如果你看過動畫片《冰河時代》，一定對那番冰天雪地的場景記憶猶新。最新研究顯示，數萬年之后，地球又將進入冰川期，整個北半球都將被厚厚的冰雪覆蓋。那么現在最讓人們頭疼的溫室效應能阻止下一個冰川期的來臨嗎？

Emissions of greenhouse gases—such as the carbon dioxide, or CO2, that comes from power plants and cars—are heating the atmosphere to such an extent that the next ice age, predicted to be the deepest in millions of years, may be postponed indefinitely.

In about 10,000 to 100,000 years, the study suggests, Antarctic-like "permanent" ice sheets would shroud much of Canada, Europe, and Asia. "I think the present carbon dioxide levels are probably sufficient to prevent that from ever happening," said Crowley, whose study will appear tomorrow in the journal Nature.

For the past three million years, Earth's climate has wobbled through dozens of ice ages, with thick ice sheets growing from the poles and then shrinking back again. These ice ages used to last roughly 41,000 years. But in the past half a million years, these big freezes each stretched to about a hundred thousand years long. Meanwhile, the temperature swings during and between these ice ages became more extreme, soaring to new highs and lows. These extreme climate swings don't appear to be easing anytime soon, according to evidence recorded in Earth's rocks, Crowley said. "The latest two glaciations were two of the biggest we've seen."

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. 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 non-glacial atmosphere with polar ice caps, presents a bizarre scenario. 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.

（來源：美國國家地理 實習生許雅寧）