Published online 14 November 2008 | Nature | doi:10.1038/news.2008.1230

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Rising carbon dioxide levels will make oceans more hostile to life.

Oxygen content of the world's oceans. Regions with low oxygen are shown in red. IFM-GEOMAR

Rising levels of carbon dioxide could increase the volume of oxygen-depleted 'dead zones' in tropical oceans by as much as 50% before the end of the century — with dire consequences for the health of ecosystems in some of the world's most productive fishing grounds.

At depths between several tens and hundreds of metres, large parts of the tropical oceans are poorly supplied with dissolved oxygen, and are therefore hostile to most marine life. Scientists suspect that these zones are sensitive to climate change, but previous studies have arrived at conflicting conclusions regarding exactly how and why a more CO 2 -rich world affects oceanic oxygen content.

A team led by Andreas Oschlies of the Leibniz Institute of Marine Sciences in Kiel, Germany, has now used a global model of climate, ocean circulation and biogeochemical cycling to extrapolate existing experimental results of the effects of altered carbon and nutrient chemistry on dissolved oxygen to the global ocean1. They found that a CO 2 -rich world will only have a small impact on waters at middle and high latitudes. But in all tropical oceans the volume of 'oxygen-minimum' zones will substantially increase as ocean bacteria feed on the algae that will flourish as a result of the elevated CO 2 levels.

"Carbon dioxide fertilizes biological production," says Oschlies. "It's really like junk food for plants. When the carbon-fattened excess biomass sinks it gets decomposed by bacteria which first consume the oxygen, and then the nutrients."

Dramatic result

Sporadic measurements in the tropical Atlantic and Pacific suggest oxygen-depleted zones have been slowly expanding over the past 50 years2. But none of the previously assumed physical causes, such as ocean warming and reduced circulation, completely accounts for the effect. This prompted Oschlies and his colleagues to examine how the ocean's biology would be affected by rising CO 2 levels. Their results are published in the journal Global Biogeochemical Cycles.

"Nobody really has ever modelled the feedback of rising CO 2 on oceanic oxygen concentrations in such a credible way," says Gian-Kaspar Plattner, a carbon-cycle modeller at the Swiss Federal Institute of Technology Zürich (ETH). "A 50% volume increase of oxygen-poor zones is much more than I would have expected. But further studies, with different climate parameters, are needed to add robustness to these results and reduce uncertainty."

Meanwhile, researchers aboard two German research vessels, the Meteor and the Maria S. Merian, are investigating the matter further. In the waters off western Africa and Peru, which are rich in marine life, teams from the University of Kiel and the Leibniz Institute of Marine Sciences, also in Kiel, will study the physical and biological processes that are thought to drive oxygen loss in tropical oceans.

The coastal economies in these regions rely heavily on fishing. For now, says Oschlies, local fisheries may not feel any downturn because fish stocks can probably evade the dead zones by moving further up in the water column. But if oxygen and nutrient levels continue to drop, that could hit the region hard within a few decades.

Oceanic oxygen levels have varied considerably throughout Earth's past. During the end of the Permian period, around 250 million years ago, catastrophic oxygen losses triggered mass extinction of terrestrial and marine life.