THE SUMMIT, GREENLAND -- High above the Arctic Circle, in a flat white world of snow and ice, scientists are drilling a hole through time.

In the most ambitious ice drilling project ever attempted, two teams of European and American researchers are boring down through more than two miles of ice -- the compressed snow of ancient winters -- searching for the trapped air bubbles and entombed crystals that will reveal what the weather was like during the past 200,000 years.

The scientists hope that by understanding the past, and learning what pushes the swinging pendulum of climate, they can predict the future, which many fear will bring a rapid and unprecedented warming caused by the accumulation of pollution in the atmosphere.

Here on the summit of Greenland -- the highest point on the vast ice mound that almost completely covers the world's largest island -- they seek an answer to the most pressing question in climatology: will rising levels of carbon dioxide and methane heat the Earth and melt the polar ice caps? And if so, how fast?

Already there is preliminary evidence from ice cores drilled by Soviet researchers in Antarctica that rising levels of carbon dioxide gas at the end of the last ice age preceded an increase in temperature, a discovery that could bolster the most popular scenarios of global warming. But the record is still hazy and the measurements are imprecise enough that it is possible the warming did not come until thousands of years after the carbon dioxide rose. As such, the relationship between carbon dioxide and climate remains shrouded in mystery.

"There's great urgency in the work," said Paul Mayewski of the University of New Hampshire. "The ice has trapped the past and held it for us to rediscover."

Mayewski is the scientific coordinator for the second Greenland Ice Sheet Project, a $15 million venture, involving more than 30 U.S. investigators, assisted by the Polar Ice Coring Office at the University of Alaska and funded by the National Science Foundation. Collaborating with the Americans is a group of Europeans who are drilling a companion core 20 miles from the Americans' site. Together their work should provide some of the most detailed records ever gathered linking past climate to rising and falling levels of carbon dioxide.

The Americans and Europeans have come to a world that is as difficult as it is beautiful. It is a place where the summer sun wheels around the sky, but never sets, and flakes of snow called ice diamonds glitter in the air like a hallucination. It is so cold that even in the laboratory, built in a trench in the snow, the computers must be swaddled in heating pads to protect them from subzero temperatures.

And because the summit is 10,200 feet above sea level, the air is so thin that newly arrived researchers stumble around for a few days like zombies, chewing aspirin and sucking on oxygen bottles to kill the pounding headaches and nausea caused by the sudden ascent to high altitude.

The plan is to drill to the bedrock of Greenland, down through nearly two miles, or more than 10,000 feet, of ice. Begun last summer, the work proceeds this year. The cutting is done by a hollow tube with a sawtoothed end. It spins, grinding a circle around the ice that becomes the core as it is surrounded by the descending drill. Piece by piece, the researchers and technicians then pull six-foot and twelve-foot lengths of ice core to the surface.

Like new babies, the ice cores must be handled with care. Upon delivery from the hole at the drill site, the cores are rushed to a frozen laboratory dug into the snow itself, a long trench 12 feet deep and several feet wide, where the samples are processed.

Researchers bundled in parkas and snow pants bombard the ice with laser beams and electric jolts. They sift and sniff the ice for traces of volcanic acids and dust, for greenhouse gases such as carbon dioxide and methane and for the special species of heavy hydrogen and heavy oxygen whose abundances tell the temperature when the snow fell.

Last week the Americans' main drill reached 412 feet and brought to the surface ice that researchers estimate was deposited in the year 1569, about half a century before the Pilgrims landed in the New World and right in the middle of a period of worldwide coolness known as the Little Ice Age.

Such precise dating to the exact year is possible for at least the first 5,000 years. The scientists date the core by looking for evidence of known events, such as historically dated volcanic eruptions, which they compare with known rates of ice accumulation and the flow and dynamics of the glacier itself.

By summer's end, the researchers hope to reach past ice laid down when Jesus was born, to the time of the pyramids about 2800 B.C. At the close of the project three years from now, the ice could be as much as 200,000 years old -- snow that fell before anatomically modern human beings evolved.

For scientists clever enough to read the clues, ice can reveal evidence that helps them date the core with considerable precision.

Researchers, for example, can see in the dust trapped in the ice evidence of annual summer winds that have swept the Gobi Desert for thousands of years, thereby giving them a dusty equivalent to the annual growth rings of trees. They can watch the sulfur levels begin to rise at the dawn of the industrial age. Layers of radioactive fallout pinpoint the fire at Chernobyl in 1986 and atomic bomb tests on the Bikini Atoll in 1946. By watching for traces of methanesulfonic acid, a substance one smells at low tide, the scientists can also gauge and time the biological productivity of marine plants in the North Atlantic, which themselves might play an important role in altering climate, according to Eric Saltzman of the University of Miami, who is examining the ice in Greenland.

The scientists are interested in dates because they want to time the Earth's cooling and warming cycles.

For a glaciologist such as Richard Alley of Pennsylvania State University, the Greenland ice can be read like geological sediment. Alley can back-light a segment of core and see light and dark layers left by the summer melts and the winter storms. He believes that he can date ice accurately by counting the seasons back to about 750 years ago. After that, dating by eye becomes almost impossible.

One reason for the difficulty is that deep down, where the pressure becomes greater, ice begins to compress and flow. "It stretches and thins like taffy," Alley said. "It shears and twists and flows. The visual data gets really flakey and shakey the further back you go." The Greenland ice sheet flows toward the sea like a ponderous river. Indeed, the drilling site itself may be moving toward the sea at 10 feet a year. This motion can make it difficult to date older ice precisely. But researchers are devising ways around the problem.

Ken Taylor of the University of Nevada, for example, runs electric probes across the ice to measure acidity. When more electricity passes through the ice, it usually means the ice is more acid and that is usually the result of hydrogen sulfate being dumped by volcanoes. So with his electric probes, Taylor can see signs of eruptions. He already has detected the eruptions of dozens of known volcanoes, from Mount St. Helens in 1980 to the Laki eruption, which blew in Iceland in 1783. Eventually, Taylor hopes to find the eruption that buried Pompeii in 79 A.D.

Another investigator, Michael Ram of the State University of New York in Buffalo, is attempting to date the ice by the dust trapped in it. Using a laser beam to scatter light through melted ice, Ram said he can spot the seasonal increases in a certain type of grit known to blow off the Gobi Desert during spring and summer storms. Though his technique is unproven, Ram believes he and his colleagues will be able to date the ice by Gobi dust back as far as 10,000 years.

Researchers know the planet moves through a cycle of ice ages and warmer interglacial periods based on the timing of the slow wobble in Earth's axis. But they also know that increases and decreases in carbon dioxide and other greenhouse gases accompany climate change. Are these gases causing the temperature to rise or are they simply a byproduct? Will the current burning of fossil fuels and forests put enough extra carbon dioxide into the atmosphere to significantly alter climate? "That's the million-dollar question," Alley said.

Preliminary evidence from ice cores retrieved by the Soviets at Vostok station in Antarctica, the coldest spot on Earth, suggests increases in carbon dioxide preceded warming. More than a mile of Vostok ice examined by Todd Sowers and Michael Bender of the University of Rhode Island suggest that carbon dioxide levels rose 3,000 to 7,000 years before continental ice began melting at the close of the great ice age that ended 140,000 years ago, presumably as a result of warming temperatures. A similar finding accompanied the end of the last ice age some 18,000 years ago.

"It is a question of thresholds," Sowers said. "Is the carbon dioxide driving the system or responding? It may be that things happen in different ways depending on whether or not the Earth is moving into or out of glacial periods."

At present, the uncertainities loom. In addition to the apparent lag between a rise in carbon dioxide and warming, there are signs that climate may resist change for many years and then suddenly jump to a warmer phase. Perhaps, some scientists argue, such a sudden surprise is in store -- perhaps sooner, perhaps much later -- as a result of humanity's currently growing output of greenhouse gases.

"Ice is an amazing scientific medium," said Alley of Penn State. "If we were just clever enough to understand what it is telling us."