Global warming could be decreasing the availability of oxygen to marine organisms living in the intermediate depths of oceans. According to a study by Ralph F Keeling and Hernan E Garcia from the Scripps Institution of Oceanography, University of California, USA, a dip in oxygen concentrations has been observed in the intermediate layers of water in North Pacific, North Atlantic, South Pacific and South Indian Oceans.

The study attributes such a reduction in oxygen content to enhanced layering (stratification) of ocean waters caused by global warming. According to the researchers, the upper three kilometres of the oceans have become warmer over the past few decades, with an associated increase in the stratification of the upper ocean: as the upper layers get warmer, they become less dense and float better. As a result, they do not mix very well with the colder water in the deeper layers. In this way, stratification limits the downward transport of oxygen from the well-oxygenated surface waters into the ocean interior, bringing down the concentration of the gas in the intermediate depths. "The chemistry and biology of the oceans is changing significantly as a consequence of global warming," says Keeling.

Ocean warming is also resulting in the release of oxygen from the seas to the atmosphere. The colder a liquid is, more gas it can hold. At middle and high latitudes during spring and summer, when the atmosphere heats the upper layer of the oceans, they become a source of oxygen to the atmosphere; while in the fall and winter, when the upper layer is cooler, the oceans take in oxygen. Consequently, with the oceans warming, they are now not able to hold more oxygen.

Changes in oxygen content of the oceans, no matter how small, can influence the spread of regions with less oxygen in coastal areas and the open ocean. This affects the distribution of marine organisms, especially those living at intermediate depths where oxygen is already quite low.