Geoscience Reference
In-Depth Information
Climate change in alpine environments
NEW DEVELOPMENTS
The high diversity of mountain climates renders General Climate Models (GCMs) less useful in predicting the
impact of climate change without high-resolution regional models (RCMs) at 1-5 km
2
grid scales nested inside
them (IPCC 2007). However, strong thinning and retreat of most alpine glaciers and small ice caps provides the
clearest evidence of global warming and accords with forecasts of 4-10
C temperature increases in mid- and
high-latitude mountains by 2100. The immediate hydrometeorological impact is 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 will increase the ratio of rain to snow. Winter snow accumulation increases
but is more than offset by increased summer ablation, with a 50 per cent increase in regional snowfall necessary
to offset just 0.5
C warming. Moreover, changes in albedo and, consequently, energy absorption and moisture
budgets may generate complex, localized positive feedbacks. This may explain recent positive mass balances in some
Svalbard, Norwegian and New Zealand maritime glaciers and some Karakoram glaciers against the global trend,
although both may also be short-lived responses to perturbations in their hemispheric NAO, monsoon and ENSO
conditions.
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
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 disappearance
of almost all alpine permafrost and also threatens major glacier retreat over decadal response times. There has already
been a 30 per cent and 46 per cent decrease in the area of Swiss and Austrian glaciers respectively since the end
of the Little Ice Age c.
AD
1850. 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 glaciers may disappear by 2100. The strongest areas of glacier
retreat elsewhere are in Alaska, Patagonia, western North American coastal and Rocky Mountain ranges and the
Canadian Arctic. Low latitude/high-altitude glaciers experience different mass balance seasons, generally experiencing
both summer accumulation and ablation seasons instead of winter accumulation/summer ablation seasonality at higher
latitudes. However, most are also in general retreat in the Andes and Himalaya and those on east African equatorial
mountains may soon vanish altogether (
Plate 15.3
).
Over 80 per cent of 500,000 km
2
of thin, tabular icefields on the
Tibetan plateau are expected to have melted by 2040.
150 m for every 1
Glacio-meteorological consequences for alpine hydrology include a shortening of the snow melt season, earlier spring
floods and drought later in the growing season. Snow cover in the European Alps is forecast to fall by 95 per cent
below 1,000 m elevation and by 50 per cent around 2,000 m. This also binds in lowland population centres, since
most rivers rise in mountains. Hydrological extremes of flood and drought, and altered sediment flux regimes, are
likely to disrupt water and hydro-electric supplies and agriculture. Eighty per cent of the already over-allocated water
used in the western United States, for example, originates in snowbound and glaciated mountain catchments. Shorter
seasonal and less reliable snow conditions will have an adverse impact on winter tourism.
Impacts on alpine ecology may be equally dramatic, with a forecast upward 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 upwards 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 rock falls, debris flows, snow and ice avalanches and jökulhlaupur, although the spatial patterns of change may
be more difficult to predict.
