The Frozen Underground
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In northernmost America and Eurasia, the frigid specter of the last Great Ice Age lurks underground. These are the lands of the permafrost. Hi, I'm Dave Thurlow. This is The Weather Notebook.

Over hundreds of millennia ago, while the great ice sheets covered Northern Hemisphere, the surface beneath could not rise above freezing. The cold seeped ever deeper into the earth until its frigid advance was finally stopped by the outward flow of heat from the Earth's interior. In this manner, thick layers of permanently frozen ground known as permafrost were formed.

Today, permafrost covers 24 percent of the Earth's landmass today. Most Northern Hemisphere permafrost lies under northern Asia and North America. Eighty-five percent of Alaska and half of Canada have permafrost underground. A layer even underlies the summit cone of our very own Mount Washington!

In the coldest regions, permafrost's surface never thaws, but where the sun provides enough heat to warm the surface, the topmost layer can thaw down a few feet during the short Arctic summer.

And that's a clue to the current threat to this subterranean ice. Warming. Global warming. Observations indicate that the Arctic permafrost has been melting over the past several decades due to the warming.

Normally, permafrost is hard as concrete. But when permafrost melts, the resulting silty mixture resembles thick soup! Oil pipelines that swing through Alaska and the Yukon rest on permafrost. Extensive melting could destabilize the pipeline's footing, and cause major oil flow disruptions and possible environmental disaster for the fragile arctic ecosystems.

Thanks today to contributing writer Keith Heidorn of Victoria, British Columbia. The Weather Notebook is a production of the Mount Washington Observatory. It is supported by a grant from the National Science Foundation.