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Ocean Dead Zones Could Approach Mass Extinction Levels

January 25, 2009
Discovery News

The future of Earth's oceans is beginning to look a lot like a mass extinction, according to new research.

Today, approximately 2 percent of the seas qualify as 'dead zones' -- naturally desolate, oxygen-starved regions where higher life forms either can't breathe or find enough food to subsist. But according to a computer simulation out to 100,000 years in the future, those zones could engulf one-fifth of the seas within a few millennia if humans don't change their carbon-emitting ways soon.

Gary Shaffer of the University of Copenhagen in Denmark led the study published yesterday in the journal Nature Geoscience, which examined the oceans' long term reaction to several emissions and warming scenarios.

In the worst case, business-as-usual scenario, society keeps burning fossils fuels unabated until 2100. Carbon dioxide levels in the atmosphere quadruple, and the planet warms by as much as 5 degrees Centigrade (9 degrees Fahrenheit).

Assuming humanity stops with its emissions after that, air temperatures should reach their maximum around the year 2200, and then start a slow decline. But the oceans will just be getting warmed up -- literally. It will take another 2000 years or so before ocean circulation fully mixes warm surface waters down into the depths.

Water loses its ability to dissolve oxygen as it warms, so hotter oceans mean larger dead zones. And some research has suggested that ocean circulation will slow down as the oceans heat up, too, causing an effect that Shaffer estimates could deplete oxygen content by up to 54 percent worldwide by around the year 5000.

Of course, there is a lot of uncertainty in just how dire the consequences will be. For instance, it's possible that the carbon we emit may have less of an effect on the oceans than his model predicts.

"This is proposing some interesting hypotheses, that if true, would be a big deal," Anand Gnanadesikan of the National Oceanic and Atmospheric Association's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey said. "But that's all they are right now -- hypotheses."

And humans could undertake to cut emissions, which would improve the outlook.

"Whatever we do in the next few generations will affect hundreds or thousands of generations to come. So it's a big responsibility," Shaffer said. "Hopefully people will understand and take up measures to stop burning fossil fuels and to start using renewable forms of energy."

But if emissions continue unabated, ecosystems could collapse in huge swaths of ocean, turning them into noxious soups of cyanobacteria. The bugs may begin feasting on nitrate and eventually sulfur, which would produce the poisonous gas hydrogen sulfide.

The same process on an even larger scale is thought to have contributed to the Permian mass extinction 250 million years ago, when 90 percent of life on the planet was snuffed out.

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