Environmental Engineering Reference
In-Depth Information
forecasts of 4°-10° C temperature increases in mid- and high-latitude mountains by 2100
adopt this procedure. This accords with general forecasts that warming will increase
globally with latitude. The immediate hydrometeorological impact will be on sensitive
snow, permafrost and ice systems - but the
direction
of change is not so predictable.
Warming will increase winter precipitation in mid to high latitudes but temperature
determines the ratio of snow and rain. Even if winter snow accumulation increases, rising
temperatures may increase summer ablation even more. A 50 per cent increase in
regional snowfall may be necessary to offset just 0·5° C warming. Moreover, changes in
albedo and, consequentially, energy absorption and moisture budgets may generate
complex positive feedbacks.
Seasonal snow cover and snowlines respond first to changing snowfall. Mid-latitude
snowpacks are close to melting point and thus extremely vulnerable. Snow is likely to lie
for twenty-five fewer days per year, and snowlines rise by 100-200 m for every 1° C of
warming in the European Alps. Regional snowlines lie at altitudes between 2·4 km and
3·5 km - already astride the mean summit altitude of 2·5 km. The same warming will
probably trigger the disappearance of almost all alpine permafrost and also threatens
major glacier retreat, with longer response times. Since the end of the Little Ice Age
c
.
AD 1850 there has been a 30 per cent and 46 per cent decrease in the area of Swiss and
Austrian glaciers respectively. Their area is forecast to decline further, to just 25 per cent
of their Little Ice Age extent by 2025. Thirty to fifty per cent of all European Alpine
glaciers may disappear by 2100.
The glacio-meteorological consequences for alpine hydrology include a shortening of
the snow melt season, raising spring flood peaks and promoting drought later in the
growing season. This also binds in lowland population centres, since most rivers rise in
mountains. Hydrological extremes of flood and drought, and altered sediment flux
regimes, may disrupt water and hydroelectricity supplies and agriculture. Eighty per cent
of the water used in the western United States, for example, originates in snow-bound and
glaciated mountain catchments. Less reliable snow conditions will also have an adverse
impact on winter tourism.
Impacts on alpine ecology may be equally dramatic, with a forecast upwards shift of
vegetation belts and ecotones by 500-700 m for a 3° C warming. As a simple rule,
vegetation zones will be replaced by the currently subjacent zone in 75 per cent of cases.
Most arctic-alpine plants can tolerate only 1°-2° C sustained temperature change, and it
is possible that rising timberlines will reduce the current extent of the alpine zone by 40-
60 per cent, driving it right off many lower peaks. Species diversity in individual zones
may increase at first, sometimes dramatically, but the potential for upward migration
diminishes with time. Many plants, failing to migrate, will become extinct. Other
implications of cryosphere and ecosystem change will follow. In geomorphic terms, there
will be an intensification of rockfalls, debris flows, snow and ice avalanches and
jökulhlaups, although the spatial patterns of change may be more difficult to predict.
ongoing tectonic activity combine to make it one of Earth's most geomorphically active
environments, in which catastrophic events may disguise continuous but less dramatic
denudation.
