Denver, Colo. -- Increasing amounts of oxygen in the atmosphere could have triggered the first of three past episodes when the Earth became a giant snowball, covered from pole to pole by ice and frozen oceans, according to a Penn State researcher.
"We have convincing evidence that at least six of the seven continents were once glaciated, and we also have evidence that some of these continents were near the equator when they were covered with ice," says Dr. James F. Kasting, professor of geosciences and meteorology. "Two of these global glaciations occurred at 600 and 750 million years ago, but the earliest occurred at 2.3 billion years ago."
According to Kasting, if it is assumed that the magnetic evidence for glaciation at the equator is correct, then only two possible explanations for equatorial glaciation exist.
One is that the Earth's tilt, which is now at 23.5 degrees from vertical, was higher than about 54 degrees from vertical. This would have positioned Earth so that the poles received the most solar energy and the equator would receive the least, creating a glacier around the middle but still leaving the poles unfrozen.
The other possibility, which is the one that Kasting leans toward now, is that the greenhouse gases in the atmosphere fell low enough so that over millions of years, glaciers gradually encroached from the poles to 30 degrees from the equator. Then, in about 1,000 years, the remainder of the Earth rapidly froze due to the great reflectivity of the already ice-covered areas and their inability to capture heat from the sun. The entire Earth became a snowball with oceans frozen to more than a half mile deep.
"For the latest two glaciations, carbon dioxide levels fell low enough to begin the glaciation process. However, for the earliest glaciation, the key may have been methane," Kasting told attendees at the annual meeting of the Geological Society of America today (Oct. 27) in Denver. "The earliest known snowball Earth occurred around the time that oxygen levels in the atmosphere began to rise," says Kasting, who is a member of the Penn State Astrobiology Center. "Before then, methane was a major greenhouse gas in the atmosphere in addition to carbon dioxide and water vapor."
As oxygen levels increased, methane levels decreased dramatically and carbon dioxide levels had not built up enough to compensate, allowing the Earth to cool. Oxygen levels need only reach a hundredth of a percent of present-day oxygen levels to convert the methane atmosphere completely. Once the Earth is snow covered, it takes 5 to 10 million years for the natural activity of volcanos to increase carbon dioxide enough to melt the glaciers.
Regardless of the greenhouse gas involved, the pattern of freezing and defrosting would be the same. Because the sun has been constantly increasing in brightness, it would take more greenhouse gas in the past to compensate for the fainter sun. For the glaciations at 600 and 750 million years ago, estimates are that carbon dioxide levels equal to recent pre-industrial levels or up to three times pre-industrial levels would have been sufficient for snowball Earth to occur.
Because many continents existed in the warm equatorial areas during the most recent glaciations, Kasting believes that rapid weathering of calcium and magnesium silicate rocks, which consumes carbon dioxide, lowered levels sufficient to cool things.
"It would have taken nearly 300 times present levels of carbon dioxide to bring the Earth out of its ice cover," says Kasting. "Then, once the high reflectivity ice was gone, the carbon dioxide would have overcompensated and the Earth would become very warm until rapid weathering would remove carbon dioxide from the atmosphere."
One reason that many scientists initially rejected the snowball Earth theory was that biological evidence does not suggest that the various forms of life on Earth branched out from the latest total glaciation. A variety of life forms had to survive from before the glaciation, which is difficult to imagine on an ice-covered world. Perhaps the ancestors of life today survived in refuges like hot springs or near undersea thermal vents.
"The biological puzzle of snowball Earth is very interesting," says Kasting. "Events suggest that life was more robust than we thought and that the Earth's climate was much less stable than we assumed."
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