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Speedy spin kept early Earth from freezing.

In the beginning, there was heaven and Earth, and a weak young sun shining faint rays into the newborn solar system. The juvenile sun was so dim, in fact, that it could not have kept the early Earth from freezing solid, causing scientists to wonder what steered our watery planet from a frigid fate that would have precluded life's development. A new study suggests that the Earth may have stayed warm in part because of its dizzying spin and lack of land at the time.

The problem is known as the "faint young sun" paradox. Calculations suggest the sun had only 70 percent of its current strength early in Earth's history, around 4 billion years ago. But earth scientists know that the planet had liquid water during this time, called the Archean Era, indicating that some factor must have compensated for the weak sun. One popular theory holds that the early atmosphere contained up to 1,000 times the current level of carbon dioxide, providing a much stronger greenhouse effect then.

Gregory S. Jenkins from the National Center for Atmospheric Research in Boulder, Colo., however, contends that other aspects of the early Earth would have helped warm the planet. "The bottom line is you don't need that much carbon dioxide," Jenkins says.

He and his colleagues decided to simulate basic features of the early Earth using a general circulation model on a supercomputer, making this the first time researchers have aimed a sophisticated climate model at this early period, says Jenkins. The team focused on how the planet's rotation rate and land coverage influence climate.

Scientists know the Earth's spin has slowed through time because of tidal friction - energy lost as the moon causes water to slosh around the globe. Over the same period, the Earth has gained more continental land through volcanic eruptions and plate tectonic collisions. To simulate the ancient climate, Jenkins' team ran the model using a 14-hour4ong day and a world with no land masses.

They found that a faster-spinning Earth has smaller weather systems lying closer to the equator -- changes that caused a 20 percent decrease in cloud cover relative to today. The reduction in clouds warmed the planet because more sunlight could reach its surface.

The absence of land also raised temperatures because water absorbs more sunlight than the light-colored land. In total, the two factors raised temperatures 5* to 6*C above what they would have been, the scientists report in the May 20 JOURNAL OF GEOPHYSICAL RESEARCH.

On their own, the fast rotation and lack of land would not warm the climate enough to keep the ocean liquid. But Jenkins says they would provide a significant portion of the warming, indicating that the atmosphere need not have held as much carbon dioxide as proposed.

One of the authors of the carbon dioxide theory, James E Kasting of Pennsylvania State University in University Park, agrees that the rotation rate and land area could have helped keep the world unfrozen. Even so, he says, carbon dioxide levels must still have reached high values - some 30 to 300 times current concentrations - to provide the rest of the warming power required to augment the rays from a faint young sun.
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Title Annotation:faster spin and absence of land explains "faint young sun" paradox
Author:Monastersky, Richard
Publication:Science News
Article Type:Brief Article
Date:Jun 12, 1993
Words:534
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