Human warming hobbles ancient climate cycle

By Deborah Zabarenko, Environment Correspondent

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.