For the last 900,000 years, mile-thick ice sheets have waxed and waned in the Northern Hemisphere with remarkable regularity - building over periods of about 100,000 years and retreating in the space of only a few thousand years, only to repeat the cycle.
Now, a team of scientists from Japan, the US, and Switzerland suggests that the North American continent is the breeding ground for these cycles. It's a region where climate and the ice's effect on the Earth's crust play off each other to draw out the length of a glacial cycle triggered by changes in solar radiation that come with changes in Earth's orbit.
This feedback between climate and ice becomes most dramatic at the end of the cycle, when an ice sheet that has bulldozed its way too far south and gotten too heavy for its own good meets up with a warming climate.
"When the ice sheets get to this huge state, they are very hard to keep that way," says Maureen Raymo, a paleoclimatologist at Columbia University's Lamont-Doherty Geophysical Observatory in Palisades, N.Y., and a member of the team conducting the study.
Once a change in Earth's orbital characteristics brings on the next warming event, "the whole system just goes into catastrophic collapse," she explains. "It melts back a little, seas start to flow into the depressed crust, this floats the ice and melts it from below."
In a tiny fraction of the time it takes to build continental- scale ice sheets, the sheets retreat to high-latitude havens atop Greenland and the northern reaches of the Canadian archipelago.
The study, led by Ayako Abe-Ouchi, a climate scientist at the University of Tokyo and the National Institute of Polar Research, resulted from a unique approach to modeling ice ages.
The study provides "good insights that clearly advance our understanding" of ice ages, notes Penn State University glaciologist Richard Alley, who was not involved in the study, in an e-mail. It also confirms aspects of ice ages that researchers have well in hand, he says.
The approach linked individual atmosphere, crust, and ice models in a way that needed only information on the amount of sunlight reaching Earth to generate ice-sheet behavior over the past 400,000 years that geologists have gleaned from more than a century of field studies.
Changes in the amount of solar radiation striking Earth come with changes in Earth's orbit occurring at intervals of 41,000, 23,000, and 19,000 years.
The study reaffirms that changes in the amount of summer sunlight striking northern high latitudes sets the process in motion. Indeed, changes to the shape of Earth's orbit over time, as well as long- term changes in the orientation of its axis, and their impact on solar radiation at high northern latitudes were the most significant astronomical influences in the team's simulations.
The team also weighed the relative contributions of changes in atmospheric concentrations of carbon dioxide, a greenhouse gas, to the 100,000-year glacial cycles.
While carbon dioxide decreased as the ice sheet expanded and cooled the climate and increased again as the climate warmed, CO2 levels did not determine the overall sequence of events during each 100,000-year cycle, the researchers concluded. …