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Ice ages: Why North America is key to their coming and going


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An iceberg at the Ilulissat fjord in west Greenland. An international team of scientists suggests that the North American continent is the breeding ground for the ebb and flow of ice sheets over hundreds of thousands of years.

Ian Joughin/AP

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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.




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.”





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The simulations also showed how the interplay between orbital and climate influences kept North America's ice sheets growing, even as a significant but smaller ice sheet building in northern Eurasia waxed and waned based on orbital changes happening at 40,000- and 20,000-year intervals.

Looking closely at the last glacial cycle, which ended about 10,000 years ago, the team estimates that by the time the North American ice sheet reached what is now Kansas, its southernmost point, it held enough water to raise sea levels by 90 meters (295 feet). But the sheet also depressed the crust.

The team estimates that a 3,000 meter thick sheet would have depressed the crust by about 1,000 meters. This would have lowered the altitude of the top of the sheet, bringing it into contact with warmer air. In addition, as the sheet melted, a slowly rebounding crust would gather melt water along the sheet's edges, providing the lifting and melting from underneath that would accelerate its retreat.

This aspect of the work alone was worth the price of admission, suggests Shawn Marshall, a climate scientist at the University of Calgary in Alberta, Canada. He notes that previous modeling studies couldn't deliver the onset of ice sheets growth and advance without conditions colder and snowier than climate models predict for an Earth in the cold phase of its long-term orbital variations.

Getting out of an ice age was even tougher, he writes in an outsider's analysis of the study, set for publication in Thursday's issue of the journal Nature.

This is the first free-running model that appears to get the ice ages right without a lot of tweaking along the way to include factors like ocean circulation or the influence of dust on ice melting in order for the onset or retreat of ice sheets to happen at the right times, he notes.

The work still has room for improvement, he continues. The model treats some processes a bit too simplistically.

Still, the researchers "make a convincing case" that North America's shape and location on the globe, as well as the slow recovery of the crust as the ice begins to melt turn variations in solar radiation occurring in cycles measured in a few tens of thousands of years into a 100,000-year glacial cycle.

What makes North America so special?

"Maybe that's because the mountains deflect the polar jet stream farther south," says Dr. Raymo, referring to a high-altitude river of air that forms the boundary between cold polar air and warmer air to the south. Deep southward meanders in that river during modern winters can bring snow to regions where it's rare, such as the US Southeast.

Or perhaps during the last glacial maximum the ice sheet covering Scandinavia was vulnerable to warm Atlantic Ocean water, she adds.

"We don't know, in fact that's the next work we're doing – experiments to try to investigate that," she says.

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