The chill would induce a long, stable period of glaciation in the mid-latitudes, smothering Europe, Asia and North America to about 45-50 degrees latitude with a thick sheet of ice.

However, there is now so much CO2 in the air, as a result of fossil-fuel burning and deforestation, that this adds a heat-trapping greenhouse effect that will offset the cooling impacts of orbital shift, said Crowley.

"Even the level that we have there now is more than sufficient to reach that critical state seen in the model," he said. "If we cut back [on CO2] some, that would probably still be enough."

In September, a scientific research consortium called the Global Carbon Project (GCP) said that atmospheric concentrations of CO2 reached 383 parts per million (ppm) in 2007, or 37 percent above pre-industrial levels.

Present concentrations are "the highest during the last 650,000 years and probably during the last 20 million years," the report said.