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\begin{document}

\title{Global-scale evaluation of coastal ocean alkalinity enhancement in a
fully-coupled Earth system model}



\author[1]{Julien Palmiéri}%
\author[1]{Andrew Yool}%
\affil[1]{National Oceanography Centre, Southampton}%


\vspace{-1em}



  
  \date{May 18, 2024}


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\begin{abstract}
The Paris Climate Accords plan for ``net-zero'' carbon dioxide (CO2) by
2050. However, reducing emissions from some sectors is challenging, and
``net-zero'' permits carbon dioxide removal (CDR) activities. One CDR
scheme is ocean alkalinity enhancement (OAE), which proposes to dissolve
basic minerals into seawater to increase its total alkalinity (TA) and
buffering capacity for CO2. While modelling studies have often
investigated OAE by adding TA to the ocean's surface at basin or global
scale, some proposals focus on readily-accessible coastal shelves, with
TA added through the dissolution of olivine sands. Critically, by
settling and dissolving sands on shallow seafloors, this retains the
added TA in near-surface waters in direct contact with atmospheric CO2.
To investigate this, we add dissolved TA to the global shelves
(\textless{}100m) of an Earth system model (UKESM1) running a high
emissions scenario. As UKESM1 is fully-coupled, wider effects of
OAE-mediated increase in ocean CO2 uptake --e.g. atmospheric xCO2, air
temperature and marine pH-- are fully-quantified. Applying OAE from
2020-2100 decreases atmospheric xCO2 \textasciitilde{}10 ppm, and
increases air-to-sea CO2 uptake \textasciitilde{}8\%. Due to advection
of added TA, \textasciitilde{}50\% of this uptake occurs remotely from
OAE operations. In-line with other studies, CO2 uptake per unit of TA
added occurs at a rate of
\textasciitilde{}0.8\textasciitilde{}mol\textasciitilde{}C\textasciitilde{}(mol\textasciitilde{}TA)\$\^{}\{-1\}\$,
though this is elevated in enclosed regions. Meanwhile, changes in air
temperature and marine pH are indistinguishable from natural
variability. While practical uncertainties and model representation
caveats remain, this analysis estimates the effectiveness of this
specific OAE scheme to assist with net-zero planning.%
\end{abstract}%



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