|Résumé||(disponible en anglais seulement)|
With climate warming, changing vegetation, and recurrent forest fires, permafrost in many arctic regions has been warming or thawing, and these trends are
likely to continue. The question is: what impacts result on the natural and built environment around Yellowknife? This summer, a team of UK and Canadian scientists has been investigating the frozen foundations of the Northwest Territories.
Permafrost is ground that remains at or below 0 C for two years or more. A dynamic expression of climate, permafrost ranges in thickness from a few centimetres to about 1,500 metres, and in temperature from 0 C to about ?20 C. Permafrost is important
to Canada's environment and economy, as about half of the country is located in permafrost regions. In Yellowknife, where annual air temperature is about ?4.6 C, and winter temperatures frequently drop to below ?40 C, permafrost is a part of life.
The climate is cold enough to maintain permafrost beneath many areas of spruce and hardwood forest and peatlands, but permafrost tends to be absent beneath bedrock outcrops, fens and bogs. As a result, permafrost in and around Yellowknife has a
patchy and variable distribution, with abrupt changes from frozen to unfrozen ground. Some impacts of permafrost thaw are well understood. For example, melting of ice within permafrost often leads to gradual settling of the ground surface and
formation of bumps and ruts in roads. Near Yellowknife, there is a lot of ice in silty sediments deposited by a vast Ice-Age lake known as glacial Lake McConnell. The lake sediments filled in hollows and basins in the bedrock, and after Lake
McConnell drained, creating the present-day Great Bear, Great Slave and Athabasca lakes, permafrost developed in the sediments and made them ice-rich. If this ice melts, the ground surface will settle, which can happen if the top of permafrost warms
because the vegetation is disturbed or removed, or the surface darkened with tarmac. Because of local permafrost thaw, many stretches along Yellowknife roads have had to be repeatedly re-graded to fill in low spots, and really problematic sections
have been realigned to make best use of the ice-free outcrops of bedrock. In addition to ice, permafrost in the northern hemisphere contains a lot of carbon, about twice as much as the atmosphere. However, unlike icy permafrost, the impacts of
thawing carbon-rich permafrost are poorly understood, particularly in terms of carbon stocks, transfers and links to climate. On the one hand, if the permafrost thaws, the frozen carbon may start to decompose and produce more greenhouse gases (carbon
dioxide and methane), which may accelerate climate warming. Conversely, vegetation generally absorbs carbon from the atmosphere, and a warming Arctic is predicted to support more plant life, which could offset rising carbon emissions. So what will
actually happen to greenhouse gas transfers as permafrost warms and vegetation changes?