The researchers found that between 10,000 and 100,000 years from now, Earth would enter into a period of permanent ice sheets—more severe than any seen in millions of years.
In some ways the ice age would be like those in the past few hundred thousand years, with a thick ice sheet covering North America, the study predicted.
But in the model, Europe and Asia also succumbed to ice sheets up to 2 miles (3.5 kilometers) thick, stretching from England to Siberia—something never before seen in models of past ice ages.
"We were surprised," Crowley said. "There's no evidence for this in Asia" during ice ages in the past few million years.
Hard to Know
Though this extreme ice age would be unusual, so is the climate that people are creating by emitting huge amounts of greenhouse gases, Crowley said (global warming fast facts).
"It's hard to say what's going to happen," Crowley said. "The very fact that you have this nonglacial [warming] atmosphere with polar ice caps [still present], presents a bizarre scenario.
"I don't know that we have a comparable analogy for it in the geologic record."
Prehistoric-climate expert Lorraine Lisiecki said, "This is the only study of which I am aware that suggests the next ice age could be much more extreme than those of the previous one million years."
Many more tests are needed to see if the study's prediction seems correct, said Lisiecki, of the University of California, Santa Barbara.
But she agreed that we might never find out what would have happened naturally, due to human-caused global warming.
"Current greenhouse gas concentrations are probably similar to those that occurred three million years ago and are high enough to prevent an ice age for hundreds of thousands of years," she said.
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