When the next glaciation of Earth takes place some time in the Next thousand years, it will double the ice cover over both hemispheres in less than 300 years.

That was the message delivered today to the 145th meeting of the American Association for the Advancement of Science by Columbia University's James D. Hays, who said that sediments drilled from deep in the Antarctic sea floor show that sea ice gorws dramatically and quickly at the onset of a glacial period.

"The change is abrupt," Hays told a press conference today. "They take place within 300 years, during which time the Antarctic ice overgrows from it s normal winter mass of 20 million square kolometers to 40 million square kilometers."

Dr. Hays said Earth is in the late stages of a period between glaciations, jprobably in the final thousand years of it. Since cooling and warming periods take place roughly every 11,000 years, the start of a new glacial period is probably no more than a thousand years away.

Ironically, Earth now appears to be undergoing a slight warming trend that could increase dramatically in the next 75 years, scientists said. The reason is that increased burning of fossil fuels, such as coal and oil, if sending carbon dioxide into the atmosphere at such a rate that Earth's heat is being trapped in the atmosphere befire it can escape to space.

"There is some evidence that we might see a doubling of carbon dioxide in the atmosphere in less than 75 years," said Dr. Roger Revelle of Harvard University. "That could mean a five to six degree increase in the average temperature of Earth, which would have strong impact on nature and on the ways human beings live."

Warming of Earth from carbon dioxide has been much discussed in recent years.

Consequences of a warming trend would include a melting of much of the world's permafrost, a northward spreading of forests, and a lengthening of the growing season in countries like Canada and the Soviet Union.

"The Corn Belt might even move from Irwa into Canada," Revelle said.

Scientists see no contradiction in Earth's warming up even as it approaches a period where things will almost surely cool down.

"I don't think there is any doubt there is a long-term trend toward cooling," Columbia's Hays said. "There may be a carbon dioxide blip on that trend but it won't go on forever. We'll run out of fossil fuels some day."

Formation of Antarctic sea ice is being watched closely by weather satellites.

"The main thing we'll be looking at is the length of the season sea ice stays in the Antarctic," Hays said. "One sign that glaciation is approaching is when Antarctic sea ice retreats less and less during the Antarctic summer."

Even when Earth goes into a period of glaciation, it does not mean the start of another Ice Age. The catastrophic ice ages, where ice covered all of New England, occur in only one out of every 10 glaciations, Hays said.

Most scientists believe that glacial periods begin every 22,000 years when Earth wobbles on its axis in a way that changes the seasonal distribution of heat and light from the sun. The changes are slight but often bring about what Hays called "profound effects" on the climate.