Abstract
As atmospheric carbon dioxide (CO2) levels continue to rise, increasing attention is focussed mitigation techniques that could enhance the natural draw down of CO2. One such mitigative intervention is ocean alkalinity enhancement (OAE). OAE involves dissolving alkaline materials into ocean surface waters to increase its natural CO2 buffering capacity. Limestone and lime have received the most attention given their widespread availability. Here, we address the order one policy-relevant question of whether OAE represents a viable CO2 removal solution to global warming. We use the UVic Earth System Climate Model to explore the potential of OAE interventions under representative concentration pathways (RCPs) 2.6, 4.5, 6.0, and 8.5. For each RCP, we undertake three OAE interventions. First, we assume that the global annual production of limestone is crushed and uniformly distributed across and immediately disassociates in the surface waters of the global ocean. Second, we assume that the global production of limestone is converted to lime with the CO2 released in this process being added to the atmosphere. In the third intervention, we repeat the second intervention but sequester the CO2 arising from lime production. Our results suggest that CaCO3-based OAE interventions have little potential for mitigating global warming given the restraint of the current world’s output of limestone from mining.
Introduction
In 1990, the Intergovernmental Panel on Climate Change (IPCC) published its first Scientific Assessment of climate change (IPCC 1990). At the time, the atmospheric carbon dioxide (CO2) concentration was 354 ppm and human activity emitted 6.2 Gt/year of fossil carbon to the atmosphere. Despite an increased understanding of the causes and profound consequences of unchecked global warming (IPCC 1992, 1995, 2001, 2007, 2013, 2021), global fossil carbon emissions in 2024 have increased 65% over 1990 levels to 10.2 Gt/year (Friedlingstein et al. 2024). Compared to the 1750 pre-industrial level of 277 ppm, the atmospheric concentration of CO2 has increased by 145 ppm as of 2024 to 422 ppm and is growing by about 2.4 ppm/year. In total, human activities have emitted approximately 494 Gigatonnes of cumulative fossil carbon emissions over this 274-year period (Friedlingstein et al. 2024).
If humanity chooses to stabilize the atmospheric level of CO2 at any level, then net emissions of fossil carbon need to go to zero (Weaver et al. 2007; Weaver 2008; Allen et al. 2025). This could be accomplished through either the decarbonization of the world’s energy systems or some combination of decarbonization alongside the widespread adoption of negative emission CO2 removal (CDR) technology. As the ocean currently absorbs about 2.9 Gt/year (26%) of anthropogenic carbon emissions (Friedlingstein et al. 2024), it makes sense to explore the potential for, and consequences of, artificially enhancing the natural process of oceanic CO2 sequestration.
As CO2 is absorbed by the ocean, it reacts with water to form carbonic acid (eq. 1), which subsequently dissociates leading to an increase in hydrogen (H+) and bicarbonate (HCO3−) ions (eq. 2). If present in sufficient quantity, carbonate ions (CO32−) in surface waters neutralize the hydrogen ions leading to an increase in bicarbonate ions at the expense of CO32− (eq. 3; Caserini et al. 2022). The net reaction (eq. 4) does not affect pH. However, the rate of carbonate flux to the ocean is limited, so that in practice, the uptake of anthropogenic CO2 by the ocean leads to increasing hydrogen and bicarbonate ion concentrations and decreasing carbonate concentrations. The resulting decrease in ocean pH is known as Ocean Acidification.
