Meeting Documents

Differences in Calculated OAE-Efficiency Between the CESM2/MARBL and ECCO-Darwin Circulation Models

Abstract

Induction of a sea-surface CO₂ deficit through alkalinity-based (OAE) or direct CO₂ removal methods has been recognized as a promising approach to meet the projected need for negative emissions. The difficulty of directly measuring the counter-factual CO₂ flux due to the rapid spreading of the DIC-deficient plume has put circulation models in the center of the MRV (Measurement, Reporting and Verification) challenge. Confidence in the results of such models is essential for the emerging industry to access carbon credit markets and grow at the required pace, to reach substantial negative emissions by 2050, as envisioned by the IPCC.

The kinetics and equilibration time of such a deficit has been shown to vary substantially depending on the location and season of the initial induction point.

A major component of this variance is the subduction and remixing of the DIC-deficient plume, however sea-surface gas exchange and carbonate chemistry are also important.

Currently, it is poorly understood how much the results of such DIC-deficit pulse simulations depends on the models chosen. To close this knowledge gap, we investigate two GCMs, the ECCO-Darwin model (0.3°) and the CESM2/MARBL model (1°). We perform a number of pulse injections at 16 locations with both models, matched precisely in terms of injection patch, release year and season. We analyze the differences in the CO2 uptake curves, vertical mixing, gas exchange and carbonate chemistry.

We show that in some locations, such as subtropical areas, very substantial differences exist between these two models, well beyond the expected intrinsic variation of each model. Further, we demonstrate that the majority of the differences are attributable to the modelling of vertical transport, whereas the effect of wind forcing and carbonate chemistry is less pronounced. In some locations, there exists good agreement between the models. In general, the largest differences are found 2–6 years post alkalinity injection, followed by slow convergence towards the expected theoretical maximums, determined by the carbonate chemistry.