practically no ice at all; the defining characteristic of permafrost is the temperature
of the soil material rather than the amount of water. Depending on local climates,
permafrost can occur in continuous, discontinuous, and sporadic concentrations. In
North America, some small pockets of permafrost occur as far south as the moun-
tains of New Mexico. Permafrost may be quite deep with record depths approaching
1,500 m in northern Siberia. Of great importance is that the large majority of the area
underlain by permafrost undergoes thaw in the surface materials during the cool
summers. The top of the permafrost is, therefore, known as the active layer and is
usually thawed to depths ranging between a half-meter to 4 m. This thawing allows
the existence of plant life, but the plant life is limited because of the short duration
of summer, the poor drainage of moisture out of the active layer, and the rock-hard
barrier of still-frozen permafrost inhibiting rooting underneath the active layer.
The resulting biome is tundra and human use of the landscape is severely
restricted. The activities of humans must take into account the permafrost’s sea-
sonal transformation from a frozen surface to a series of mushy bogs. Telephone
poles, road surfaces, and house foundations can be ruined. Witness the extraordi-
nary engineering measures of the Trans-Alaskan Pipeline stretching 1,300 km
southward from the Arctic Ocean. The pipeline is carried on a series of supports76 Cryosphere
The area covered by ice and snow is declining on our warming Earth. The Mendanhall Glacier near
Juneau, Alaska, has retreated 4 km since 1500. (iStockPhoto)