Efficiency metrics for ocean alkalinity enhancement under responsive and prescribed atmosphere conditions

Abstract. Ocean alkalinity enhancement (OAE) and direct ocean capture (DOC) are emerging as promising technologies for enacting negative emissions. Due to the delayed air-sea gas exchange, potential for premature subduction of surface water parcels and extensive horizontal transport on timescales for equilibration, the direct experimental measurement of induced CO₂ uptake remains elusive. Therefore, the problem of measurement, reporting and verification (MRV) hinges on general circulation models. A number of recent studies have assessed the efficiency of OAE using different model setups and different metrics. Some models use prescribed atmospheric CO₂ levels, while others use fully coupled earth-system models. The former ignores atmospheric feedback effects, while the latter explicitly models them. In this paper it is shown that, even for very small OAE deployments, which do no substantially change atmospheric pCO₂, the change in oceanic CO₂ inventories differs significantly between these methods, due to atmospheric feedback causing some ocean CO₂ offgassing. However, an analogous offgassing occurs during direct air capture (DAC). Due to these feedback effects, care must be taken to compute the correct metrics when assessing OAE efficiency with respect to determining negative emissions credits, as opposed to determining the effect on global temperatures. This paper examines the commonly used metrics of OAE efficiency, their exact physical meanings, the assumptions inherent in their use and the relationship between them. It is shown that the efficiency metric η(t), used in prescribed pCO₂^atm simulations, equals the equivalent schedule of a gradual direct air capture (DAC) removal in a fully coupled system.