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.
Several explanations for this shift have been proposed, but one of the most popular ones — a long-term decline in carbon dioxide levels — isn’t t