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Decline in global oceanic oxygen content during the past five decades


Ocean models predict a decline in the dissolved oxygen inventory of the global ocean of one to seven per cent by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean1,2. It is thought that such a decline in the oceanic oxygen content could affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries and coastal economies3,4,5,6. Regional observational data indicate a continuous decrease in oceanic dissolved oxygen concentrations in most regions of the global ocean1,7,8,9,10, with an increase reported in a few limited areas, varying by study1,10. Prior work attempting to resolve variations in dissolved oxygen concentrations at the global scale reported a global oxygen loss of 550 ± 130 teramoles (1012 mol) per decade between 100 and 1,000 metres depth based on a comparison of data from the 1970s and 1990s10. Here we provide a quantitative assessment of the entire ocean oxygen inventory by analysing dissolved oxygen and supporting data for the complete oceanic water column over the past 50 years. We find that the global oceanic oxygen content of 227.4 ± 1.1 petamoles (1015 mol) has decreased by more than two per cent (4.8 ± 2.1 petamoles) since 1960, with large variations in oxygen loss in different ocean basins and at different depths. We suggest that changes in the upper water column are mostly due to a warming-induced decrease in solubility and biological consumption. Changes in the deeper ocean may have their origin in basin-scale multi-decadal variability, oceanic overturning slow-down and a potential increase in biological consumption11,12.

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Figure 1: Amount of dissolved oxygen and changes per decade since 1960.
Figure 2: Oxygen content and change per decade over the past five decades.
Figure 3: Oxygen time series for ocean basins.

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S.S. was supported by the German Federal Ministry of Education and Research project MIKLIP, and L.S. and M.V. by the German Research Foundation (DFG) as part of research project SFB-754. We thank R. Keeling for providing expertise on chemical processes of oxygen measurements and calculating impacts on carbon budgeting.

Author information

Authors and Affiliations



S.S. designed the experiment and did the computations and data analysis; L.S. and S.S. evaluated the analysis; and M.V. provided expertise on ocean ventilation and computations. All authors discussed the results and wrote the manuscript.

Corresponding author

Correspondence to Sunke Schmidtko.

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Competing interests

The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks S. Doney, D. Gilbert and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Dissolved oxygen and apparent oxygen utilization changes per decade since 1960.

a, Change of dissolved oxygen (DO) per square metre per decade (in units of percentage of local dissolved oxygen); these data are similar to those in Fig. 1. b, Change of apparent oxygen utilization (AOU) in units of mol per square metre per decade.

Extended Data Figure 2 Oxygen solubility changes.

ac, Zonal upper 2,500 m mean oxygen solubility changes in the Atlantic (a), Indian (b) and Pacific (c) oceans. No substantial changes are observed below 1,000 m. Contour lines represent oxygen concentrations at 20 μmol kg−1 and every 30 μmol kg−1.

Extended Data Figure 3 Temperature and salinity changes.

af, Zonal mean temperature (T; ac) and salinity (S; df) changes in the Atlantic (top row), Indian (middle row) and Pacific (bottom row) oceans per decade. Data locations and handling are identical to those used for the oxygen computation and for the results in Extended Data Fig. 1. This is only a small subset of data available for global temperature and salinity trend computations. Contour lines represent the mean fields. Observed trends are similar to trends described in the literature and thus confirm that no artificial trend is created because of sparse or irregular data locations or through the mapping method. Distortions in the North Atlantic around 40° N are due to the Mediterranean Sea, which creates a discontinuity in the zonal means.

Extended Data Figure 4 Oxygen concentration and changes.

af, Zonal mean oxygen concentrations in the Atlantic, Indian and Pacific oceans (ac, respectively), and respective changes in oxygen concentration per decade (df). Contour lines are at 20 μmol kg−1 and every 30 μmol kg−1.

Extended Data Figure 5 Oxygen profile data coverage since 1900.

Blue indicates locations of oxygen profiles over 5-year data intervals, given at the top of each panel.

Extended Data Figure 6 Time span of observations and time of last observation in data sets.

aj, Time span in years is colour coded (key at right of each panel); each panel shows results for the indicated depth layer. kt, Year of last observation of data for each grid point mean state and trend computation is colour coded; each panel shows the results for a particular depth layer.

Extended Data Figure 7 Expected oxygen loss distribution from artificial bias.

ac, Zonal mean changes in dissolved oxygen (colour key at right) of an induced systematic bias of 0.5% for historic measurements (Methods) for Atlantic (a), Indian (b) and Pacific (c) oceans. Note the different order of magnitude of colour scales compared to Extended Data Fig. 4. Solid lines represent the mean oxygen field, as in Extended Data Fig. 4.

Extended Data Figure 8 Trend of zonal oxygen loss using reduced data sets.

af, Change in dissolved oxygen of reduced oxygen data distributions for 20,000 profiles per decade (ac) and 30,000 profiles per decade (df), validating the robustness of the mapping with ‘strongly reduced’ and ‘reduced’ data sets in comparison with the full data set as presented in this Letter (see Methods for details). The global mean trends related to these maps are 946 ± 526 Tmol per decade (ac) and 988 ± 459 Tmol per decade (df). Solid lines represent the mean oxygen field, as in Extended Data Fig. 4.

Extended Data Table 1 Volume, oxygen content and change and solubility related changes per basin
Extended Data Table 2 Data sources with date of access of data used

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Schmidtko, S., Stramma, L. & Visbeck, M. Decline in global oceanic oxygen content during the past five decades. Nature 542, 335–339 (2017).

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