Part of the Climate Change exhibition.

Permafrost—a soil layer that's frozen not just in the winter, but year-round—underlies an amazing 25 percent of the Northern Hemisphere. Much is near the Arctic Circle (see map), in regions already strongly affected by rising temperatures. In many of those places, permafrost is melting.

Why should we care about something happening under the ground, far from where most of us live? Because it presents a global risk whose exact dimensions are uncertain. Permafrost holds enormous amounts of carbon, so its melting could release huge quantities into the atmosphere as CO2 or methane, increasing atmospheric warming everywhere.

A Vast Expanse

Permafrost—technically, soil that's been frozen for at least two years straight—encircles much of the Arctic. It is often tens to hundreds of meters thick, but in some places may be much thicker.

"Drunken" trees

Trees growing at crazy angles are one sign of changes in the Arctic. As the underlying permafrost melts, the unfrozen soil above—where trees have their roots—buckles and sinks, causing the trees to topple. Without the permafrost seal, water drains quickly through the soil, drying it out in some places.

Twisting Tracks

This newly built railroad in western China—the highest in the world—crosses long stretches of a type of permafrost whose average temperature is only 1–2°C below freezing (28–30°F). While climate change was factored into construction plans, permafrost has melted much faster than projected, leaving engineers scrambling to reinforce the tracks to prevent buckling. Some permafrost layers in the region have thinned 5 to 7 meters (16 to 23 feet) in only 20 years.

Competing Impacts

Arctic plants are feeling the impact of climate change in two competing ways. On the one hand, warmer temperatures and higher CO2 levels have produced a greening effect, as the range of some vegetation expands. These photos of the same Alaskan location, one (left) taken in July 1950 and the other in August 2002, demonstrate the effect. On the other hand, those warmer temperatures also melt the underground permafrost "seal," causing surface water to drain away and the soil to dry. This produces drought stress in other plants.