BERKELEY, California - Climate models that predict the Earth's average temperature could rise as much as 10 degrees by the end of the century may have underestimated the increase by as much as four degrees.

New research at UC Berkeley and Lawrence Berkeley Laboratory suggests that as carbon dioxide emissions heat the globe, hotter oceans and soils will release stored carbon dioxide, which will in turn kick up the thermostat an extra notch.

"We've probably underestimated the problem," said UC Berkeley ecologist John Harte.

Current models predict that a doubling of the carbon dioxide in the atmosphere, which at current emissions rates should happen within the next 50 years, would raise global temperatures between 2.7 and 8.1 degrees.

Harte and biogeochemist Margaret Torn of the Berkeley lab predict that if humans double the carbon dioxide, that will actually lead to more carbon dioxide being released naturally, which in turn will push the global thermostat up between 2.9 and 11 degrees, with the higher temperatures more likely.

By the end of the century, the increase could be as much as 14 degrees.

"It's a vicious cycle where more warming causes more greenhouse gas emissions, and more greenhouse gas emissions cause more warming," Torn said. "That could have serious consequences both for human populations and biodiversity."

Global temperatures fluctuate naturally with the intensity of sunlight hitting Earth.

Harte and Torn studied past warm periods in a 360,000-year climate record contained in Antarctic ice cores. Using air bubbles trapped in the ice, they estimated past global temperatures using the ratios of oxygen isotopes and deuterium, which vary with temperature. They then checked the effect of hotter temperatures on the level of carbon dioxide and methane in the bubbles.

They found that as temperatures rose, these two greenhouse gases increased more than would be expected just from the increased sunlight that initiated the warming.

Though the ice can't tell them why this happened, the scientists suspect that heating up the Earth's soil and oceans caused them to let go of stored carbon dioxide. Warmer soils decompose faster which releases carbon dioxide faster, and warmer oceans are known to lose more carbon dioxide as well.

Harte and Torn took what they learned about this feedback loop from the past and added it into the most sensitive state-of-the art climate model to find out what the globe might be headed for in the future.

"The extra greenhouse gas emissions that occurred with past warming would lead to a significant amount of extra warming in the future," Torn said.

Harte is also experimenting with artificially heating soils in the Rocky Mountains. His research shows the warmer soils do indeed discharge significantly more carbon dioxide than soils that aren't heated. He cautions, however, that different soils in different climates may not behave exactly the same. And the long-term effect isn't known either.

We may not be able to rely on the oceans and soils to absorb more carbon dioxide as atmospheric levels increase, as some scientists have predicted will happen.

"Just the opposite is actually more likely," Harte said.

Currently, 6.5 billion tons of carbon dioxide are released into the atmosphere from fossil fuel combustion each year. Two-thirds of it stays in the atmosphere.

The other third is absorbed by the ocean and the soil. But it's possible that this amount will go down in the future, Harte said.

In addition to these "carbon sinks" becoming saturated or clogged, the increased heating will also cause more of the carbon stored in the sinks to come back out.

© 2006 KRT Wire and wire service sources

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