Geography Reference
In-Depth Information
glacier. These tongue-shaped
rock glaciers
have a steep front and
usually descend from cirques that they have created through
downslope movement of the angular debris. Ice within the pore
spaces assists the flow processes.
While there are no perfectly symmetrical valleys, periglacial
valleys can often show a distinct asymmetry. Areas which are no
longer periglacial often have asymmetric valleys that are relicts from
former periglacial times. The asymmetry can be caused by south-
facing slopes having longer exposure to the Sun's energy in the
northern hemisphere and this promotes prolonged thawing, more
frequent freeze-thaw, as there are more days and nights with
freeze-thaw conditions, more melting and more rapid mass move-
ments. Therefore south-facing slopes in the northern hemisphere
and north-facing slopes in the southern hemisphere will experience
a quicker reduction in slope angle while the deposited material at
the bottom pushes the stream toward the opposite facing slope
undercutting it and keeping it steep.
Impacts of recent climate change
Evidence gathered by the IPCC has shown that ice masses and per-
mafrost coverage is declining rapidly. Satellite information collected
since 1978 shows that Arctic sea-ice extent has decreased at a rate
of 2.7 per cent per decade, with larger decreases in summer of 7.4
per cent per decade. There are even predictions that the Arctic
might be free of sea-ice in the summers at the North Pole within
the next few decades. Mountain glaciers and snow cover on
average (but not everywhere) have declined in both hemispheres.
The maximum area of permafrost has decreased by about 7 per
cent since 1900 while the temperatures at the top of the permafrost
layer have generally increased since the 1980s in the Arctic by up
to 3°C. While permafrost melt produces more thermokarst land-
scapes and adds to problems of ground subsidence and infrastruc-
ture damage, melting permafrost may also release more stored
methane and carbon dioxide into the atmosphere as the soil organic
matter is warmed and can be decomposed. On the other hand this
may be balanced by faster plant growth that could take up addi-
tional carbon dioxide. The full impacts of permafrost melt due to
climate change are, as yet, uncertain.
