Science 320, 655–658 (2008)

Credit: NOAA

Oxygen, one of the essential elements for most of life on Earth, has become increasingly scarce across large expanses of the tropical oceans over the past half century. A new study, based on real-world observations, supports the predictions of climate models that the ocean will become oxygen-depleted as a consequence of global warming.

Lothar Stramma of the University of Kiel, Germany, and colleagues combined historical records with recent data taken from ships and buoys to reconstruct oxygen concentrations since 1960 at selected tropical ocean sites. They focused their study on waters of intermediate depth, where oxygen supply is weak and where changes would be expected to have larger consequences than in the oxygen-rich areas. Over the past 50 years, oxygen decreased by 0.09–0.34 micromoles per kilogram per year in the layer at 300–700 metres depth. Depletion was most severe in areas with the greatest increase in temperature.

Oxygen depletion is likely to worsen in the tropical oceans as temperatures rise, creating large 'underwater deserts'. The authors warn that this could decrease the survival of large mobile marine species such as tuna and could disrupt nutrient cycles in the tropical oceans, reducing the productivity of entire marine ecosystems.