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What did we do?
We analyzed the trend of dissolved oxygen in the global ocean since 1958 using the World Ocean Database. This is the most comprehensive historic dataset of ocean observations, there are many data gaps and irregularities in sampling frequency. We applied quality control and mapped the original data onto a regularly spaced grid, then analyzed the long-term trend and its spatial patterns. We also analyzed the relationship between the ocean’s heat content and the oxygen level.
What did we find?
We found a significant declining trend of oxygen, both northern and southern hemispheres’ upper oceans above the depth of 1,000m. This trend became apparent and consistent after mid-2000s beyond the envelope of year-to-year fluctuations. The trends are particularly strong in the tropics, eastern margins of each basin and the subpolar North Pacific.  We also found a significant relationship between the global heat content and oxygen inventory of similar magnitude as predicted by the comprehensive climate models that are used for climate projections.
What is this important?
Oxygen is an essential element for life. As ocean absorbs heat, it is expected that its oxygen level goes down. Oxygen is a dynamic properties and it’s concentration can change with natural climate variability. The important aspect of our result is that the rate of global oxygen loss appears to be exceeding the level of nature’s random variability. Also the magnitude of oxygen trend is about 2 to 3 times faster than what we predicted from the temperature dependence of solubility associated with the ocean warming. This is most likely due to the changes in ocean circulation and mixing associated with the heating of the near-surface waters and melting of polar ice.