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Another Knob for Climate Models?

The rain in Spain--and everywhere else--may have had a significant impact on ancient global warming spells, according to a study published in this month's issue of Geology. In a computer model of Earth's oceans and atmosphere millions of years ago, climatologists have shown that rainfall and runoff patterns can be key factors in the development of what are presumed to be climate-warping ocean currents.

Water temperature can play a major role in setting up ocean currents, with colder and thus denser water sinking and sliding along the sea floor. But salts are also important, because if warm water is loaded with enough salt, it will sink deep. Salinity of the surface waters can be influenced by the amount of river water flowing into the oceans, yet no computer models of ancient ocean circulation had included this variable.

To test the effect, a team led by Karen Bice, a paleoclimatologist at Pennsylvania State University, in State College, modified an existing computer model called GENESIS, which is designed to recreate the ocean conditions of the Eocene, some 55 million years ago. When the team ran the model with rivers, the freshwater diluted the salty surface waters of the tropics and subtropics and shut off the deep currents. Bice says the finding demonstrates the need to consider river systems, which are "a very important factor."

But experts say the pattern of Eocene rainfall and rivers, while perhaps important for ancient climate changes, may not help understand future climate changes. "It doesn't follow that it's relevant to today," says George Philander, an ocean scientist at Princeton University, who points out that today's ocean currents are very different from those of the Eocene. James Kennett of the University of California at Santa Barbara adds that salinity changes might have had a greater influence on the circulation of Eocene oceans than on that of today's oceans.