Equatorial waters hold undercurrent to global warming
December 4, 2000
By Environmental News Network staff
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."
Article by Environmental News Network (ENN)
SOURCE: CNN
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