Ground Thaw

Geographer Christopher Burn explains why permafrost is thawing

Few of us will ever lay foot on the frozen soil condition known as permafrost, which covers one quarter of the world's landmass, yet it promises to effect us all as climate change sends it into retreat. The rate and magnitude of its thawing is hard to predict - as is its toll on Northern communities and the global carbon budget. One scenario predicts that the massive amounts of carbon frozen within its depths will decompose and enter the atmosphere as it thaws.

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Christopher Burn, a geographer at Carleton University in Ottawa, has been keeping a keen eye on soil temperatures throughout Canada's Yukon and western Arctic since 1983. To that end, he ventures to the far North 3-4 times each year to monitor permafrost's demise across some odd 50 sites. His fieldwork provides detailed documentation of the environmental and ecological changes that ensue climate change in northern soils, information he hopes will help northern communities adapt to the changes underfoot.

What's at stake as permafrost thaws?
There are two principal impacts of thawing permafrost. Locally, when ice in the ground melts, the soil loses its strength, and infrastructure built on the surface may settle into the ground, creating, for example, the undulating road surfaces so common in the north.

Globally, a catastrophic consequence of permafrost thawing may be the decomposition and release to the atmosphere of the vast quantities of carbon now stored as frozen peat. Considerations such as this are part of the reason some scientists have warned that we may be close to a climatic "tipping point."

You've been keeping tabs on the ground temperatures in northwest Canada and maintaining a long record of active-layer development in the tundra. What have you found?
The Mackenzie delta area is the most rapidly warming part of northwest North America, or has been over the last 35 years. Air temperatures have risen by over 2.5°C since 1970. In the outer Mackenzie delta it appears that ground temperatures have risen by 1.5 to 2°C over the same period—and the warming extends to over 50 meters in depth.

So what does that mean?
With the warming of ground temperatures, the surface layer of ground above permafrost that is thawed in the summer time thickens. We call this layer the active layer. It's the zone where plants have their roots and take their nutrients.

In the last five years in the western Arctic we've seen a considerable proliferation of the shrub vegetation as the active layer has increased in depth. These bushes trap more snow in the winter, and that in turn keeps the ground warmer. As the ground becomes warmer, the active layer becomes deeper, which means that the bushes can become taller.

There's a positive feedback loop that is leading to the changing of the nature of the surface of the ground. So there is much more subsidence occurring now than I would have anticipated a few years ago, because the deepening of the active layer is leading to melting of near-surface ground ice.

Does this all stem from climate change then?
Nobody will stick their neck out and say with 100 percent certainty it's this or that.

In the mid-70s we began to develop computer models that suggested that as greenhouse gas concentrations in the atmosphere increased there would be consequences for the behavior of the climate. And we're now seeing those predictions reach maturity and become evident.


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