Rising Oxygen Levels Triggered Glacial Episodes, Scientist Says

Increasing amounts of oxygen in the Earth’s atmosphere produced by early oxygen-releasing bacteria may have triggered the first of three episodes when the Earth was covered from pole to pole by ice, a phenomenon known as snowball Earth. One episode occurred 2.3 billion years ago, while the second and third occurred 750 million and 600 million years ago. Evidence suggests that glaciation occurred even at the equator during those periods.

Geoscientist James F. Kasting of Pennsylvania State University reported Wednesday at a Denver meeting of the Geological Society of America that immediately prior to the first glaciation episode, Earth’s temperature was maintained primarily by methane, a so-called greenhouse gas that absorbs the sun’s heat. Release of oxygen, he said, would have destroyed the methane without producing enough carbon dioxide--a less effective greenhouse gas--to offset its loss. Earth fell into a glacial period, he said, until volcanoes belched out enough carbon dioxide to restore the heat balance.

Compiled by Times medical writer Thomas H. Maugh II