Global oxygen levels in the oceans have been falling for 50 years, a new study finds, and are predicted to fall further still by the end of the century.

The declining oxygen of the oceans is a key prediction made by climate scientists as a result of global warming, as gases such as oxygen dissolve less in warmer water. A study published in the journal Nature has found that there has already been a two per cent decline in global oxygen levels in the seas since 1960s. Decreases are expected to reach the seven per cent mark by the end of the century.

Climate change models predict that increasing the temperature of surface waters will decrease their oxygen content. However, only about 15 per cent of this decrease is directly attributed to this factor, the study authors say.

A further decrease is because the warmer surface waters are less likely to mix with deeper water. Just as warm air tends to rise above colder air, warm water rises above cold water because it is less dense. Warm air at the surface will have a tendency to sit there, blocking off lower waters from exposure to oxygen in the atmosphere and decreasing circulation.

Some oceans have become more oxygen depleted than others, with effects also varying by depth below the surface. The worst affected areas are the tropical and northern Pacific Ocean, the Southern ocean and the South Atlantic Ocean.

Many areas of these oceans are already low in oxygen, with "dead zones" in mid-depth waters in which marine life struggles to inhabit. The authors note in the paper that already there are "more abundant and growing dead zones" in the oceans. These are likely to expand and extend into areas where marine animals are living currently as oceans warm, they say.

"[The study authors] synthesise all available observations to show a global-scale decline in oxygen that conforms to the patterns we expect from human-driven climate warming," ocean scientist Matthew Long of the National Center for Atmospheric Research, who was not involved in the study, told the Washington Post.

"They do not make a definitive attribution statement, but the data are consistent with and strongly suggestive of human-driven warming as a root cause of the oxygen decline," he said. "It is alarming to see this signal begin to emerge clearly in the observational data."