strongly influenced by, and vary as a function of, pH (and to a lesser extent, temperature, salinity, and pressure), where the pH represents the amount of hydrogen ions (H⁺) present (i.e., pH is the inverse logarithm of the H⁺ activity, -log₁₀[H⁺]). The more H⁺ ions, the lower the pH, and the more acidic the water. As the oceans absorb anthropogenic CO₂ from the atmosphere, they become more acidic, decreasing the saturation state of carbonate minerals and threatening shell-building species and ecosystems. At typical sea surface conditions (seawater pH of ~8.1), about 90% of DIC is in the form of bicarbonate, 10% is in the form of carbonate and less than 1% is in the form of dissolved CO₂.³ Importantly, bicarbonate and carbonate cannot directly exchange with the atmosphere, so carbon stored in this form is considered out of contact with the atmosphere and cannot contribute to further global warming over a 1000 year timescale.

There are multiple CDR approaches that utilize or enhance the ocean’s ability to store atmospheric CO₂ as DIC. Some examples of such approaches include:

• Ocean Alkalinity Enhancement: this approach increases the Total Alkalinity of the ocean, which enhances the ocean’s ability to absorb more CO₂ from the atmosphere that is then stored as bicarbonate and carbonate.
• Direct Ocean Removal: this approach directly removes CO₂ from seawater and durably sequesters it, and the DIC-depleted seawater is able to then absorb more CO₂ from the atmosphere, which is then primarily stored as bicarbonate and carbonate.
• Direct Air Capture: one way to store CO₂ captured through Direct Air Capture is reacting it with calcium carbonate and water, generating calcium cations and bicarbonates which can be durably stored in the ocean⁴.
• Enhanced Weathering in Agriculture: the bicarbonate generated through weathering of alkaline feedstock in fields is carried via rivers to the ocean as its final storage reservoir.

The durability of the ocean DIC reservoir can be described by its residence time, which is the average amount of time a substance stays in a particular reservoir. Residence time is defined by dividing the total inventory of a substance by the inflows or outflows, and assuming near-steady state conditions. The durability of CDR projects whose final storage reservoir is DIC in the ocean is expected to be between 10,000 and 100,000 years^5,6. This is based on decades of research that estimates the global ocean DIC inventory to be between 37,000 and 39,000 GtC,^7,8,9 with the most recent estimate being 37,200 ± 200 GtC¹⁰. There is significant uncertainty around the total amount of the global riverine DIC inputs, with values ranging from 0.23 GtC/yr¹¹ to 0.78±0.41 GtC/yr¹², with the latest estimate being 0.65±0.3 GtC/yr^13,14. It is typically assumed that the riverine input approximately balances the loss of DIC through carbonate precipitation and burial on the seafloor at steady state.¹⁵ Note that this estimate does not include minor sources and sinks such as submarine groundwater supply¹⁶ or reverse weathering¹⁷, because their relatively small contributions are dwarfed by the uncertainties in the riverine input and would not change the overall outcome.

Beyond 10,000-100,000 years, the true permanent storage of marine alkaline carbon on multimillion year timescales is the precipitation of solid carbonates to the sea floor:

$$Ca^{2+} + 2HCO_3^- \rightarrow CaCO_{3(s)} + CO_2 + H_2O.$$

Equation 2

So over geologic timescales, roughly half of of the captured carbon stored as DIC will be returned back to the atmosphere. However, for the purposes of quantifying durable storage on the 1000 year timescale in line with the Isometric Standard, carbonate precipitation is not considered.

The following factors could contribute to a decrease in the expected durability of ocean DIC storage.

1. Changes to the residence time of CO₂ stored in the global DIC reservoir if large-scale CDR is deployed.
   
   In the near-term when CDR is operating on small scales (i.e. not Gt), it is unlikely that CDR activities will result in meaningful changes to the global ocean DIC inventory or its input/output fluxes. Longer-term, elevated alkalinity in the ocean may lead to increased carbonate mineral production, which would remove alkalinity and decrease the residence time and durability of the global DIC reservoir. More research is needed to better understand these potential effects.¹⁸
   
   

2. A significant reduction in atmospheric CO₂ due to climate mitigation strategies leading to net outgassing of carbon from the ocean back into the atmosphere.
   
   While bicarbonate and carbonate ions cannot directly exchange with the atmosphere, in the surface ocean, they are in equilibrium with the dissolved CO₂ which can directly exchange with the atmosphere. Thus, leakage back to the atmosphere will occur if when atmospheric CO₂ becomes lower than in the ocean.^19,20,21 This redistribution of carbon between the different Earth system reservoirs is a natural process that is always occurring – for example, approximately half of the CO₂ emitted into the atmosphere is taken up by the ocean and land.²² It is assumed that this coupled climate-carbon cycle response is symmetric, such that the response to a 1 Gt CO₂ removal is equal and opposite to that of a 1 Gt CO₂ emission. However, recent studies find an asymmetric response for large perturbations (on the order of a few hundred Gt).²³ It should be noted that once the atmospheric CO₂ concentration decreases, the net amount of CO₂ that degasses will be less in scenarios with marine CDR than in scenarios without because marine CDR decreases the surface ocean pCO₂. Presently, this is not a concern since CO₂ emissions are still increasing and land-based CDR is not operating at a scale large enough to noticeably alter global carbon fluxes, but this will be revisited in the future to be aligned with the latest research on CDR impacts on the carbon cycle.

1. Poulter et al. (2022). Balancing Greenhouse Gas Budgets: Accounting for Natural and Anthropogenic Flows of CO₂ and other Trace Gases. https://www.sciencedirect.com/book/9780128149522/balancing-greenhouse-gas-budgets ↩

2. Gruber et al. (2019) The oceanic sink for anthropogenic CO₂ from 1994 to 2007. Science. https://www.science.org/doi/10.1126/science.aau5153 ↩

3. Hartmann et al. (2013). Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification. Review of Geophysics.https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/rog.20004 ↩

4. De Marco et al. (2023). Techno-economic evaluation of buffered accelerated weathering of limestone as a CO2 capture and storage option. Mitigation and Adaptation Strategies for Global Change. https://link.springer.com/article/10.1007/s11027-023-10052-x ↩

5. Renforth and Henderson (2017). Assessing ocean alkalinity for carbon sequestration. Review of Geophysics. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RG000533 ↩

6. Middelburg et al. (2020). Ocean Alkalinity, Buffering and Biogeochemical Processes. Review of Geophysics. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019RG000681 ↩

7. Siegenthaler & Sarmiento (1993). Atmospheric carbon dioxide and the ocean. Nature. https://www.nature.com/articles/365119a0 ↩

8. The Carbon Cycle and Atmospheric Carbon Dioxide. TAR Climate Change 2001: The Scientific Basis. https://www.ipcc.ch/site/assets/uploads/2018/02/TAR-03.pdf ↩

9. Friedlingstein et al. (2022). Global Carbon Budget 2021. Earth System Science Data. https://essd.copernicus.org/articles/14/1917/2022/ ↩

10. Keppler et al. (2020). Seasonal Carbon Dynamics in the Near-Global Ocean. Global Biogeochemical Cycles. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GB006571 ↩

11. Lacroix et al. (2020). Oceanic CO₂ outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modeling approach. Biogeosciences.https://bg.copernicus.org/articles/17/55/2020 ↩

12. Resplandy et al. (2018). Revision of global carbon fluxes based on a reassessment of oceanic and riverine carbon transport. Nature. https://www.nature.com/articles/s41561-018-0151-3 ↩

13. Regnier et al. (2020)The land-to-ocean loops of the global carbon cycle. Nature. https://www.nature.com/articles/s41586-021-04339-9 ↩

14. Friedlingstein et al. (2023). Global Carbon Budget 2023. Earth System Science Data. https://essd.copernicus.org/articles/15/5301/2023/ ↩

15. Middelburg et al. (2020). Ocean Alkalinity, Buffering and Biogeochemical Processes Review of Geophysics. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019RG000681 ↩

16. Zhou et al. (2019). Fresh Submarine Groundwater Discharge to the Near-Global Coast. Geophysical Research Letter. https://doi.org/10.1029/2019GL082749 ↩

17. Isson & Planavsky (2018). Reverse weathering as a long-term stabilizer of marine pH and planetary climate. Nature. https://www.nature.com/articles/s41586-018-0408-4 ↩

18. Renforth and Henderson (2017). Assessing ocean alkalinity for carbon sequestration. Review of Geophysics. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RG000533 ↩

19. Keller et al. (2018). The Effects of Carbon Dioxide Removal on the Carbon Cycle. Current Climate Change Reports. https://link.springer.com/article/10.1007/s40641-018-0104-3?fl=4&error=cookies_not_supported&code=631eb573-f6e7-469f-a51e-1475654053d8#Sec3 ↩

20. Mathesius, S., Hofmann, M., Caldeira, K. & Schellnhuber, H. J. (2015). Long-term response of oceans to CO₂ removal from the atmosphere. Nature Climate Change. doi: 10.1038/nclimate2729 ↩

21. Vichi, M., Navarra, A. & Fogli, P. G. 2013. Adjustment of the natural ocean carbon cycle to negative emission rates. Climate Change. doi: 10.1007/s10584-012-0677-0 ↩

22. Keller et al. (2018). The Effects of Carbon Dioxide Removal on the Carbon Cycle. Current Climate Change Reports. https://link.springer.com/article/10.1007/s40641-018-0104-3?fl=4&error=cookies_not_supported&code=631eb573-f6e7-469f-a51e-1475654053d8#Sec3 ↩

23. Zickfield et al. (2021) Asymmetry in the climate–carbon cycle response to positive and negative CO₂ emissions. Nature. https://www.nature.com/articles/s41558-021-01061-2 ↩