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estimates of river discharge from gauging stations to estimates of meltwater fluxes and
annual mass changes of all glaciers draining to the Arctic Ocean including the Greenland
ice sheet. Annual glacier runoff (Concept 5, Table
3
) was found to have increased sub-
stantially from 1961 to 1992 to 1993-2006 (from 146 ± 338 to 202 ± 48 Gt a
-1
) while
glacier mass loss more than doubled. The increase in glacier runoff was the same order of
magnitude as the observed increase in river runoff (Bring and Destouni
2011
), suggesting
an important role of glacier melt in Arctic freshwater budgets.
Neal et al. (
2010
) adopted a water balance approach to estimate the contribution of glacier
runoff to freshwater discharge into the Gulf of Alaska, a 420,230 km
2
watershed covered
18 % by glaciers. Glacier runoff (Concept 1) contributed 47 % of the total runoff (870 km
3
a
-1
), with 10 % originating from glacier net mass loss alone (Concept 4, Table
3
).
Dyurgerov (
2010
) analyzed all available mass-balance profiles, which describe the
distribution of mass change with altitude, and found an increase in both accumulation and
ablation in the observed period (1961-2006), with major increases since the late 1980s, and
a steepening of the mass-balance gradient. The latter was attributed to an increase in
meltwater production at low elevations combined with more snow accumulation at higher
elevations and interpreted as evidence of an intensified hydrological cycle in times of
global warming.
Huss (
2011
) assessed the contribution of glaciers to runoff from large-scale drainage
basins in Europe with areas up to 800,000 km
2
over the period 1908-2008 based on
modeled monthly mass budget estimates for all glaciers in the European Alps. The glacier
runoff defined as the water due to glacier mass change (Concept 4, Table
3
) was computed
for each month and compared to monthly river runoff measured at gauges along the entire
river lengths. Although glacierization of the investigated basins did not exceed 1 % of the
total area, the maximum monthly glacier contributions during summer ranged from 4 to
25 % between catchments, indicating that seasonal glacier contributions can be significant
even in basins with little ice cover. Comeau et al. (
2009
) analyzed annual runoff in a large
catchment in Western Canada and found that reductions in glacier volume due to receding
glaciers (Concept 4, Table
3
) contributed 3 % to total runoff during 1975-1998.
Kaser et al. (
2010
) performed the only global-scale study on the effects of glaciers on
freshwater resources and provided a first-order estimate of the role of glaciers to water
availability and their societal importance. For 18 large glacierized river basins, the fraction of
runoff that is seasonally delayed by glaciers was computed based on gridded climatologies
and theoretical considerations rather than glacier mass balance and runoff data. Monthly
accumulation was computed as a function of elevation using gridded climatological data from
the Climatic Research Unit (CRU). Assuming the glaciers to be in equilibrium with climate,
an equal amount of annual ablation was distributed to each month based on monthly air
temperatures. Any excess ablation beyond accumulation, for a given month, was considered
seasonally delayed glacier runoff and weighted with population to assess the societal impact
of delayed runoff. Results showed that seasonally delayed glacier runoff is most significant in
seasonally arid regions and of moderate importance in midlatitude basin, but negligible in
lowland basins affected by monsoon climates. This underlines the importance of climate
regimes in determining the importance of glaciers on runoff.
6 Synthesis and discussion
Accelerated glacier wastage in many parts of the world and the resulting impacts on sea-
level rise and water resources is a topic of global concern. Mass losses from the Earth's
