# Substantial Limitations of Ocean Alkalinity Enhancement in Mitigating the Negative Impacts of Ocean Acidification on Marine Calcifiers

## TL;DR

Ocean Alkalinity Enhancement may not fully restore marine calcifier health despite its potential to reduce ocean acidification.

## Contribution

Quantifies the limited effectiveness of OAE in reversing calcification losses in marine calcifiers under various scenarios.

## Key findings

- Historical calcification losses in marine calcifiers range from 7% to 44% due to ocean acidification.
- Restoring calcification rates via OAE requires large alkalinity additions (up to 104 μmol kg–1) and is limited to 0–52.2% recovery.
- Higher CO2 drawdown efficiency reduces the potential for biological ocean acidification mitigation.

## Abstract

Ocean Alkalinity Enhancement (OAE) is increasingly considered
as
a marine carbon dioxide removal (mCDR) strategy with the potential
cobenefit of mitigating ocean acidification (OA), but this remains
poorly constrained. Here, we evaluate these biological cobenefits
for 27 marine calcifiers whose calcification has declined under OA,
by quantifying both historical OA-driven calcification losses and
the potential of OAE to reverse them under scenarios with and without
air–sea equilibration. Regression models describing calcification
as a function of TA-DIC reveal substantial declines since preindustrial
times, particularly in linear responders (mean 22%, range 7–44%),
such as gastropods and pteropods, while threshold responders show
minimal decline (∼3%). A realistic addition of 50 μmol
kg–1 of OAE alkalinity restores species-specific
calcification rates maximally only between 0 and 52.2%, with the largest
benefits in OA-sensitive taxa. However, restoring preindustrial calcification
requires far larger TA additions (mean 104 ± 58 μmol kg–1 without equilibration and more than triple this amount
when equilibration with atmospheric CO2 is considered).
While higher CDR efficiency enhances atmospheric CO2 drawdown,
it simultaneously reduces the potential for biological OA mitigation.
Thus, restoration of marine calcifiers through the OAE will not necessarily
align with its climate goals, complicating its application in ocean
management and CDR policy.