Environmental Engineering Reference
In-Depth Information
(Bahr et al., 2009). A future equilibrium with the current climate implies a
change in sea level of 0.089±0.015 m due to glaciers and 0.095±0.029 m
due to ice caps, with a total change in sea level of 0.184±0.033 m.
The ratio of accumulation area (AAR) at the end of the melt season and
total glacier area dropped from roughly 0.54 in 1961 to 0.44 in 2007, and it
is expected to drop further to 0.31 by 2050. This is a conservative estimate—
observations indicate a faster than linear decrease in global ice mass balance
over the past 40 years (Kaser et al., 2006). Although the actual decrease in
AAR may be faster than linear, this conservative estimate represents a 30%
decrease from the current value. As a rough approximation, we can assume
that the AAR of every glacier decreases by the same percentage, giving an
estimate of the fractional volume change for each glacier. In that case, the
minimal sea level rise from glaciers and ice caps will be 0.373±0.021 m
over the next 100 years (Bahr et al., 2009).
Greenland and Antarctic Ice Sheets
On the polar ice sheets, there is observational evidence of accelerating
flow from outlet glaciers both in Greenland and in west Antarctica. Both
inland snow accumulation and marginal ice melting have increased over
the Greenland ice sheet, but there is little evidence for any significant ac-
cumulation trend over the Antarctic ice sheet. Antarctica and Greenland
maintain the largest ice reservoirs on land. For 1993-2003, the estimated
contributions for the Greenland and Antarctic ice sheets are 0.21±0.07 mm
y
-1
and 0.21±0.35 mm y
-1
, respectively (Bindoff et al., 2007). There is little
information to constrain ice sheet contributions for previous decades, but
it is thought that the Greenland contribution has increased significantly in
recent years (Lemke et al., 2007). Since IPCC AR4, there have been a number
of new studies on ice sheet mass budget that have considerably enhanced
our understanding of ice sheet vulnerabilities (Figure 4.24) (Allison et al.,
2009a). Recent observations have shown that changes in the rate of ice
discharge into the sea can occur far more rapidly than previously suspected
(e.g., Rignot, 2006).
The pattern of ice sheet change in Greenland is one of near-coastal
thinning, primarily along fast-moving outlet glaciers. Accelerated flow and
discharge from some major outlet glaciers (also called dynamic thinning) is
responsible for much of the loss (Rignot and Kanagaratnam, 2006; Howat
et al., 2007). Pritchard et al. (2009) used high-resolution satellite laser
altimetry to show that dynamic thinning of fast-flowing coastal glaciers is
now widespread at all latitudes in Greenland. Figure 4.24 shows estimates
