07 Aug 2023
 | 07 Aug 2023
Status: this preprint is currently under review for the journal BG.

The additionality problem of Ocean Alkalinity Enhancement

Lennart Thomas Bach

Abstract. Ocean Alkalinity Enhancement (OAE) is an emerging approach for atmospheric carbon dioxide removal (CDR). The net climatic benefit of OAE depends on how much it can increase carbon sequestration relative to a baseline state without OAE. This so-called ‘additionality’ can be calculated as:

Additionality = COAE - ∆Cbaseline

So far, feasibility studies on OAE have mainly focussed on enhancing alkalinity in the oceans (COAE) but not primarily how such anthropogenic alkalinity would modify the natural alkalinity cycle (∆Cbaseline). Here, I present incubation experiments where materials considered for OAE (sodium hydroxide, steel slag, olivine) are exposed to beach sand to investigate the influence of anthropogenic alkalinity on natural alkalinity sources and sinks. The experiments show that anthropogenic alkalinity can strongly reduce the generation of natural alkalinity, thereby reducing additionality. This is because the anthropogenic alkalinity increases the calcium carbonate saturation state, which reduces the dissolution of calcium carbonate from sand, a natural alkalinity source. I argue that this ‘additionality problem’ of OAE is potentially widespread and applies to many marine systems where OAE implementation is considered – far beyond the beach scenario investigated in this study. However, the problem can potentially be mitigated by dilute dosing of anthropogenic alkalinity into the ocean environment, especially at hotspots of natural alkalinity cycling such as in marine sediments. Understanding a potential slowdown of the natural alkalinity cycle through the introduction of an anthropogenic alkalinity cycle will be crucial for the assessment of OAE.

Lennart Thomas Bach

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-122', Matthew Eisaman, 14 Aug 2023
  • RC2: 'Comment on bg-2023-122', Anonymous Referee #2, 28 Aug 2023
  • RC3: 'Comment on bg-2023-122', Anonymous Referee #3, 29 Aug 2023
  • RC4: 'Comment on bg-2023-122', Adam Subhas, 30 Aug 2023

Lennart Thomas Bach

Lennart Thomas Bach


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Short summary
Ocean Alkalinity Enhancement (OAE) is a widely considered marine carbon dioxide removal method. OAE aims to accelerate chemical rock weathering, which is a natural process that slowly sequesters atmospheric carbon dioxide. This study shows that the addition of anthropogenic alkalinity via OAE can reduce the natural release of alkalinity and therefore the efficiency of OAE for climate mitigation. However, this problem can be mitigated through dilute dosing of alkalinity into the environment.