Limits and CO2 equilibration of near-coast alkalinity enhancement

<p>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. Here, 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 <span class="inline-formula">Ω<sub>Arag</sub></span> (aragonite saturation) changes? What is the interference distance between adjacent OAE projects? How much <span class="inline-formula">CO<sub>2</sub></span> is absorbed per unit of alkalinity added? How quickly does the induced <span class="inline-formula">CO<sub>2</sub></span> deficiency equilibrate with the atmosphere? Choosing relatively conservative constraints on <span class="inline-formula">Δ</span>pH or <span class="inline-formula">Δ</span>Omega, we examine the limits of OAE using the ECCO LLC270 (0.3<span class="inline-formula"><sup>∘</sup></span>) 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 <span class="inline-formula">km</span> 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 <span class="inline-formula">CO<sub>2</sub></span> yr<span class="inline-formula"><sup>−1</sup></span> 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 <span class="inline-formula">CO<sub>2</sub></span> per mol of alkalinity after 3–4 years, after which there is only slow additional <span class="inline-formula">CO<sub>2</sub></span> 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.</p>