When I was a girl of 5, my father left me alone on the summit of a mountain in the Alps while he fetched my mother and infant brother from the trailhead below. All around me was ice, snow and sky. The world vibrated with light. Ever since, I have taken comfort in the cold places on earth and embraced their bright and solid beauty. Now it troubles me to think that some of those cold regions are warming, ice frozen underground is melting and the stark glory of whole ecosystems is changing. I am talking of those subarctic and arctic regions that are based on permafrost — permanently frozen ground — that supports forests of spruce and birch and tundras of feathery moss, the stomping places of moose and caribou.

To see the melting for myself, I travel to Fairbanks, Alaska, and from there north to Prudhoe Bay, where two cold-adapted scientists have been probing the permafrost. Tom Osterkamp, a geophysicist at the University of Alaska, Fairbanks, began the studies in 1977, drilling 60-meter holes into the frozen ground and measuring the temperature at one-meter intervals. He was joined a few years ago by Russian-born scientist Vladimir Romanovsky, with whom I drive up from Fairbanks.

We stop half a dozen times to take the permafrost's vital signs, trekking out across the scarlet-and-gold tundra, armed with probe, computer and shotgun (in case of wayward grizzlies). When we reach the array of pipes and flags that mark an automated field station, Romanovsky checks the site's small solar panel and downloads four seasons' worth of air, surface and permafrost temperatures. Then we begin a paced walk. After every eight strides, Romanovsky thrusts a metal probe through the soft surface peat until it strikes impenetrable permafrost.

After years of doing this at some 30 sites around the state, Osterkamp and Romanovsky are discovering patterns of change. Between the late 1980s and 1998, the mean annual temperature of the active layer and the top of the permafrost rose as much as 6.3 degrees Fahrenheit north of the Yukon River. South of the Yukon, the permafrost warmed between 1 and 2.7 degrees F. Here the permafrost is discontinuous and already warm — usually within one or two degrees of melting, so just a slight rise in temperature can tip the balance. It is melting at the rate of about a meter a decade. Farther north, the permafrost is colder to begin with, requiring more heat to initiate a thaw.

The data raise haunting questions. If warming continues, how will melting permafrost transform landscapes? Will forest grow in place of tundra? Will wetlands rise where forests once grew? And will these changes affect the carbon balance, as the now-unfrozen soil emits greater quantities of carbon dioxide (CO2), a greenhouse gas that traps heat in the atmosphere?

Osterkamp believes answers will be most readily found in Alaska's interior, from the banks of the Tanana River to the flanks of Mount McKinley, where we visit one of his favorite sites. The scent of Arctic sage suffuses the air as we navigate high clumps of grass and pools of water. In less than 15 years, this once-level meadow has become a mosaic of ponds and pits and water-bound plants. The new topography, called thermokarst, got its name from karst terrain first described in Poland, where weathered limestone is pocked with sinkholes, pits and troughs. Thermokarst is karst produced by heat. While North America's highest peak rises as regally as ever, ground around its base is sinking.

We jump across a trough that formed when underground ice melted. It provides a clue to the processes beneath our feet. Within permafrost is ice: sometimes countless crystals in the pores between soil particles; sometimes huge interlocked wedges that began life in small surface cracks; sometimes lenses the size of a skating rink. The melting of ice-rich permafrost can destroy the physical foundation of everything above: tundra and forests, houses and roads.

In Alaska, permafrost has warmed in response to air temperatures that have risen since the late 1800s, and particularly in the past 20 years. During much of the past decade, change has also come in the form of deeper snow that can insulate permafrost from cold winter air, allowing it to keep its summer glow. Climate models predict that air temperatures in the Arctic will rise as much as 9 degrees F in the next half-century, fueled in part by increasing levels of greenhouse gases.

What of other permafrost regions? In Canada, Russia, China and Mongolia, scientists also see widespread permafrost degradation. In China, the southern limits of permafrost are believed to be moving northward at about a mile a year. Vast areas — up to 25 percent of the exposed land surface on earthm — are underlain by permafrost, including isolated pockets in the Alps and Pyrenees, and on mountain slopes in southern and eastern Africa, Chile and Argentina. All this frozen ground stores huge quantities of carbon; northern ecosystems alone contain the equivalent of 60 percent of the carbon currently in the atmosphere as CO2.

Eighty-year-old black spruce, scrawny as pipe cleaners, grow on north-facing slopes near Fairbanks. In their shadowy understory, mosses and lichen thrive. Here the active layer above the permafrost is less than two feet deep. Turn over a spadeful of peat, and there is the carbon source: black soil, bits of branches, insects, roots living and dead, an organic mat you can squeeze like a sponge. Warming of this active layer would increase microbial activity and hasten the decomposition of dead organic matter, releasing its carbon. Furthermore, if warming deepens the active layer, peat that was previously permanently frozen would also be exposed to decay, releasing yet more CO2. These dynamics could create a simple cycle: increased air temperature, deeper thaw, more CO2, increased air temperature.

But nothing in nature is so simple. As tundra warms, the tree line may migrate north, and the fresh growth of forest may soak up more carbon from the atmosphere than the soil releases. In areas of discontinuous permafrost, where forests topple as the ground collapses, emerging wetlands may also sequester more carbon than the forest that preceded them. On the other hand, climatic warming is already increasing the acreage of Northern forest that burns every year, a trend likely to accelerate — and burning forest releases huge amounts of CO2. We just can't be sure how these forces will play out.

One thing we can be sure of is the critical role of the peat and moss that cover permafrost. Destroy that layer of insulation, and the ground will be as exposed to the sun as is bare skin. Yet, in our zeal to build, we often ignore the guardian role of moss and peat. All around Fairbanks, land has been cleared of forest and moss to build homes, hundreds of which are now settling at odd angles as permafrost thaws. In Kotzebue, a hospital damaged by melting permafrost was abandoned. So was a radio transmitter site near Fairbanks, and miles of roadway across interior Alaska.

It is earliest autumn, and as I drive with Tom Osterkamp on the road to Mount McKinley, he exclaims with delight whenever he sees a yellow birch. The road heaves and dips and falls away at the shoulder into newly formed pits and pools of meltwater. "Thermokarst," he points out, again and again. I have grown to recognize it, as I suspect will many others, as climate and land change.

By Bernice Wuethrich