The global climate appears to be warming, but the consequences of that change aren't always simple to predict. Permafrost in the Yukon is a case in point.

Whether and how Yukon permafrost is affected by a general warming might have a lot to do with local geography, says Chris Burn, who has been studying permafrost near Mayo and in the Takhini Valley since 1982.

The Carleton University geographer points out that, in some ways, Yukon permafrost is an oddity. There are substantial areas of permafrost in south and central Yukon. However, permafrost is not found in northwest Europe at similar latitudes.

The difference is due to the mountains. The St. Elias Mountains and Coast Mountains tower above northern European mountains. Large areas of the mountains that separate most of the Yukon from the Pacific Ocean reach heights of more than 3500 metres above sea level. In contrast, the summits of the spectacular mountains of Norway, also rising from the ocean, are generally less than 2000 metres above sea level.

Because of their height, the St. Elias and Coast mountains cast a huge rain shadow, limiting the amount of precipitation and warm maritime weather that reaches their eastern slopes. And precipitation, particularly snow, is an important factor in the development of permafrost.

Snow is an excellent insulator, Burn says. A thick layer of snow slows the ground's cooling as the air temperature drops. A thin layer of snow allows the ground temperature to drop more quickly in the fall, and to stay cool.

The climate record in Mayo over the last few decades shows how strongly ground temperature is linked to snow levels.

The year 1992, says Burn, marked 20 years of ground warming in the Mayo area. The top 15 metres of ground had warmed roughly from minus 3 degrees to minus 1.5 degrees over that period.

That amount of warming was not enough to melt the permafrost, which is up to 40 metres thick near Mayo, but it came fairly close. Ice in soil begins to melt at about minus 1 degree or a bit cooler, Burn says.

However, the warming trend didn't continue. Since 1992 the ground temperature near Mayo has dropped.

"It's cooled down by half a degree over the last seven years," says Burn.

He links the fluctuation in ground temperature to weather patterns during the same period. In the late 1980s, Mayo experienced a series of warm summers and snowy winters. In the winters of 1990-91 and 1991-92, there were up to 60 centimetres of snow on the ground.

Since 1992, there has been far less snow around Mayo. In the winter of 1995-96, for example, the snow depth only reached 30 centimetres at Mayo Airport.

The low snow levels coincided with cold winters. On December 5, 1995, Mayo was the coldest place on the planet, with a recorded low of -54.1 degrees Celsius, Burns recalls.

"Short-term climate fluctuation has a strong impact on ground temperature and permafrost," he says. "It's hard to predict long-term change because our thin permafrost is sensitive to short-term fluctuations."

Comparing the ground temperature fluctuations at Mayo to a research site in the Takhini Valley makes the problem clearer. The Takhini Valley hasn't had the strong shifts in snow cover and temperature that Mayo has experienced in the last couple of decades.

"In the Takhini Valley, permafrost temperatures at undisturbed sites in the forest haven't changed in the past 16 years," Burn says.

Burn's findings do not imply that global climate change won't affect permafrost. They simply mean that predicting the impact of climate change on Yukon permafrost is far from simple.

"We might see a general warming trend," he says, "but several cold or low-snow years might have a greater impact on permafrost than the general trend, because it is easier to take heat out of the ground when frozen than pump it in when thawed."

"Trying to predict for Yukon what will happen in the long term really is a speculative exercise, because the climate (not the weather) forecasts for our region have been made without acknowledging sufficiently the influence of the mountains."

For more information about permafrost and climate change, contact Chris Burn at the Department of Geography, Carleton University, Ottawa, Ontario, K1S 5B6. For information about global warming, contact Environment Canada, Whitehorse.