Andrea Thompson, livescience.com 28 Jan 09;
In recent years, massive fertilizer runoff from big farms has depleted oxygen in parts of the ocean, choking off life in these aptly-named dead zones.
That much is well known and widely studied.
But in the future, pollution won't be the only thing stripping the oceans of oxygen, as global warming's effects could leave the seas deprived of oxygen for thousands of years, a new computer model simulation suggests.
The research is not just based on pure prognostication. Already, oxygen levels in the world's seas have been declining for decades as the water has, on average, become warmer.
As carbon dioxide from fossil fuel combustion accumulates in Earth's atmosphere, warming the planet, the oceans warm in response. This warming in turn alters the chemistry of the ocean, specifically, decreasing the waters' ability to hold oxygen. Several studies in recent years have shown this relationship.
But some of the carbon dioxide already in the atmosphere is going to be hanging around for thousands of years, and "no studies have really looked at the effects on the Earth system itself over such long times scales," said the leader of the new study, Gary Shaffer of the Niels Bohr Institute at the University of Copenhagen in Denmark.
Shaffer and his colleagues did just that, using a model to project the changes in dissolved ocean oxygen over the next 100,000 years.
"So we were able to sort of get a complete picture of how the oxygen depletion develops and how it recovers," Shaffer told LiveScience. The results of the study were detailed on Jan. 25 in the online issue of the journal Nature Geoscience.
Anoxic expansion
While some anoxic areas (those with oxygen levels too low to support fish and shellfish) are created by man-made fertilizer runoff from rivers, others exist naturally at intermediate depths in the ocean - these are called oxygen minimum zones. These low-oxygen areas can be found in the eastern Pacific Ocean and the northern Indian Ocean, Shaffer said.
"About 2 percent of the ocean is covered by these zones already at 500 meters [1,600 feet] depth," he said.
As the ocean water warms and becomes less soluble to oxygen, these areas will expand, with the greatest effects off the coasts of Peru, Chile, and California and in the seas on both sides of India, the model projections show.
In fact, a May 2008 study in the journal Science already found that ocean oxygen levels have been decreasing in parts of the open ocean since the 1950s as a result of warming ocean waters.
Ocean oxygen levels globally "have been on the decline for a long time," said Peter Brewer of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. Brewer was not involved with the study.
It will take some time, a few thousand years, for the full effect of oxygen depletion to be felt. This is because the ocean takes much longer to warm than the air or land.
After the surface waters have become depleted in oxygen, oxygen levels in the deep ocean could also dip if warming slows down the ocean's circulation, as some models predict. Shaffer acknowledges some skepticism on this point, but says that he and his colleagues "wanted to cover all the possibilities which have been brought up before."
The team's work was supported in part by the Danish Natural Science Research Foundation and CONICYT-Chile.
Biological effects
As oxygen levels drop, the ocean waters become unable to support many marine species. Extreme ocean oxygen depletion events are one of the theories proposed to explain some of Earth's mass extinctions, including the largest such event at the end of the Permian 250 million years ago.
As anoxic zones expand, nitrate - a form of nitrogen and an essential nutrient for life - is stripped from the ocean. This shifts the biological production in sunlit ocean surface waters. Fish and shellfish that would normally populate an area give way to plankton species, which don't need nitrate because they can draw dissolved nitrogen directly from the water. This is the same situation that creates the familiar algae blooms seen in the Gulf of Mexico and Baltic Sea dead zones, though these are a result of fertilizer runoff.
Such a shift can lead to large, unpredictable changes in the food chains of these ecosystems, threatening fisheries that we humans depend upon for food.
"[Ocean oxygen depletion is] a very, very important possible consequence of global warming, and one should be aware of all possible consequences of global warming," Shaffer said.
Brewer agrees, noting that ocean effects of global warming have been less well-constrained than effects to the land and atmosphere in reports such as those from the Intergovernmental Panel on Climate Change (IPCC). Shaffer and his team's work adds to a growing body of research shedding light on oxygen depletion in the ocean, Brewer told LiveScience.
But unlike the fertilizer-fueled dead zones, which could be recovered relatively quickly once the pollution is stopped, anoxic zones created by global warming will take far longer to bounce back. What was thousands of years in the making will take thousands of years to undo.
"Once you get things going, they're going to be around for thousands of years," Shaffer said.
The only way to stop the problem is to reduce fossil fuel emissions over the next few generations, Shaffer said.
"I used to like to say that what we do in the next few generations affects the next few thousand generations," he said.
But even if we do curtail emissions, there is still some warming already set by current emissions that would cause anoxia, but it would last for only a few hundred years, instead of a few thousand, Shaffer said.
Global Warming to Create "Permanent" Ocean Dead Zones?
Ker Than, National Geographic News 28 Jn 09;
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 worse case-scenario, dead zones could encompass more than a fifth of the world's oceans, the team says.
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.
"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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