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Abstract

Ocean Alkalinity Enhancement (OAE) simultaneously mitigates atmospheric concentrations of

CO2 and ocean acidification; however, no previous studies have investigated the response of the

non-linear marine carbonate system sensitivity to alkalinity enhancement on regional scales. We

hypothesise that regional implementations of OAE can sequester more atmospheric CO2 than a

global implementation. To address this, we investigate physical regimes and alkalinity sensitivity

as drivers of the carbon-uptake potential response to global and different regional simulations of

OAE. In this idealised ocean-only set-up, total alkalinity is enhanced at a rate of 0.25 Pmol a-1 in

75-year simulations using the Max Planck Institute Ocean Model coupled to the HAMburg Ocean

Carbon Cycle model with pre-industrial atmospheric forcing. Alkalinity is enhanced globally and

in eight regions: the Subpolar and Subtropical Atlantic and Pacific gyres, the Indian Ocean and

the Southern Ocean. This study reveals that regional alkalinity enhancement has the capacity to

exceed carbon uptake by global OAE. We find that 82-175 Pg more carbon is sequestered into the

ocean when alkalinity is enhanced regionally and 156 PgC when enhanced globally, compared

with the background-state. The Southern Ocean application is most efficient, sequestering 12%

more carbon than the Global experiment despite OAE being applied across a surface area 40

times smaller. For the first time, we find that different carbon-uptake potentials are driven by

the surface pattern of total alkalinity redistributed by physical regimes across areas of different

carbon-uptake efficiencies. We also show that, while the marine carbonate system becomes less

sensitive to alkalinity enhancement in all experiments globally, regional responses to enhanced

alkalinity vary depending upon the background concentrations of dissolved inorganic carbon

and total alkalinity. Furthermore, the Subpolar North Atlantic displays a previously unexpected alkalinity sensitivity increase in response to high total alkalinity concentrations.