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Looking far ahead into the greenhouse.

Most forecasts of greenhouse warming take a relatively shortsighted view by focusing on how the climate will react to a doubling of atmospheric carbon dioxide - an increase expected sometime in the middle of the next century under current emission rates. But unless nations drastically curtail their use of fossil fuels, carbon dioxide will continue to accumulate even after doubling. That raises the question of how greater amounts of carbon dioxide will affect the climate.

A new computer simulation of quadruple carbon concentrations suggests that major climate changes could lie ahead.

At the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J., Syukuro Manabe and Ronald J. Stouffer used a general circulation model to compare how a computerized climate evolves under three different scenarios. In one run, carbon dioxide concentrations increase 1 percent per year until they double in 70 years, and then remain constant. In a second run, concentrations climb until they quadruple in 140 years. A "control" simulation lets the climate develop with constant carbon dioxide levels.

The double and quadruple cases show similar behavior up to a point. As global temperatures rise, both show a weakening in the conveyer-belt system of ocean currents that transports heat around the globe. In the doubling simulation, the currents gradually regain strength over several centuries. But they remain weak in the case of quadrupled carbon dioxide.

"This is a fundamental change in the ocean circulation," says Manabe. The diminished circulation would reduce mixing between the deep ocean and surface waters, he and Stouffer report in the July 15 Nature. This would limit the amount of nutrients brought up from the depths. It would also curtail the supply of life-sustaining oxygen to the deep ocean.

By the end of the 500 years, the quadruple-carbon simulation shows temperatures over the continents reaching 7[Degrees]C to 10[Degrees]C warmer than today. The last time Earth saw such warmth was during the Cretaceous period, more than 65 million years ago.
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Title Annotation:forecasting carbon dioxide increases in the air
Publication:Science News
Article Type:Brief Article
Date:Aug 14, 1993
Words:330
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