From fire comes ice.

A global warming from greenhouse gas pollution could send the planet spiraling into an ice age over the next few thousand years, two geologists assert.

Gifford H. Miller of the University of Colorado at Boulder and Anne de Vernal at the University of Quebec in Montreal reached that conclusion after examining how glaciers and ice sheets behaved through the past 130,000 years, a time that includes the last ice age. Geologic records compiled by various researchers indicate that ice sheets began building in the high latitudes of North America, Europe and Asia at a time when the climate was at least as warm as today. In the Jan. 16 NATURE, Miller and de Vernal explain that the mild conditions - especially during winter - led to increased evaporation from the ocean, stimulating more snowfall over the far North. The snow survived during summer because shifts in Earth's orbit had reduced the amount of sunlight reaching the Arctic during that season.

The two researchers suggest that a greenhouse warming may bring the same sort of conditions that stimulated the growth of glacial sheets 120,000 years ago. Computer models of Earth's climate predict that a doubling of carbon dioxide concentrations will warm the winter Arctic by 8[degrees]C to 14[degrees]C, while raising summer temperatures much less. Some simulations predict a 20 to 40 percent increase in Arctic precipitation.

Earth's orbit may serve as an accomplice in the building of ice sheets. Over tens of thousands of years, slight variations occur in the orientation of the Earth's axis and the planet's distance from the sun. Because of such changes, the amount of solar radiation reaching key regions in the Arctic is now decreasing - one of the conditions believed to stimulate the growth of ice sheets. The radiation has already dropped to a level on a par with conditions at the start of the last ice age, Miller and de Vernal note.

Their theory counters the idea that a greenhouse warming will raise global sea levels by hastening melting of ice sheets and glaciers. The two researchers suggest that ice accumulation around the world could lower sea levels at the significant rate of 7 millimeters per year.