Abstract
Global ocean oxygen loss - deoxygenation - is projected to persist in
the future. Previous generations of Earth system models (ESMs) have,
however, failed to provide a consistent picture of how deoxygenation
will influence oxygen minimum zones (OMZs; O2<= 80 μmol/kg),
in particular the largest OMZ in the tropical Pacific Ocean. The
expansion of the Pacific OMZ would threaten marine ecosystems and
ecosystem services such as fisheries and could amplify climate change by
emitting greenhouse gases. Here, we use the latest generation of ESMs
(CMIP6) and a density framework that isolates oxygen changes in the
thermocline and intermediate waters. We show that the Pacific OMZ
expands by the end of the century in response to high anthropogenic
emissions (multi-ESM median expansion of 2.4 * 10^15 m^3m, about
4% of the observed OMZ volume). The expansion is driven by a reduction
of the shallow overturning circulation in the thermocline and a robust
weakening of the oxygen supply to the upper OMZ in all ESMs. The
magnitude of this expansion is, however, uncertain due to the less
constrained balance between physical and biological changes in the lower
OMZ. Despite uncertainties in the biological response, our results
suggest that models with more complex biogeochemistry project weaker
changes in the lower OMZ, and therefore stronger overall OMZ expansion.
The fact that the OMZ largely expands in the upper ocean maximizes its
ecological, economic, and climatic impacts (release of greenhouse
gases).