Limits and CO₂equilibration of near-coast alkalinity enhancement

Abstract

Abstract. Ocean alkalinity enhancement (OAE) has recently gained attention as a potential method for carbon dioxide removal (CDR) at gigatonne (Gt) scale, with near-coast OAE operations being economically favorable due to proximity to mineral and energy sources. In this paper we study critical questions which determine the scale and viability of OAE. Which coastal locations are able to sustain a large flux of alkalinity at minimal pH and ΩArag (aragonite saturation) changes? What is the interference distance between adjacent OAE projects? How much CO2 is absorbed per unit of alkalinity added? How quickly does the induced CO2 deficiency equilibrate with the atmosphere? Choosing relatively conservative constraints on ΔpH or ΔOmega, we examine the limits of OAE using the ECCO LLC270 (0.3∘) global circulation model. We find that the sustainable OAE rate varies over 1–2 orders of magnitude between different coasts and exhibits complex patterns and non-local dependencies which vary from region to region. In general, OAE in areas of strong coastal currents enables the largest fluxes and depending on the direction of these currents, neighboring OAE sites can exhibit dependencies as far as 400 km or more. At these steady state fluxes most regional stretches of coastline are able to accommodate on the order of 10s to 100s of megatonnes of negative emissions within 300 km of the coast. We conclude that near-coastal OAE has the potential to scale globally to several Gt CO2 yr−1 of drawdown with conservative pH constraints, if the effort is spread over the majority of available coastlines. Depending on the location, we find a diverse set of equilibration kinetics, determined by the interplay of gas exchange and surface residence time. Most locations reach an uptake efficiency plateau of 0.6–0.8 mol CO2 per mol of alkalinity after 3–4 years, after which there is only slow additional CO2 uptake. Regions of significant downwelling (e.g., around Iceland) should be avoided by OAE deployments, as in such locations up to half of the CDR potential of OAE can be lost to bottom waters. The most ideal locations, reaching a molar uptake ratio of around 0.8, include North Madagascar, California, Brazil, Peru and locations close to the Southern Ocean such as Tasmania, Kerguelen and Patagonia, where the gas exchange appears to occur faster than the surface residence time. However, some locations (e.g., Hawaii) take significantly longer to equilibrate (up to 8–10 years) but can still eventually achieve high uptake ratios.

Global‐Scale Evaluation of Coastal Ocean Alkalinity Enhancement in a Fully Coupled Earth System Model

“…Meanwhile, the millennial-scale, global box model study of Köhler (2020) found that the maximum efficiency of 0.81 coincided with the peak of CO 2 emissions, with efficiency declining to around half this over the following two millennia. In terms of comparable global studies with coastal releases, Feng et al (2017) found an efficiency of 0.60 rising to 0.72 with lower TA additions, while He and Tyka (2023) found that, after initially lower efficiency, this rose to 0.8 after 3-4 years of simulation. Feng et al (2017)'s elevated efficiency with lower TA addition suggests the near-linear response to OAE found here may be a consequence of the lower TA addition used.…”

“…Aside from the primary consideration of the availability of olivine, its processing, transportation and distribution at coastal locations imply further practical constraints that are beyond this study. Some aspects of these are quantified in the coastal OAE study of He and Tyka (2023).…”

Toward a consensus framework to evaluate air–sea CO2 equilibration for marine CO2 removal

“…This includes but is not limited to Ocean Alkalinity Enhancement where CO 2 is “removed” by conversion to bicarbonate or Iron Fertilization and Seaweed Sinking where CO 2 is “removed” by photosynthetic CO 2 ‐fixation with subsequent export into the deep ocean. The simulation was performed using the ECCO LLC270 0.3 × 0.3° model (Forget et al 2015; He and Tyka 2023) using a simple five‐tracer biogeochemical model (Dutkiewicz et al 2005). The simulations were compared to a control run where no CO 2 deficit was simulated.…”

“…These arguments are based on two reasons. First, there is a concern that the seawater CO 2 deficit generated through an mCDR deployment will not be equilibrated for decades to centuries if mCDR is employed in some unfavorable oceanic regions, such as deep‐water formation regions where water loses contact with the atmosphere for a long time (Jones et al 2014; He and Tyka 2023; Fig. 1).…”

Identifying the Most (Cost‐)Efficient Regions for CO2 Removal With Iron Fertilization in the Southern Ocean

“…There is a risk that OIF reduces CO 2 concentrations in seawater, but the water parcel carrying this CO 2 deficit subducts below the sea surface before CO 2 equilibration with the atmosphere has been completed. In such a case, atmospheric CO 2 removal is delayed potentially far into the future when the CO 2 -deficient water is re-exposed to the atmosphere (Bach et al, 2023;He & Tyka, 2023). To investigate this risk, we simulated a Lagrangian experiment for the temporal evolution of a 35 μmol kg −1 deficit in dissolved inorganic carbon (DIC), which is typical of OIF experiments with shallow mixed layers of ∼40 m during summer (de Baar et al, 2005;Krishnamurthy et al, 2008).…”