Did Carbon Save Earth From a Deep Freeze?

By Phil Berardelli
ScienceNOW Daily News
5 December 2007

As half of the United States struggles through the first major snow and ice storm of the season, it might be useful to consider what the whole planet would be like if not for the effects of carbon. Researchers are postulating that carbon in the ocean, dissolved from mineral deposits on the sea floor, has prevented Earth from becoming a giant snowball at critical junctures in its history. The findings also lend credence to a hypothesis that climate change and the global carbon cycle are tightly linked, with each influencing the other.

Life on Earth depends on the greenhouse effect. Without carbon dioxide and other gases in the atmosphere to capture a significant portion of the sun's heat, the planet would have plunged into subfreezing temperatures long ago--and perhaps never warmed enough to allow complex, heat-dependent creatures such as humans to flourish. Geological data show that Earth has come perilously close to this scenario in the distant past. One viewpoint, called the snowball Earth hypothesis, suggests that the planet required millions of years to recover from these deep freezes--the last one occurring about 550 million years ago--and could do so only via the accumulation of atmospheric carbon dioxide from volcanic eruptions.

Now a team from the University of Toronto in Canada has used a model of the carbon cycle to show that the presence of the element--in the form of a class of minerals called carbonates--in the sea bottom arrested the deep freeze before it completely overcame the planet. Instead of snowball Earth, the team reports in the 6 December issue of Nature, the effect was more of a "slushball Earth." The researchers, led by geophysicist W. Richard Peltier, found that cooling global temperatures allow the oceans to absorb more oxygen from the atmosphere. The oxygen reacts with carbonates deposited from the skeletons of tiny marine organisms as well as the action of photosynthesis by plankton, releasing carbon dioxide that helps temperatures bounce back quickly, at least on a geological timetable. The greenhouse effect has played an important role, "even in the deep past, in determining the surface climate of the planet," Peltier says. And from 1 billion years ago to about 550 million years ago, the period that was the subject of the study, "this influence apparently acted to protect the biosphere."

It is clear from the research that Earth's feedback systems kept the planet from plunging permanently into a snowball state, says geologist Alan Jay Kaufman of the University of Maryland, College Park. In principle, the feedback system might spare the planet from a runaway greenhouse effect if the oceans take up enough carbon from the atmosphere. But the effect takes place over many thousands of years, Kaufman says, so there's little hope that the carbon cycle could temper the effect of increased atmospheric CO₂ on human time scales.