Human warming hobbles ancient climate cycle

WASHINGTON Sun Apr 27, 2008 2:37pm EDT

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WASHINGTON (Reuters) - Before humans began burning fossil fuels, there was an eons-long balance between carbon dioxide emissions and Earth's ability to absorb them, but now the planet can't keep up, scientists said on Sunday.

The finding, reported in the journal Nature Geoscience, relies on ancient Antarctic ice bubbles that contain air samples going back 610,000 years.

Climate scientists for the last 25 years or so have suggested that some kind of natural mechanism regulates our planet's temperature and the level of carbon dioxide in the atmosphere. Those skeptical about human influence on global warming point to this as the cause for recent climate change.

This research is likely the first observable evidence for this natural mechanism.

This mechanism, known as "feedback," has been thrown out of whack by a steep rise in carbon dioxide emissions from the burning of coal and petroleum for the last 200 years or so, said Richard Zeebe, a co-author of the report.

"These feedbacks operate so slowly that they will not help us in terms of climate change ... that we're going to see in the next several hundred years," Zeebe said by telephone from the University of Hawaii. "Right now we have put the system entirely out of equilibrium."

In the ancient past, excess carbon dioxide came mostly from volcanoes, which spewed very little of the chemical compared to what humans activities do now, but it still had to be addressed.

This antique excess carbon dioxide -- a powerful greenhouse gas -- was removed from the atmosphere through the weathering of mountains, which take in the chemical. In the end, it was washed downhill into oceans and buried in deep sea sediments, Zeebe said.

14,000 TIMES FASTER THAN NATURE

Zeebe analyzed carbon dioxide that had been captured in Antarctic ice, and by figuring out how much carbon dioxide was in the atmosphere at various points in time, he and his co-author determined that it waxed and waned along with the world's temperature.

"When the carbon dioxide was low, the temperature was low, and we had an ice age," he said. And while Earth's temperature fell during ice ages and rose during so-called interglacial periods between them, the planet's mean temperature has been going slowly down for about 600,000 years.

The average change in the amount of atmospheric carbon dioxide over the last 600,000 years has been just 22 parts per million by volume, Zeebe said, which means that 22 molecules of carbon dioxide were added to, or removed from, every million molecules of air.

Since the Industrial Revolution began in the 18th century, ushering in the widespread human use of fossil fuels, the amount of carbon dioxide in the atmosphere has risen by 100 parts per million.

That means human activities are putting carbon dioxide into the atmosphere about 14,000 times as fast as natural processes do, Zeebe said.

And it appears to be speeding up: the U.S. government reported last week that in 2007 alone, atmospheric carbon dioxide increased by 2.4 parts per million.

The natural mechanism will eventually absorb the excess carbon dioxide, Zeebe said, but not for hundreds of thousands of years.

"This is a time period that we can hardly imagine," he said. "They are way too slow to help us to restore the balance that we have now basically distorted in a very short period of time."

(Editing by Eric Walsh)

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