Alkalinity biases in CMIP6 Earth system models and implications for simulated CO2 drawdown via artificial alkalinity enhancement
Authors: Hinrichs Claudia , Köhler Peter , Völker Christoph , Hauck Judith 
Published: September 2023

Abstract. The partitioning of CO2 between atmosphere
and ocean depends to a large degree not only on the amount of dissolved
inorganic carbon (DIC) but also on alkalinity in the surface ocean. That is
also why ocean alkalinity
enhancement (OAE) is discussed as one potential approach in the context of negative emission technologies. Although
alkalinity is thus an important variable of the marine carbonate system,
little knowledge exists on how its representation in models compares with
measurements. We evaluated the large-scale alkalinity distribution in 14
CMIP6 Earth system models (ESMs) against the observational data set GLODAPv2
and show that most models, as well as the multi-model mean, underestimate
alkalinity at the surface and in the upper ocean and overestimate it in the
deeper ocean. The decomposition of the global mean alkalinity biases into
contributions from (i) physical processes (preformed alkalinity), which
include the physical redistribution of biased alkalinity originating from
the soft tissue and carbonates pumps; (ii) remineralization; and (iii) carbonate formation and dissolution showed that the bias stemming from the
physical redistribution of alkalinity is dominant. However, below the upper
few hundred meters the bias from carbonate dissolution can gain similar importance to physical biases, while the contribution from remineralization
processes is negligible. This highlights the critical need for better
understanding and quantification of processes driving calcium carbonate
dissolution in microenvironments above the saturation horizons and
implementation of these processes into biogeochemical models. For the application of the models to assess the potential of OAE to increase
ocean carbon uptake, a back-of-the-envelope calculation was conducted with
each model's global mean surface alkalinity, DIC, and partial pressure of CO2 in seawater (pCO2) as input
parameters. We evaluate the following two metrics: (1) the initial pCO2 reduction at
the surface ocean after alkalinity addition and (2) the uptake efficiency
(ηCO2) after air–sea equilibration is reached. The relative biases
of alkalinity versus DIC at the surface affect the Revelle factor and
therefore the initial pCO2 reduction after alkalinity addition. The
global mean surface alkalinity bias relative to GLODAPv2 in the different
models ranges from −85 mmol m−3 (−3.6 %) to +50 mmol m−3
(+2.1 %) (mean: −25 mmol m−3 or −1.1 %). For DIC the relative
bias ranges from −55 mmol m−3 (−2.6 %) to 53 mmol m−3
(+2.5 %) (mean: −13 mmol m−3 or −0.6 %). All but two of the CMIP6
models evaluated here overestimate the Revelle factor at the surface by up
to 3.4 % and thus overestimate the initial pCO2 reduction after
alkalinity addition by up to 13 %. The uptake efficiency, ηCO2,
then takes into account that a higher Revelle factor and a higher initial
pCO2 reduction after alkalinity addition and equilibration mostly
compensate for each other, meaning that resulting DIC differences in the models are small
(−0.1 % to 1.1 %). The overestimation of the initial pCO2 reduction
has to be taken into account when reporting on efficiencies of ocean
alkalinity enhancement experiments using CMIP6 models, especially as long as
the CO2 equilibrium is not reached.