Ocean Deoxygenation: Future Projections And Ecological Implications.pdf

This chapter, authored by Matthew C. Long, Takamitsu Ito, and Curtis Deutsch, explores the future of ocean deoxygenation due to climate warming. Key points include:

- Cause: Warmer ocean waters hold less oxygen, and reduced vertical mixing due to increased upper-ocean buoyancy stratification further depletes oxygen at depth.

- Future Loss: By 2100, under a "business-as-usual" scenario (RCP8.5), the ocean is projected to lose about 3-4% of its oxygen inventory, with most losses concentrated in the upper 1000 meters.

- Regional Variations: There will be distinct regional differences in deoxygenation intensity and ecological impacts. Mid and high-latitudes are expected to experience strong oxygen loss, while projections for the tropics are more ambiguous.

- Consequences:
- Habitat contraction and fragmentation for marine macro-organisms as oxygen levels drop below metabolic requirements.
- Expansion of suboxic zones, potentially disrupting nitrogen cycling with increased denitrification rates.
- Uncertainty in nitrous oxide production changes.

- Mitigation: Reducing human-driven warming is crucial to prevent widespread ocean oxygen loss. While ocean ventilation can recover partially from reduced emissions, earlier mitigation action will yield maximum benefit due to the long persistence timescales of climate drivers.

- Uncertainties: Models may underestimate actual oxygen loss rates, and future projections for tropical oxygen distributions are highly uncertain. Comparisons with observations suggest that deoxygenation could occur more rapidly and severely than anticipated.