Geoscience Reference
In-Depth Information
give an indication of future mass changes, and projections should preferably be addressed
by
models
describing
the
physical
processes
involved
and
using
transient
climate
scenarios.
2.4.2 Multi-method approach
Gardner et al. (
2013
) synthesized a consensus global mass-balance estimate for the period
October 2003-October 2009 by standardizing existing and generating new, regional esti-
mates for 19 individual glacierized regions (Fig.
1
) while investigating the large dis-
crepancies between the estimates obtained from GRACE and those from interpolating local
glaciological records (Table
1
). The analysis is based on a new globally complete glacier
inventory (Randolph Glacier Inventory, RGI, Arendt et al.
2012
). ICESat and GRACE
estimates agreed well in large glacierized regions, where results from spatial interpolation
of local records tended to give considerably more negative mass budgets. Their analyses
suggest that available local glaciological records are negatively biased in larger regions,
indicating that previous assessments based on spatial interpolation (Sect.
2.1
) may have
overestimated mass losses. GRACE results tend to have large uncertainties in regions with
little ice cover. Therefore, averages of available ICESat and GRACE estimates were
generally used for the larger glacierized areas while results from spatial interpolation of
local measurements updated from Cogley (
2009a
) were adopted for the smaller
(\ 5,000 km
2
) regions where the density of in situ measurements tends to be high.
Results show that all glaciers other than the ice sheets lost 259 ± 28 Gt year
-1
accounting for 29 ± 13 % of the observed sea-level rise of 2.50 ± 0.54 mm SLE year
-1
during 2003-2009, thus matching approximately the combined contribution of the two
large ice sheets (Shepard et al.
2012
). Glacier mass was lost in all 19 regions during this
period with the largest losses from Arctic Canada, Alaska, and peripheral Greenland.
3 Modeling glacier mass balance on regional and global scales
State-of-the-art simulations and projections of global mass changes of glaciers and ice caps
have relied on low-complexity models of surface mass balance and glacier dynamics.
These modeling studies have commonly assumed that the main drivers of glacier mass
balance are air temperature and precipitation, while glacier dynamics, involved in changes
of glacier area and thickness, are assumed to be successfully simulated by scaling methods
(Bahr et al.
1997
). In the following sections, we will briefly discuss a selection of the
modeling studies (listed in Tables
1
and
2
), narrowing our review to the most recent studies
(last few years) and to those that used some type of meteorological/climate data. Meth-
odological approaches fall broadly into two categories: (1) models based on mass-balance
sensitivities to temperature and precipitation changes (e.g., Hock et al.
2009
; Slangen et al.
2012
), and (2) direct modeling of transient surface mass balance (e.g., Raper and Brai-
thwaite
2006
; Radi´ and Hock
2011
; Marzeion et al.
2012
). Most of the latter studies have
used an ensemble of global climate model (GCMs) to provide climate forcing for their
models. The use of a multimodel ensemble is a common way to provide a range of
projections and uncertainties in any climate change impact studies. Also, studies that
evaluated GCM simulations of mean climate on global and regional scales have shown that
the multimodel ensemble average is superior to any individual model (e.g., Gleckler et al.
2008
; Pierce et al.
2009
).
