Observed decreases in oxygen content of the global ocean

Abstract

Comparing the high-quality oxygen climatology fromthe World Ocean Circulation Experiment to earlier data wereveal near-global decreases in oxygen levels in the upperocean between the 1970s and the 1990s. This globallyaveraged oxygen decrease is −0.93 0.23 mol l<sup>−1</sup>, whichis equivalent to annual oxygen losses of −0.55 0.13 10<sup>14</sup> mol yr<sup>−1</sup> (1001000 m). The strongest decreases inoxygen occur in the mid-latitudes of both hemispheres,near regions where there is strong water renewal andexchange between the ocean interior and surface waters.Approximately 15% of global oxygen decrease can beexplained by a warmer mixed-layer reducing the capacityof water to store oxygen, while the remainder is consistentwith an overall decrease in the exchange between surfacewaters and the ocean interior. Here we suggest that thisreduction in water mass renewal rates on a global scale is aconsequence of increased stratification caused by warmersurface waters. These observations support climate modelsimulations of oxygen change under global warmingscenarios.