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Global Warming to Create "Permanent" Ocean Dead Zones?

Ker Than
for National Geographic News
January 28, 2009
 
Fish and other marine life could be left gasping for breath in oxygen-poor oceans for thousands of years to come if global warming continues unchecked, scientists warn in a new study.

While previous studies have established a link between climate change and low-oxygen areas known as "dead zones," new computer simulations by Danish researchers suggest the dead zones could persist for millennia and lead to a considerable purge and restructuring of ocean life.

"Any increase in dead zones from global warming will last for thousands of years. They will be a permanent fixture" of our oceans, said lead researcher Gary Shaffer of the University of Copenhagen.

The new model tracked the effects of global warming on ocean dead zones in the eastern Pacific and northern Indian oceans for the next 100,000 years.

Dead zones currently make up less than 2 percent of the world's ocean volume. The model predicts that global warming could cause dead zones to grow by a factor of ten or more by the year 2100.

In the worst-case scenario, dead zones could encompass more than a fifth of the world's oceans, the team says.

(Related: "Ocean Dead Zones Growing; May Be Linked to Warming" [May 1, 2008].)

Waves of Death

While many dead zones today are transient and reversible, those expanded by greenhouse gases such as carbon dioxide would last for millennia, Shaffer said.

"Quite a bit of the carbon dioxide emitted by human activity into the atmosphere will stay there for tens of thousands of years."

"Thus global warming, ocean warming, and oxygen depletion will also have this long time scale," he said.

According to the model, global warming will reduce the oceans' ability to store oxygen while simultaneously decreasing the amount of oxygen available in the ocean depths.

Warmer water can hold less oxygen compared with cooler waters. "As the ocean is heated by a warmer atmosphere, oxygen concentrations decrease, Shaffer said.

Furthermore, as Earth's icy poles gradually transform into open oceans, new organisms, from plankton to shellfish, will move in.

This biological boom further decreases the available oxygen in the ocean interior because when organisms die, their bodies drift to the sea floor and are broken down by bacteria which require oxygen to survive.

If biological production in the ocean increases, so too does the population of oxygen-consuming bacteria, said Shaffer, whose research is detailed in the current issue of the journal Nature Geoscience.

Doomsday Scenario

Francis Chen is a marine ecologist at Oregon State University who was not involved in the study.

By modeling how the ocean system will respond to and in turn, influence climate change, Shaffer and his colleagues have provided a means of systematically looking at how multiple factors will play out, Chen said.

"Such studies are an important tool for examining what the future may hold for the world's oceans," he said.

Paul Wignall, a professor of paleoenvironments at the University of Leeds in the United Kingdom, called the new predictions "the doomsday scenario for the oceans."

Wignall, who was not involved in the current research, has studied the Permian extinction 250 million years ago.

Also known as the "Great Dying," the event is considered the worst mass extinction in Earth's history, affecting 95 percent of all marine species. Growing evidence suggests global warming and oxygen-poor oceans—possibly stemming from volcanic eruptions—were major factors.

(Related: "Did Million-Year-Long Eruptions Cause Mass Extinction?" [May 2, 2006].)

"The proposed mechanism for these ancient crises is exactly the same as modeled in this paper," Wignall said.

In the case of the Permian period, up to 80 percent of the Earth's oceans were not just oxygen poor, but actually anoxic, or completely lacking in oxygen. Scientists think the oceans remained in this lifeless state for about 20 million years.

"We do not predict ocean anoxia," Shaffer said, "but a worst-case scenario could possibly lead to ocean anoxia."
 

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