Dissolved Inorganic Carbon Storage in Oceans

1.0 Introduction

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.

2.0 Background

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).

[math: 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.

3.0 Durability of DIC storage

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:

[math: 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.

4.0 Reversal Risks

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

4.1 Buffer Pool

Projects using this Storage Module are typically deemed to have No Observable Risk of reversal, according to the ++Isometric Standard Risk Assessment Questionnaire (also found in the relevant Protocol), as all reversal risk is accounted for using the reversal risk modelling and Uncertainty Discount approach. This results in a 0% ++buffer pool for Projects using this Storage Module. This reversal risk will be reassessed at the renewal of the Crediting Period, or when new scientific research and knowledge are produced.