Geoscience Reference
In-Depth Information
to resolve individual components of the mass changes, the signal needs to be decomposed
in order to identify the signal due to glacier mass changes. The decomposition is relatively
complex and, because it relies on the accuracy of models used to simulate Earth system
processes (isostatic rebound, tectonics, hydrology, atmosphere), it may introduce large
uncertainties into the derived mass balances (e.g., Jacob et al.
2012
).
GRACE-derived regional-scale mass balances have been reported for the Canadian
Arctic (Gardner et al.
2011
), Alaska (Tamisiea et al.
2005
; Chen et al.
2006
; Luthcke et al.
2008
; 2013; Wu et al.
2010
), Patagonia (Chen et al.
2007
; Ivins et al.
2011
), and High
Mountain Asia (Matsuo and Heki
2010
). Jacob et al. (
2012
) were the first to compute
GRACE-derived mass-balance estimates for all glacierized regions outside Greenland and
Antarctica, followed by Gardner et al. (
2013
) who updated their estimate and generated a
new one based on the methods of Wouters et al. (
2008
). These two analyses report a total
mass budget for these regions of -170 ± 32 Gt year
-1
and -166 ± 37 Gt year
-1
,
respectively, for the period 2003-2009. Jacob et al. (
2012
) note that their results are
roughly 30 % smaller than the most recent available estimate at that time, obtained from
the interpolation of glaciologically derived in situ observations by Dyurgerov (
2010
).
2.4 Assessments by other approaches
2.4.1 AAR method
Bahr et al. (
2009
) derived global glacier mass changes using an approach based on the
observations of the accumulation area ratio (AAR), i.e., the ratio of the accumulation area
to the total glacier area. AAR is closely related to the mass balance of a glacier in the case
when calving and submarine melt are negligible (Dyurgerov and Meier
2005
). AARs can
be relatively easily approximated from aerial and satellite observations of the end-of-
summer snowline. For a glacier in balance with the climate, the AAR is equal to its
equilibrium value, AAR
0
, whose average value from a sample of * 100 glaciers has been
found to be 58 % (Dyurgerov et al.
2009
).
Glaciers with AAR \ AAR
0
will retreat to higher elevations, typically over several
decades or longer, and the AAR may return to the equilibrium value. Using AAR obser-
vations of * 80 glaciers collected during 1997-2006, Bahr et al. (
2009
) computed a mean
AAR of 44 ± 2 %, with AAR \ AAR
0
for most glaciers in the dataset. Mernild et al.
(
2013
) revised the methodology, expanded and updated their data, and found an average
AAR of 34 ± 3 %, for the period 2001-2010. Using the empirical relationship between the
ratio AAR/AAR
0
and annual glacier mass balance, Mernild et al. (
2013
) reconstructed
pentadal global glacier mass balances for 1971-2010, showing a good agreement with
estimates from Cogley (
2009b
). However, they also found much larger uncertainties in the
global estimate than in the original study by Bahr et al. (
2009
).
This AAR-based approach has also been used to provide estimates of future glacier area
and volume changes assuming that the future climate resembles the one of the recent few
decades. Bahr et al. (
2009
) estimated that, even without additional atmospheric warming,
the volume of glaciers must shrink by 27 ± 5 % to return to a balanced mass budget.
Assuming that the total volume of the Earth's glaciers and ice caps is 650 mm SLE
(Dyurgerov and Meier
2005
), the fractional losses would raise global mean sea level by
184 ± 33 mm. With the updated AAR dataset and updated estimate of total glacier volume
(430 mm SLE by Huss and Farinotti
2012
) and accounting for the larger errors due to
regional and global undersampling, Mernild et al. (
2013
) revised this estimate to
163 ± 69 mm. We note that, because of its simplicity, the AAR-based approach may only
