Thousands of years ago -- perhaps as much as 30 thousand years ago -- the place that is now Whitehorse was buried under a layer of ice as much as two kilometres thick.

At the same time, the land that now forms the bottom of the Bering Sea was part of a dry, windswept plain that reached from the central Yukon all the way to central Siberia.

That dry plain was Beringia, a cool grassland that supported herds of muskoxen, caribou, bison, and woolly mammoths, along with predators like lions, wolves, saber-toothed cats, and wolverine. It was what scientists call a glacial refugium, a place where plants and animals survived through the long periods when ice took over much of North America.

Beringia was also a spectacular example of the effects of climate change -- not just varying temperatures, but the other changes that accompany shifts in temperature.

It wasn't warmer temperatures that saved Beringia from the ice, says John Storer, Yukon government palaeontologist. In fact, Beringia was not significantly warmer than surrounding areas that succumbed to ice.

To some extent, the ice itself saved Beringia. So much of the planet's supply of water was locked up in ice that the sea level dropped as much as a hundred metres, exposing the bed of the Bering Sea. Precipitation also diminished because so little water was available to evaporate into the atmosphere.

What little snowfall there was in those days was trapped by the high mountain ranges along the coast, just as it is now. Inland, in the shadow of those mountains, there simply wasn't enough snowfall to create ice to cover Beringia.

"The unglaciated area was colder and drier and generally more extreme than today," says Storer.

The ground temperature, however, was probably slightly warmer. Because there wasn't much snow cover, the dark soil could absorb more solar energy, causing the permafrost that currently underlies most of the region to retreat.

Beringia was a lusher place than the same land is today. The dry conditions resulted in large amounts of blowing dust, Storer explains, and dust blown from more productive areas fertilized the mineral soil of Beringia, resulting in nutritious plant growth that supported the large and varied animal population.

Beringia wasn't just grassland. Besides the grassy plains, there were shrubs, patches of forest, and peaty lowlands that were flooded when the ice began to melt and the water levels rose.

"It was a mosaic of environments," says Storer. " Just like today, you had a variety of habitats."

But how do we know what it was like? There are no written climate records from Beringia, and no survivors to tell the tale.

The record, says Storer, is in the fossils and in the rocks. For the past 50 thousand years or so, fossils and fragments of organic matter trapped in layers of rock, soil, and ice can provide hints about climatic conditions, as well as a general timeframe based on radiocarbon dating of the material.

Scientists look at a variety of things to piece together the puzzle of the past, Storer says. The most commonly-used fossils are plants, small mammals, and insects. Analysis of the plant and animal life that existed gives us an idea of the climatic conditions that allowed them to exist.

"In peaty deposits, you often find bits of carapaces of insects," he says. If they are closely related to modern insect species, you can look at the temperature and moisture requirements of the modern insects and deduce the kind of climate that allowed the prehistoric insects to thrive.

That's the best sort of evidence for letting modern scientists arrive at a guess about temperatures and climatic conditions of long ago. But that record only goes back a short distance into geological history.

"Beyond that, you have to get a little bit lucky," says Storer. "Once you get back more than 50 thousand years, you're not dealing with radiocarbon. You're looking at dates in rocks."

For more information about Beringia and research into the prehistoric past, visit the Yukon Beringia Interpretive Centre in Whitehorse or check out the Centre's web site at www.beringia.com.