Geoscience Reference
In-Depth Information
mountain glaciers and ice caps contribute to the freshwater influx to the ocean and make up
one-third of recent sea-level rise (the remaining parts come in equal shares from ice sheet
mass losses and thermal expansion of seawater). They also influence the runoff charac-
teristics of glacierized basins with significant effects even at low levels of glacierization.
The expected changes in glacier runoff may be larger than those generally projected for
other components of the water cycle.
The main impacts of glacier wastage vary regionally. For sea-level rise, the most
important regions are found in high-latitude regions where large ice volumes are typical,
such as the Antarctic and Greenland peripheries, Canadian Arctic, Alaska, and the Russian
Arctic (Gardner et al.
2013
). In contrast, mid- and low-latitude regions (e.g., European
Alps, Scandinavia, Tropical Andes, and Western Canada/USA) have relatively little ice
cover and therefore (except for the High Asian Mountains) less potential impact on sea-
level change. However, many of these regions have relatively large populations and the
hydrological consequences of glacier wastage are of concern.
Assessing and projecting the effects of glaciers on sea level and terrestrial hydrology
requires accurate assessments of the glacier mass balance and its components. In recent
years, much progress has been made in measuring glacier mass changes on regional and
global scales, mostly due to the launch of the ICESat and GRACE satellites in the
beginning of the twenty-first century. For the first time, regional scale mass-balance
observations were possible in regions with sparse local in situ observations. These results
will be valuable for calibration and validation of global hydrology models. Although the
traditional technique of extrapolating local observations is problematic in regions with
sparse data, as it can bias global results (Gardner et al.
2013
), in situ measurements are
essential for calibration and validation of glacier mass-balance and runoff models.
Unfortunately, the number of mass-balance monitoring glaciers has declined in recent
years.
Until recently, the lack of basic inventory data was a major impediment in global mass-
balance assessments and projections resulting in large uncertainties in the results due to
necessary upscaling procedures or other workarounds (e.g., Raper and Braithwaite
2005
;
Radi
´
and Hock
2010
). The recently completed Randolph Glacier Inventory, the first
globally complete glacier inventory (Arendt et al.
2012
), is a major step forward toward
reducing uncertainties in global-scale studies. Also, for the first time, it has become pos-
sible to model global mass balances for each glacier in the world individually (Radi´ and
Hock
2011
; Marzeion et al.
2012
; Radi
´
et al.
2013
). However, there is a large range in the
twenty-first century projections from the three independent studies (Marzeion et al.
2012
;
Radi
´
et al.
2013
and Hirabayashi et al.
2013
) that use the new inventory despite using the
same climate forcings (RCP scenarios) and largely overlapping selection of GCMs.
Hirabayashi et al. (
2013
) projections are at the low end. Results also indicate that previ-
ously found large uncertainty due to the choice of the GCMs (e.g., Radi´ and Hock
2011
)
has not been reduced. Glacier models also still suffer from the omission of frontal ablation
(calving and submarine melt) due to the inherent difficulty in modeling these processes and
the lack of data to develop parameterizations suitable for regional scales.
For sea-level change calculations, rates of regional or global glacier net mass loss are
generally converted into sea-level equivalent simply by dividing the volume of water lost
by the ocean area (362.5 9 10
12
m
2
, Cogley et al.
2011
), thus neglecting the effects of
altering ocean area and terrestrial hydrology. The effect of flow of meltwater into
groundwater aquifers or enclosed basins rather than the oceans is virtually unknown and
should be addressed by coupling glacier mass-balance models with global hydrology
models. For future scenarios, it is important that hydrology models have the capacity to
