Permafrost, or perennially frozen ground, is one of the things that defines the Canadian North. It is the ground on which all plants and animals must survive. It also determines how people must live in the North, as we adapt our buildings and roadways to handle this frozen ground.
Permafrost occurs when the ground remains at or below a temperature of 0°C (32°F) for a minimum period of 2 years. The soil above permafrost that freezes and thaws every year is called the active layer. Permafrost occurs not only at high latitudes but also at high elevations.
In central Yukon, 50 to 90 percent of the ground is underlain by permafrost, which has significant consequences on the local ecology. One of the most widespread examples is the dominance of Black Spruce, which has a rooting system that can tolerate the frozen ground.
Around Dawson, permafrost is found in valley bottoms where cold air pools in winter, and on north-facing hillsides, but not on south-facing slopes where the suns warmth keeps the ground thawed. In some places the ground contains more ice than soil, sometimes in the form of massive ice lenses or ice wedges.
On Hunker Creek in the nearby goldfields, miners recently exposed a swath of frozen mud and ice. This was the oldest ice ever uncovered in North America.
More than half of Dawson City is built on permafrost that is about 60 metres thick and many thousands of years old. It extends from Church Street to past the ferry landing at the north end of town.
The permafrost that underlies Dawson is considered warm, with temperatures ranging from -0.1°C to -2.0°C. This makes it vulnerable to thawing if there is any increase in surface temperature. Dawson's permafrost is still largely intact, but it's at risk.
Global temperatures are expected to increase throughout the 21st century. In the North, this warming will be even greater than the global average. The average annual temperature in Dawson is expected to rise by 2°C to 3°C by 2060.
Permafrost in Dawson is particularly ice-rich. When the ice melts the ground loses volume, and the surface drops. Buildings tilt and lean as their foundations sink into the ground; roads buckle and water pipes snap.
In Dawson, steps are taken in new construction to ensure that the ground will remain frozen. Buildings sit on wooden cribbing so heat will not be transferred into the ground. Front Street is paved with a light-coloured asphalt to minimize solar absorption.
Sensors have been installed in and around Dawson that measure the ground temperature at half-meter intervals. The recorded data will track changes in ground temperature, and provide warnings if the permafrost is in danger of thawing.
In order to log this earthcache, please submit the following to me:
1) Does the nearby Yukon River have permafrost beneath it? Why or why not?
2) How old is the ice that was uncovered on Hunter Creek?
3a) Find evidence around town where permafrost has affected buildings, roads or pipes. Feel free to post pictures of this evidence and/or provide a description of your observations.
3b) Take a measurement or provide an estimate of the amount of lean or buckle from any of the locations you found in question 3a.