The Oceans' Global Warming Hangover

26 January 2009 (All day)

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Trouble in the water. Warming seas could mean less dissolved oxygen--and more dead fish.

Even if humans can rein in the atmosphere's carbon dioxide content by the end of this century, large zones in the oceans could remain depleted of oxygen for hundreds or even thousands of years, researchers reveal. The lower oxygen content in the seas--a consequence of global warming--could threaten much of the world's marine life by the 22nd century, including the fish, shellfish, and other creatures on which humans depend for food.

Scientists have known for centuries that warmer water holds less dissolved oxygen: As temperatures rise, oxygen bubbles into the atmosphere. Too little dissolved oxygen and marine life begins to suffocate. Ever since global warming became a concern, researchers have been attempting to monitor the oxygen content of the seas. One such study last year reported startling expansions of oceanic low-oxygen zones over the past half-century, though part of that development is due to agricultural pollution from streams and rivers. Other research has revealed that once deoxygenation starts, it triggers a feedback loop, wherein a cascade of physical and chemical reactions can greatly prolong its effects.

To determine how long ocean oxygen levels might remain low, Danish researchers constructed a computer model to track the phenomenon over the next 100,000 years. Under Intergovernmental Panel on Climate Change scenarios predicting atmospheric temperature increases of up to 4°C by the end of this century, the resulting ocean temperature increases would expand existing low-oxygen zones, the team reports online this week in Nature Geoscience. That would create so-called dead zones where, for 2 millennia or even longer, few fish and shellfish could survive. These zones would cover tens of thousands of square kilometers in the northern Indian Ocean and in the eastern Pacific Ocean off the tropical coasts of North, Central, and South America. "We were surprised to find how ocean warming and associated oxygen depletion increased more than global warming itself," says lead author and oceanographer Gary Shaffer of the Niels Bohr Institute in Copenhagen.

The bad news doesn't end there. Shaffer says that the model does not take into account the effect of methane released by ocean sediments as the water warms. Methane reacts with oxygen and removes it from water. So it's possible, he says, that the "oxygen depletion would be much worse."

Oceanographer Gregory Johnson of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, Washington, says the model takes the complex interactions of the ocean and other climate systems and "collapses them into a simple framework." That framework, he says, will help researchers study the effects of climate over very long periods of time.

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