Current and future environmental change - Fundamentals of the Physical Environment - page 709

Plate 28.6 The Rhône Glacier, source of the river Rhône, in south-west Switzerland during 1978. The glacier reached the valley
foot in the foreground in 1860, at the end of the Little Ice Age, had retreated 5 km to the vegetation-free trimline position by the
1950s and is now out of sight.
Photo: Ken Addison

The greatest mass losses recorded are in Alaska followed
by the US/Canadian Rocky Mountain and Pacific coast
cordillera, Patagonia, the Himalayas and European Alps.
Permafrost degradation completes the geomorphic
impact of climate change on the cryosphere. Seasonally
frozen ground is the single most extensive surface
condition in the northern hemisphere, covering 48 M km 2
or 51 per cent of the land area, and almost 23 M km 2
(22 per cent) is underlain by permafrost. Warming since
the mid-twentieth century is accelerating the spatial loss
and thickness of frozen ground and parallels earlier break-
up and later freeze-over of river and lake ice in the same
essentially Arctic regions. There is considerable variation
in our knowledge of rates of degradation, partly because
extensive permafrost monitoring developed only recently,
but some common trends are already evident. The
boundaries of continuous qdiscontinuous qsporadic

zonation of permafrost (see Chapter 15, p. 372) are
inexorably retreating polewards, or upwards in the case
of mountain permafrost in north America and central
Asia ( Figure 28.7 ). Previous estimates that 16 per cent,
or 4 M km 2 , will be lost by 2050 with just 2 C regional

warming are being revised upwards. Downwards heat
transfer from the warmer atmosphere raises subsurface ice
temperature before melting occurs. Observed increases at
the top of the permafrost layer have risen by 0-3

C since
the 1980s, extending the depth of the seasonal active
layer by 0.3 m. Permafrost degradation will produce
major changes in land surface, drainage and vegetation
systems, with profound implications for the biosphere and
further atmospheric warming (see p. 720). Its geomorphic
significance is not in the mass of meltwater but in ground
instability caused by rapid extension of the active layer and
potential future collapse of the permafrost system.