A committed fourfold increase in ocean oxygen loss

The Earth's system is not in equilibrium with current atmospheric CO2 levels, which have increased rapidly due to human activities. While models predict temperature stabilization upon CO2 emission cessation, other responses, like ocean deoxygenation, will continue due to the system's inertia. This study focuses on the committed changes in marine oxygen levels, which, along with warming and acidification, significantly stress marine ecosystems. Understanding these committed changes is crucial for assessing ecological and socioeconomic impacts even after emissions stop and surface temperatures stabilize. The research investigates the extent and patterns of unavoidable ocean oxygen loss despite halting CO2 emissions.

Previous research has documented declining global ocean oxygen levels and identified ocean warming and stratification under elevated atmospheric CO2 as key drivers. Studies have explored the concept of transient climate response to cumulative CO2 emissions (TCRE) and the long-term consequences of past emissions, including continued ocean warming, sea-level rise, and changes in terrestrial ecosystems. The study builds upon these findings by specifically investigating the committed changes in marine oxygen levels and their ecological implications.

The study employs the University of Victoria Earth System Climate Model (UVic ESCM) version 2.8, an intermediate complexity Earth system model calibrated to simulate observed climatic properties and oxygen distributions. The model was run with historical CO2 emissions until 2010, followed by emissions corresponding to the RCP 8.5 high-emission scenario until 2020, and then zero emissions from 2021 onwards. The model simulates atmospheric CO2 levels, global mean surface air temperatures, and ocean heat uptake, which are compared to observational data to ensure model validity. The model incorporates a fully three-dimensional primitive-equation ocean component with a simple marine ecosystem model including two phytoplankton classes and nutrient cycling. The model considers aerobic remineralization and denitrification based on oxygen concentration thresholds. The model's horizontal resolution is 1°. The metabolic index (Φ), a function of oxygen supply and temperature-dependent species metabolic rates, is used to assess the impact of combined warming and oxygen loss on marine animal viability. The analysis extends to 2650, focusing on the period after emission cessation to understand the committed changes.

The model simulations reveal a substantial committed loss of oceanic oxygen, even under a zero-emission scenario. The total oxygen loss is four times larger than the direct solubility effect of warming. The committed oxygen loss is predominantly in deep waters (below 2000 m), representing about 80% of the total loss. This is attributed to a sluggish overturning circulation leading to increased water residence times and accumulation of respiratory oxygen demand. While surface waters show little oxygen change after emission cessation, deep waters continue to lose oxygen due to increased respiration and slower ventilation. Regional patterns of warming and deoxygenation vary significantly, with oxygen loss most pronounced in the Southern Ocean, North Atlantic, and North Pacific below the surface mixed layer. Between 2020 and 2650, the volume of hypoxic and suboxic waters increases, but less dramatically than under continued high emissions. The model indicates that the metabolic index, a measure of metabolically viable marine environments, declines by 10–25% in much of the deep ocean below 2000 m between 2020 and 2650, primarily due to oxygen decline. This decline poses a significant threat to deep-ocean ecosystems. The model shows a Southern Ocean deep convection event after 2650 could potentially lead to a major release of heat and CO2 from the ocean to the atmosphere and a subsequent re-oxygenation of the deep ocean, although this event is associated with significant uncertainties.

The findings highlight the significant and unavoidable consequences of past CO2 emissions on ocean oxygen levels, even if emissions are immediately stopped. The substantial committed oxygen loss in deep waters, where the effects are prolonged due to slow ventilation, presents a considerable challenge for deep-ocean ecosystems. The decline in the metabolic index underscores the potential for widespread reductions in marine animal viability. The study's results emphasize the need for a holistic approach to climate change mitigation that considers not only immediate temperature changes but also the long-term, committed changes in ocean biogeochemistry. Further research is needed to better understand the sensitivity of deep-ocean ecosystems to oxygen changes and to refine model projections of future ocean deoxygenation and its effects on marine life.

This study demonstrates a substantial and unavoidable fourfold increase in ocean oxygen loss committed by past CO2 emissions, even under a zero-emission scenario from 2021 onwards. The majority of this loss occurs in the deep ocean, posing a significant threat to deep-sea ecosystems. The results highlight the urgent need for rapid and substantial emission reductions to mitigate the long-term consequences of climate change on ocean biogeochemistry. Further research should focus on improving model projections, investigating the sensitivity of deep-sea ecosystems, and developing effective strategies to protect these vulnerable environments.

The study relies on model simulations, and uncertainties remain regarding the precise timing and magnitude of Southern Ocean deep convection events and their impact on ocean oxygen levels. The model's simplification of marine ecosystems might not fully capture the complexity of biological responses to oxygen decline. The metabolic index used is a simplification, and the sensitivity of individual species to oxygen decline may vary.