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

[Geoengineering News]

https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2150/

Authors 

Michael Dominik Tyka

Citations: Tyka, M. D.: Efficiency metrics for ocean alkalinity enhancement under responsive and prescribed atmosphere conditions, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-2150, 2024.

Received: 11 Jul 2024 – Discussion started: 24 Jul 2024

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 pCO2atm simulations, equals the equivalent schedule of a gradual direct air capture (DAC) removal in a fully coupled system.

Source: EGU sphere