Churchill, Manitoba, perches on the northwest shore of Hudson Bay, Canada where the rail line ends, where the trees end, and where many birds' migrations end. Beyond Churchill is nothing but north: ice, tundra, and scattered Inuit settlements.

In this remote place, ecologist and Earthwatch scientist Dr. Peter Scott has spent more than 23 years studying the environment, from hydrology to biology to geology to ornithology to botany. His goal is to understand this environmental system that sits at the confluence of several major ecological zones in Canada and lies between the major climatic zones, as well. Here, too, migration paths of birds converge, and the major indigenous cultural groups of Canada overlap. Despite the apparent barrenness of the place, Peter Scott and other scientists have found more than 600 species of plants in the immediate vicinity, three times the number expected at that latitude. Species diversity includes polar bears and wood frogs, as well as birds from every corner of the hemisphere, gathered in their millions to feast on even greater numbers of insects.

Perhaps most significant of all: Churchill sits on a global warming time bomb.

To understand the threat, we must first briefly review the Greenhouse Effect, which keeps our planet warm enough to support life as we know it. Like a greenhouse, light from the sun passes through the atmosphere and reaches Earth as short-wavelength visible and ultraviolet light. Soil, plants, rocks, and concrete absorb that light and re-radiate it as longer wavelengths of infrared light, or heat. These longer wavelengths can't easily pass through the atmosphere, and their heat is trapped, keeping the planet warm. The primary gases in the atmosphere that trap this heat include carbon dioxide and methane, with carbon dioxide being far more common.

If it were only a matter of gases and sunlight, the heat would build and build as more light was trapped, and the planet would soon be like Venus. Fortunately for us, green plants absorb the heat-trapping carbon dioxide as part of photosynthesis. Consequently, when greenhouse gases increase and warm the weather, plants grow more abundantly, thereby absorbing more carbon dioxide. That reduction in carbon dioxide reduces the greenhouse effect, and the planet cools, reducing the plant abundance, which increases atmospheric carbon dioxide and increases the temperature, and the cycle starts again. As long as the amount of plants and the amount of carbon dioxide are not tampered with, the system regulates itself, and we enjoy a relatively stable climate.

That stability is staggering. Some blame human-operated industry and cars producing enormous amounts of carbon dioxide while we concurrently reduce the planet's carbon-absorption capacity by cutting forests. However, a Yugoslavian scientist named Milutin Milankovitch -- an expert on Earth's changing elliptical orbit, changing angle of tilt and wobbling along its axis -- predicts a strong 500-year warming period beginning now. That would happen without any human contribution.

Most scientists agree that both human and natural causes are present; the debate focuses on which is the driving force. Some estimates credit humans with production of as much as 75 percent of the current carbon dioxide buildup; others say as little as 25 percent. Regardless, the majority of scientists agree that the Earth as a whole is rapidly getting warmer and that atmospheric carbon is increasing at dramatic rates.

Churchill's time bomb lies buried within one of the largest accumulations of carbon on the planet. The carbon resides in the peat that runs in a strip a few hundred kilometers wide around the globe-a huge swath of habitat that covers 11 percent of the Earth's land surface. Peat, halfway between soil and vegetation, is formed when tundra plants (including the trees that grow in the forest tundra zone) die. Unlike the tropics - where dead plants break down quickly so that the carbon and nutrients are recycled immediately - northern breakdown happens slowly, allowing the carbon to be held in the soil; at the cold edge of the tundra, breakdown all but stops. A fallen tree that lies on the ground for 200 years with hardly any decomposition eventually is incorporated into the ground as peat. Virtually all of the carbon that that plant absorbed in its lifetime remains intact. Because the peat itself doesn't break down, but simply piles up, this peat strip contains up to one-third of all the carbon stored on the surface of the Earth and is equivalent to 60 percent of the carbon held in the atmosphere, according to Scott. If that peat carbon were released into the atmosphere, it would dramatically increase the amount of greenhouse gas and thus increase the temperature of the planet.

The news gets worse. These peat bogs are water-logged, and the peat layer has very little oxygen, so if the carbon is released, much of it won't be as carbon dioxide (CO2), but rather as methane (CH4), which is, on average, 12 times as potent a greenhouse gas as carbon dioxide. Potentially, Scott says, this peat layer could increase the atmosphere's warming capacity by a factor of seven.

The carbon in this peat has stayed safely frozen a meter below the surface, but as Earth warms, the permafrost could thaw, and warm peat may release its carbon. However, the resulting warming trend could stimulate more plant growth in the tundra and increase local carbon-absorption capacity. Or the peat could release deadly methane. No one knows.

Scott and his team of Earthwatch volunteers hope to form hypotheses based on what is currently happening. On six different sites, the volunteers measure the amount of carbon currently being stored and released, both above ground and in the peat, thereby setting up a baseline against which future changes can be compared . Over the years, as carbon measurements are taken from the same sites, each of which represents a different habitat or circumstance, Scott will discover whether the peat is giving up its carbon or whether tundra plants are absorbing it.

Most global warming warnings sound dire with vanishing polar ice and islands, shortages of water and food, and extreme destruction from violent storms. However, Scott doesn't believe that global warming has to be disastrous. "Change isn't good or bad; it's just change. The question is, can we adapt to it?" And that is where his research is so important. We may not be able to avoid the coming change, but if we can see it coming, if Peter Scott can give us that early warning, perhaps we will have time to adapt.

Want to help? You, too, can be an Earthwatch volunteer. Check it out at: http://www.earthwatch.org