Unabated global warming could lead to a serious depletion of oxygen in the world's oceans, creating "dead zones" that could remain for thousands of years, purging some areas of advanced marine life, a new study says.

Low-oxygen areas known as dead zones are now typically limited to coastal areas, where excess fertilizer runoff causes greater growth of algae in water. When the algae decomposes, it consumes much of the oxygen in the water, leaving the area unable to support much aquatic life.

The study, published Sunday in the journal Nature Geoscience by researchers at the University of Copenhagen, says global warming could expand these low-oxygen areas.

"Such expansion would lead to increased frequency and severity of fish and shellfish mortality events, for example off the west coasts of the continents like off Oregon and Chile," lead researcher Gary Shaffer said in a statement.

Warming leads to reduced ocean water circulation and diminishes the ability of oxygen to dissolve in sea water, the study says. As time goes on, these effects are amplified by the Earth's progressively increasing sensitivity to temperature and emissions.

"We conclude that substantial reductions in fossil-fuel use over the next few generations are needed if extensive ocean oxygen depletion for thousands of years is to be avoided," the study says.

The study used a computer model to simulate global warming in over the next 100,000 years.  The worst-case scenario — if emissions continue at their current rate — would have carbon dioxide concentrations increase to 1,168 ppm by 2100, a three-fold increase.

In this situation, dead zones could grow to a fifth of the world's oceanic area. Even if humankind stops emissions after 2100, it could take 2,000 years for the oceans to recover, the study says.

The depletion of oxygen causes the ocean to be stripped of nutrients, the study says. This leads to large, unpredictable changes in the oceans' ecosystem structure, perhaps even calling into doubt the ocean as a source of food, the authors say.