Environmental Engineering Reference
In-Depth Information
Chapter
15
Ice at the land surface
The revolutions in glacial and atmospheric sciences are now fully integrated through the
nature of ice sheet- ocean-atmosphere interactions. Concern that atmospheric warming
may cause runaway melting of Arctic ice is tempered by the risk of at least regional
cooling by negative feedbacks on ocean thermohaline circulation. With popular attention
lavished on 'greenhouse' conditions and global warming, we sometimes forget that we
occupy a brief
temperate
or
interglacial
phase of the Quaternary Ice Age. The inheritance
of the last
cold
or
glacial
stage surrounds us. Most mid-latitude farmland and sand-and-
gravel aggregates industry is founded on glacial sediments or their derivatives. Their
complex geotechnical character tests civil engineering skills, especially in glaciated
highlands. Many slopes excavated and over-steepened by glacial erosion are still unstable
and liable to failure but the same process has created ready-made water reservoir sites.
Spectacular highland scenery, formed during intense Quaternary glacial and frost action
in Cenozoic and older orogens, continues to develop in modern alpine glacial areas.
Scenic attraction and tourism depend on them. The full impact of global warming
depends on the uncertain reaction of Earth's cryosphere.
Little more than a generation ago, glaciation was regarded as a climatic 'accident' and
the retreat of glaciers meant a return to 'normal' denudation. Ice was thought simply to
have ornamented existing fluvial valleys and even to have protected some land surfaces
from 'normal' erosion. This was a retreat from the glacial revolution which began a
century earlier, when only a brave geologist would have dared propose that glaciers once
occupied Britain. Charles Darwin missed the glacial evidence surrounding him in Cwm
Idwal (North Wales) in 1831. He made generous amends a decade later, inspired by the
Andean glaciers seen on the
Beagle
voyage and a visit to Britain by the pioneer Swiss
glaciologist Louis Agassiz. Some Victorians preferred theories involving icebergs,
bobbing around on Noah's flood, to the land-based ice sheets envisaged by more
visionary geologists. The modern glacial revolution recognizes world-wide glaciation not
only as an important, if intermittent, feature of Earth's physical environment but as
providing an
icehouse
counterpart to long periods of
greenhouse
Earth.
FORM, MASS AND ENERGY BALANCE OF ICE
Snow and ice accumulate in four different modes.
Ice sheets
and
glaciers
are permanent
ice bodies distinguished by their size and thermodynamic activity (see below). They
comprise the vast bulk of Earth's 35 million km
3
of ice which covers 11 per cent of its
land surface today, with some 98 million km
3
covering over 30 per cent at the last global
maximum 18 ka ago (Table 15.1 and Figure 15.1). They are responsible almost
exclusively for mid- and high-latitude glacial geomorphology and low-latitude alpine
