According to a new study, the oxygen content in the oceans will continue to decrease for centuries even if all CO2 emissions would be stopped immediately. The slowdown of ocean circulation and the progressive warming of deeper water layers are responsible for this process.

«In the study, a model of the Earth system was used to assess what would happen in the ocean in the long term if all CO₂ emissions would be stopped immediately,» explains the author, Professor Andreas Oschlies from GEOMAR Helmholtz Centre for Ocean Research Kiel. «The results show that even in this extreme scenario, the oxygen depletion will continue for centuries, more than quadrupling the oxygen loss we have seen to date in the ocean ,» Oschlies continues.

The long-term decrease in oxygen takes place primarily in deeper layers. According to Prof. Oschlies, this also has an impact on marine ecosystems. A so-called ‘metabolic index’, which measures the maximum possible activity of oxygen-breathing organisms, shows a widespread decline by up to 25%, especially in the deep sea (below 2000 metres). This is likely to lead to major shifts in this habitat, which was previously considered to be very stable, explains the oceanographer. These changes have already been initiated by our historical CO₂ emissions and are now on their way to the deep ocean. He recommends that a comprehensive investigation of the deep ocean habitat, which has only been studied randomly so far, should take place before this environment, that is deemed as having been stable for many millennia, is likely to change significantly due to the now expected decrease in oxygen.

In the upper layers of the ocean, the model shows a much faster response to climate action. There, a further expansion of the relatively near-surface oxygen minimum zones can be stopped within a few years if the emissions were stopped. An ambitious climate policy can therefore help to prevent at least the near-surface ecosystems from being put under further pressure by a progressive decrease in oxygen.

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Materials provided by Helmholtz Centre for Ocean Research Kiel (GEOMAR). Note: Content may be edited for style and length.