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鈥檚. These warm spells used to come every 41,000 years, but about 950,000 years ago the period shifted to every 100,000 years.
Changes in ocean circulation or in the structure of continental ice sheets have been blamed for the shift.
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But David Lea of the University of California, Santa Barbara, suspects fluctuating carbon dioxide levels might have been the real culprit.
His evidence comes from magnesium levels in the shells of fossil plankton, which reveal past sea-surface temperatures. Seabed cores taken from the western equatorial Pacific show the same temperature fluctuations as recorded in the eastern equatorial Pacific at the time of the shift. That rules out changes in ocean currents, which would affect the two areas differently. The effect driving the temperature change must have been global, says Lea (Science, DOI: 10.1126/science/1115933).
Variations in atmospheric carbon dioxide, which is uniform across the globe, fit the bill. But even if CO2 levels do explain the shift, this just pushes the mystery to another variable, says Peter Huybers of Harvard University. 鈥淲e still need to know what鈥檚 changing the CO2,鈥� he says.
