Seesawing gas kept Earth in deep freeze for longer.

This article focuses on a study according to which a puzzling shift in glacial cycles that occurred 950,000 years ago could be partially explained by changing greenhouse gas levels. Earth has spent most of the last 2.5 million years in an ice age, interrupted periodically by interglacial thaws that have warmed the planet to temperatures similar to today's. Changes in ocean circulation or in the structure of continental ice sheets have been blamed for the shift. But researcher David Lea of the University of California, Santa Barbara, suspects fluctuating carbon dioxide levels might have been the real culprit.