A committed fourfold increase in ocean oxygen loss

Andreas Oschlies

doi:10.1038/s41467-021-22584-4

Andreas Oschlies

Open Access

https://doi.org/10.1038/s41467-021-22584-4

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Abstract

Less than a quarter of ocean deoxygenation that will ultimately be caused by historical CO2 emissions is already realized, according to millennial-scale model simulations that assume zero CO2 emissions from year 2021 onwards. About 80% of the committed oxygen loss occurs below 2000 m depth, where a more sluggish overturning circulation will increase water residence times and accumulation of respiratory oxygen demand. According to the model results, the deep ocean will thereby lose more than 10% of its pre-industrial oxygen content even if CO2 emissions and thus global warming were stopped today. In the surface layer, however, the ongoing deoxygenation will largely stop once CO2 emissions are stopped. Accounting for the joint effects of committed oxygen loss and ocean warming, metabolic viability representative for marine animals declines by up to 25% over large regions of the deep ocean, posing an unavoidable escalation of anthropogenic pressure on deep-ocean ecosystems.

Highlights

Less than a quarter of ocean deoxygenation that will be caused by historical CO2 emissions is already realized, according to millennial-scale model simulations that assume zero CO2 emissions from year 2021 onwards
Ocean warming and associated thermal expansion of seawater adds to committed sea-level rise[12,13], which primarily results from committed melting of inland ice[14,15]
An emissiondriven numerical Earth system model of intermediate complexity[21], which is calibrated to simulate observed climate properties and oxygen distributions[22,23], is employed to examine what would happen if CO2 emissions were stopped by the end of year 2020

Summary

Introduction

Less than a quarter of ocean deoxygenation that will be caused by historical CO2 emissions is already realized, according to millennial-scale model simulations that assume zero CO2 emissions from year 2021 onwards. Theory and models predict that global-mean surface temperatures would essentially stop rising further and remain relatively stable for many decades to centuries once CO2 emissions are stopped[2,3,4,5]. This is implicit in the concept of transient climate response to cumulative CO2 emissions (TCRE6,7), which provides the scientific rationale for relating temperature targets to remaining carbon budgets. CO2 emissions to stabilize global-mean surface temperatures has gained substantial traction in climate politics and scenario development Accomplishing this goal must, not be regarded as automatically stopping the increase in climate damages. This study investigates committed changes in marine oxygen levels that are already declining at a previously unexpected pace in response to anthropogenic CO2 emissions[17,18], and that act, together with warming and acidification, as key stressors on marine ecosystems[19]

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