PINE FALLS, Manitoba -- Here on the edge of the silent and frozen northern tier of the Earth, the fate of the world's climate is buried beneath the snow and locked in the still limbs of aspen trees.

Nearly half of the carbon that exists on land is contained in the sweeping boreal forests, which gird the Earth in the northern reaches of Canada, Alaska, Scandinavia and Russia. Scientists now fear that the steady rise in the temperature of the atmosphere and the increasing human activity in those lands are releasing that carbon, a process that could trigger a vicious cycle of even more warming.

The prospect of the land itself accelerating climate change staggers scientists, as well as woodsmen such as Bob Austman, who stopped recently in a quiet stand of birch on the edge of the boreal forest to examine a jack rabbit's tracks.

"There are big forces out there," he said succinctly.

Those forces, which scientists are only starting to understand, could free vast stores of carbon and methane that have been collecting since the last ice age in the frozen tundra and northern forests. Their release would push the world's climate toward a heat spiral that would raise ocean levels, spawn fierce storms and scorch farmlands, scientists believe.

But the land is impartial. It could also be enlisted to help abate global warming, as both a storehouse for man-made carbon dioxide and a natural sponge for greenhouse gases. Policymakers are considering changes to protect and expand the forested areas that store carbon; outside the boreal forest, they are experimenting with techniques to bury man-made carbon dioxide in underground vaults and porous seams.

"The world is both victim of climate change and a possible solution to it," said Stewart Elgie, associate director of the Institute of the Environment at the University of Ottawa.

Carbon is freed from the land in numerous ways. Permafrost melting because of warmer weather exposes peat, deadwood and buried pine needles to decay, freeing the carbon they contain. Fires, raging through forests more often because of hotter and drier weather, send wood -- and its carbon -- up in smoke. Insects thriving in milder winters girdle trees and send them to rot on the forest floor. Miners and oilmen build roads that expose the earth and warm the land, and loggers cut down old forests and replace them with young ones that will take decades of growth to absorb and store the same amount of carbon.

As the released carbon rises, it adds to the belt of greenhouse gases in the atmosphere, trapping even more heat, which causes more warming. Scientists call it a "feedback loop." Others have a more ominous term: the carbon time bomb.

Risk Poorly Understood"We are taking risks with a system we don't understand that is absolutely loaded with carbon," said Steven Kallick, a Seattle-based expert on the boreal forests for the Pew Charitable Trusts. "The impact could be enormous." Scientists acknowledge they are not certain how the carbon time bomb will explode, or when. Many of the consequences of global warming that experts once predicted would take centuries are occurring in decades, such as the melting of the world's glaciers and ice caps. But other changes might be more gradual.

"With permafrost, it may take longer for change to get moving. But it may keep moving, even if we get our emissions under control," said Antoni Lewkowicz, a professor of geography at the University of Ottawa. "It's like a big boulder. Once you get it moving, it won't stop."

Brian Amiro, head of the department of soil science at the University of Manitoba, is part of a research team involved in a project called Fluxnet. The researchers have erected more than 400 towers throughout the world, outfitted with instruments to measure the exchange of carbon between earth and air. The boreal forest, sometimes called "the lungs of the world," breathes in more carbon in years when the forests grow, and loses more carbon in years of bad forest fires, drought or insect infestation.

Lately, there has been a string of bad years. The number of forest fires in Canada doubled in the 1980s and '90s from the previous two decades, and some scientific models indicate they will double again this century, Amiro said. Logging, mining and oil exploration have carved roads deeper into the forests. Temperatures have risen faster toward the north -- by as much as five degrees since the 1950s -- than in more temperate zones.

"The environmental triggers are going to become much more significant," said Faisal Moola, director of science at the David Suzuki Foundation, a Vancouver-based environmental organization.

There are mixed views about whether the process can be stopped. The levels of carbon dioxide in the atmosphere -- the highest in at least 420,000 years -- mean average temperatures will continue to rise, accelerating the thawing.

But humanity's footprint could be changed. Development, mining and logging account for 25 percent of the carbon loss in forests, Elgie said. Logging releases almost twice as much carbon dioxide each year as all the passenger vehicles in Canada, he said.

Credits for PreservationHere in Pine Falls, a town of 1,400 about 80 miles northeast of Winnipeg, the giant Tembec pulp mill billows steam and smoke into the crystalline sky. The 1920s-era mill makes newsprint from spruce and pine trees, and Vince Keenan, a forester for the company, said Tembec has responded to calls for change. It has set aside 12 percent of its 2 million-acre logging forest here, and up to one-fourth of its product is now made from recycled paper. Changes in mill practices have reduced greenhouse gas emissions by nearly 50 percent since 1990, he said.

But a broader step would be to set aside vast areas of the forest now designated for mining or logging and preserve them. This could be done by setting up a system of "carbon credits," in which, for example, an industrial plant would offset its pollution by paying money to preserve land in the forest that could store an equal amount of carbon.

"Right now, the only way to make money in the boreal forest is to cut trees down," Elgie said. "If you had carbon credits, you would be able to make money by keeping the trees up and storing carbon."

That system appeals to some native Indian groups, now torn between the desire to keep their traditional lands and the need for income from logging or mining.

"Preserving the land is important to us," said Carl Smith, an elder of the Brokenhead Ojibway First Nation on the Winnipeg River near Pine Falls. "Once the land is gone, you're gone."

Smith also is president of the Manitoba Model Forest, a group set up 15 years ago to balance the competing views of how the forest here should be used. One of its goals is teaching schoolchildren about the forest, a job that falls to Bob Austman, the woodsman, whose family has lived in and on the boreal forests of Manitoba for three generations.

He sees nothing but beauty here. As he and Brian Kotak, an environmental scientist, tramped in minus-10-degree cold through a stand of birch near the Winnipeg River recently, it seemed hard to see the Earth as a potential danger.

"The dilemma," Austman said, "is that we live on a planet with 6 billion people. This land is under increasing pressure."

Turning a Minus Into a PlusSouth of the great swath of forest in central Canada, the wrinkles of the land smooth out, stretching toward a straight horizon. The Great Plains are frozen and still in winter. But in Weyburn, 70 miles southeast of Saskatchewan's capital, Regina, pumps bob relentlessly amid the snowy wheat fields, sucking crude oil from a mile underground like a host of mechanical mosquitoes.

What goes back into the ground here heartens some environmentalists. The giant EnCana oil and gas company, which operates more than 700 oil pumps in this field, pumps carbon dioxide deep down to drive more oil out of the porous rocks.

Almost inadvertently, the company has become the world's largest working example of carbon storage, or sequestration, a technique being hailed by international experts as one tool to reduce greenhouse gases. Darcy Cretin, operations superintendent at the EnCana plant, is slightly amused by the environmental scientists who have flocked here to see the maze of pipes, pumps, valves and sensors planted in the prairie.

"We have to keep explaining we are doing this to make more oil," he said. "The carbon sequestration is an extra."

When the oil brought up at Weyburn dwindled after 40 years of pumping, EnCana struck a deal with the Dakota Gasification Co. It owns a plant in Beulah, N.D., that converts coal to natural gas. Combustion at the gasification plant makes carbon dioxide, which was being vented into the air. EnCana offered to buy the gas, and in 1999 the U.S. company built a 200-mile pipeline into Canada. The foot-wide pipe emerges from its underground route at a chain-link fence on the edge of EnCana's property.

The company pumps the carbon dioxide under high pressure into the oil field. The gas acts as a kind of solvent, driving the oil out of porous rock. The greenhouse gas remains underground, leaving buried nearly 5,000 tons a day that would otherwise have gone into the atmosphere.

Experts believe this scheme of carbon storage could be used more widely in cases where the gas could be easily collected at a single point and moved by pipeline to a storage field. The approach would not work where the carbon dioxide could not be collected easily, such as from the tailpipes of moving cars. But nearly 40 percent of the carbon dioxide released to the air comes from big power plants or industrial areas, where the gas could be captured.

A committee of more than 100 experts from the Intergovernmental Panel on Climate Change concluded in 2005 that carbon sequestration has "considerable potential" to help reduce greenhouse gases, and a lengthy study at Weyburn by the International Energy Agency found virtually no leakage. The British Columbia government this month announced that all its coal-fired electric plants will be required to utilize carbon sequestration to eliminate greenhouse gas emissions.

For oilmen such as Cretin, the prospect of helping reduce a greenhouse gas by pumping it underground seems a natural fit.

"This is pretty easy," he said. "It's basic stuff for us."