The reason why those cold spells now come less frequently is still unknown
Scientists have peered back in time with a new analytical technique to see atmospheric concentrations of carbon dioxide more than 2 million years into the past. The findings indicate that a long-term decline in the levels of that greenhouse gas isn’t to blame for a geologically recent shift in the frequency of ice ages, scientists say.
The record of ice ages in North America stretches back 2.4 million years (SN: 2/5/05, p. 94). Until about 1.2 million years ago, ice ages in the Northern Hemisphere occurred about every 40,000 years, says Jerry F. McManus, a paleoclimatologist at Lamont-Doherty Earth Observatory in Palisades, N.Y. But for the past 500,000 years or so, ice ages have occurred, on average, only once every 100,000 years, he notes.