Special seminar: Ocean deoxygenation: theory, observations and future projections

The loss of oxygen from the ocean, termed ``deoxygenation'', is a consequence of climate warming. In transient warming scenario, the ocean loses oxygen due to the direct effect of temperature on gas solubility: warmer waters hold less oxygen. Additionally, reductions in ocean ventilation associated with enhanced upper-ocean stratification can cause respiration-driven oxygen depletion at depth. In this presentation I first summarize insights gained through historic observations and CMIP-5 ESM simulations. The ocean as a whole is expected to lose about 3-4% of its oxygen inventory by the year 2100 under a ``business-as-usual'' scenario (RCP8.5) with most of this loss concentrated in the upper 1,000m where species richness and abundance is highest. There is a consensus across models that oxygen loss at mid and high-latitudes will be strong and driven by both solubility reductions and weakened ventilation. Projections are more ambiguous in the tropics, where there will be compensation between oxygen decline due to reduced solubility and oxygen increase caused by reductions in cumulative respiration. However, further analyses of ESMs and large ensemble of a single ESM reveal that the models severely underestimate the anthropogenic global O2 trend. Furthermore, decadal climate variability can yield a basin-scale O2 variability that are comparable to the magnitudes of ocean deoxygenation, challenging our ability to determine anthropogenic trends. In terms of mitigation, it is difficult to reverse the oxygen loss as it is intrinsically linked to ocean heat content. Stabilization of GHG emissions, however, can enable ventilation rates to recover to some degree, mitigating the ventilation-driven oxygen loss.  




13:30 h


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


Taka Ito, Georgia Institute of Technology


Yohei Takano

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