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Equatorial waters hold undercurrent to global warming
Despite the skepticism and posturing about global warming, most recently at an international conference in The Hague, evidence continues to accumulate that Earth's temperature is rising, most likely due to human activities.
Experts point to gases that prevent heat from escaping our atmosphere, particularly carbon dioxide, as prime culprits. But before scientists can predict the future climate or propose remedial action, they first need to look to the past.
By gathering data about how atmospheric carbon dioxide varied during the Ice Ages and then using that information in climate models, they get a better picture of changes to come.
One key to improving the accuracy of climate models is understanding the role oceans play. Scientists know that water bodies absorb a sizeable chunk of manmade carbon dioxide emissions but the process itself remains poorly understood. The balance of carbon dioxide between ocean and atmosphere teeters constantly, depending on the amount dissolved in chilly polar waters and outgassed in warm tropical swells and also on the amount absorbed during plankton growth and decay.
These microscopic floating plants feed in upwelling water that brings them a steady diet of nutrients, including carbon dioxide, nitrate, phosphate and iron. When they die, plankton carry carbon and waste products to the ocean floor, which keeps carbon dioxide out of the atmosphere.
But what regulates how fast the plankton grow and multiply? As much as 50 percent of biological production in global oceans occurs in the eastern equatorial Pacific, making it an ideal laboratory to study the factors involved. That's doubly true, because the region is also the primary area for release of carbon dioxide to the atmosphere.
"Until now, it's been assumed that atmospheric conditions, such as the trade winds blowing across the tropics, largely controlled ocean conditions in the eastern equatorial Pacific," says Paul Loubere, a geosciences professor at Northern Illinois University whose work appears in a recent issue of Nature. "My research presents the first evidence that there's something else to consider."
That something else is the Equatorial Undercurrent, an undersea ribbon of water that originates south of New Zealand, zigzags along the western edge of the South Pacific and stretches across the equator.
"As the water in the undercurrent moves farther east, upwelling peels off the upper layers," says J.R. Toggweiler, head of the Ocean Circulation Group at NOAA's Geophysical Fluid Dynamics Lab in Princeton, N.J. "By the time the undercurrent surfaces off the coast of Peru, the flow contains cold, nutrient-rich water from below."
The possibility that biological productivity in the eastern equatorial Pacific isn't controlled solely by tropical processes but also by a link to high latitudes intrigued Loubere. He set out to learn how the area's carbon dioxide supply has changed over time, what role biological productivity played and to what degree these relate to known changes in atmospheric carbon dioxide.
"What"s important is determining which mechanisms make the climate sensitive to change," says Alan Mix, a professor of oceanic and atmospheric sciences at Oregon State University. Biological productivity may, for example, heavily influence the amount of atmospheric carbon dioxide, leading back to the greenhouse effect.
To reconstruct a record of marine life activity over the past 130,000 years, Loubere studied organisms in sediment cores taken several hundred miles off the Peruvian coast. He based biological productivity estimates on bottom-dwelling foraminifera, microanimals that form a vital link in the marine food chain.
Although the southeasterly trade winds influenced the environment at all four core sites, the South Equatorial Current also affected two of them. This current carries water that can be traced to subantarctic origins, but that's not surprising: Winds pick up the Equatorial Undercurrent's surfacing waters and blow them back across the ocean along the equator.
By comparing what's known about the temperature over the past 100,000 years with what he learned from the productivity records, Loubere found that the pattern of biological productivity is distinct where the undercurrent exerts its greatest influence.
"If atmospheric processes controlled the productivity, then the records from all four cores should be the same," he says. Instead, the two cores influenced only by trade winds showed a pattern of more-frequent productivity change than the two also affected by the South Equatorial Current.
"It's a valuable new piece of information," says Richard Barber, professor of biological oceanography at Duke University. "Understanding why carbon dioxide varied in the last glacial maximum is the most important question facing us. If we can't explain the recent past 18,000 to 20,000 years ago, then we can't be confident of our ability to predict the future."
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