Mountain Glaciers and Ice Caps

In addition to the Greenland Ice Sheet, the Arctic contains
a diverse array of smaller glaciers ranging from small cirque
glaciers to large ice caps with areas up to 20 000 km
2
. Together,
these glaciers cover an area of more than 400 000 km
2
, over
half the global area of mountain glaciers and ice caps. Their
total volume is sufficient to raise global sea level by an average
of about 0.41 m if they were to melt completely.
These glaciers exist in a range of different climatic regimes,
from the maritime environments of southern Alaska, Iceland,
western Scandinavia, and Svalbard, to the polar desert of the
Canadian Arctic. Glaciers in all regions of the Arctic have
decreased in area and mass as a result of the warming that has
occurred since the 1920s (in two pulses – from the 1920s to the
1940s and since the mid-1980s). A new phase of accelerated
mass loss began in the mid-1990s, and has been most marked in
Alaska, the Canadian Arctic, and probably Greenland. Current
rates of mass loss are estimated to be in the range 150 to 300
Gt/y; comparable to current mass loss rates from the Greenland
Ice Sheet. This implies that the Arctic is now the largest regional
source of glacier contributions to global sea-level rise.
Most of the current mass loss is probably attributable to a
change in surface mass balance (the balance between annual
mass addition, primarily by snowfall, and annual mass loss by
surface melting and meltwater runoff). Iceberg calving is also
a significant source of mass loss in areas such as coastal Alaska,
Arctic Canada, Svalbard, and the Russian Arctic. However,
neither the current rate of calving loss nor its temporal
variability have been well quantified in many regions, so this is a
significant source of uncertainty in estimates of the total rate of
mass loss. It is, however, clear that the larger Arctic ice caps have
similar variability in ice dynamics to that of the Greenland Ice
Sheet. That is to say, areas of relatively slow glacier flow (which
terminate mainly on land) are separated by faster-flowing outlet
glaciers (which terminate mainly in the ocean). Several of these
outlet glaciers exhibit surge-type behavior, while others have
exhibited substantial velocity changes on seasonal and longer
timescales. It is very likely that these changes in ice dynamics
affect the rate of mass loss by calving both from individual
glaciers and the total ice cover.
Projections of future rates of mass loss from mountain
glaciers and ice caps in the Arctic focus primarily on projections
of changes in the surface mass balance. Current models are not
yet capable of making realistic forecasts of changes in losses by
calving. Surface mass balance models are forced with downscaled
output from climate models driven by forcing scenarios that
make assumptions about the future rate of growth of atmospheric
greenhouse gas concentrations. Thus, mass loss projections vary
considerably, depending on the forcing scenario used and the
climate model from which climate projections are derived. A
new study in which a surface mass balance model is driven by
output from ten general circulation models (GCMs) forced by
the IPCC (Intergovernmental Panel on Climate Change) A1B
emissions scenario yields estimates of total mass loss of between
51 and 136 mm sea-level equivalent (SLE) (or 13% to 36% of
current glacier volume) by 2100. This implies that there will still
be substantial glacier mass in the Arctic in 2100 and that Arctic
mountain glaciers and ice caps will continue to influence global
sea-level change well into the 22nd century.