11.2.3.4 Sensitivity to climatic change
The sensitivity of the ice sheet’s surface mass balance has been studied
with multiple regression analyses, simple meteorological models, and GCMs (Table
11.7). Most progress since the SAR has been made with several coupled AOGCMs,
especially in the “time-slice” mode in which a high-resolution AGCM
(Atmospheric General Circulation Model) is driven by output from a low-resolution
transient AOGCM experiment for a limited duration of time. Model resolution
of typically 100 km allows for a more realistic topography crucial to better
resolving temperature gradients and orographic forcing of precipitation along
the steep margins of the polar ice sheets. Even then, GCMs do not yet perform
well in reproducing melting directly from the surface energy fluxes. The ablation
zone around the Greenland ice sheet is mostly narrower than 100 km, and the
important role played by topography therefore requires the use of downscaling
techniques to transfer information to local and even finer grids (Glover, 1999).
An additional complication is that not all melt water runs off to the ocean
and can be partly retained on or in the ice sheet (Pfeffer et al., 1991; Janssens
and Huybrechts, 2000).
For Greenland, estimates of the sensitivity to a 1°C local warming over
the ice sheet are close to 0.3 mm/yr (with a total range of +0.1 to +0.4 mm/yr)
of global sea level equivalent. This range mainly reflects differences in the
predicted precipitation changes and the yearly distribution of the temperature
increase, which is predicted to be
Figure 11.4: Estimates of global sea level change over the last 140,000
years (continuous line) and contributions to this change from the major
ice sheets: (i) North America, including Laurentia, Cordilleran ice, and
Greenland, (ii) Northern Europe (Fennoscandia), including the Barents region,
(iii) Antarctica. (From Lambeck, 1999.) |
larger in winter than in summer in the GCMs, but is assumed
uniform in the studies of Van de Wal (1996) and Janssens and Huybrechts (2000).
Another difference amongst the GCM results concerns the time window over which
the sensitivities are assessed. The CSIRO9/T63 sensivities are estimated from
high-resolution runs forced with observed SSTs for the recent past (Smith et
al., 1998; Smith, 1999), whereas the ECHAM data are given as specific mass balance
changes for doubled minus present atmospheric CO2. Thompson and Pollard
(1997) report similar results to the ECHAM studies but the corresponding sensitivity
value could not be calculated because the associated temperature information
is not provided. Some palaeoclimatic data from central Greenland ice cores indicate
that variations in precipitation during the Holocene are related to changes
in atmospheric circulation rather than directly to local temperature (Kapsner
et al., 1995; Cuffey and Clow, 1997), such that precipitation might not increase
with temperature (in contrast with Clausen et al., 1988). For glacial-interglacial
transitions, the ice cores do exhibit a strong positive correlation between
temperature and precipitation (Dansgaard et al., 1993; Dahl-Jensen et al., 1993;
Kapsner et al., 1995; Cuffey and Marshall, 2000), as simulated by AOGCMs for
anthropogenic warming. Although other changes took place at the glacial-interglacial
transition, this large climate shift could be argued to be a better analogue
for anthropogenic climate change than the smaller fluctuations of the Holocene.
To allow for changes in circulation patterns and associated temperature and
precipitation patterns, we have used time-dependent AOGCM experiments to calculate
the Greenland contribution (Section 11.5.1).
For Antarctica, mass-balance sensitivities for a 1°C local warming are
close to –0.4 mm/yr (with one outlier of –0.8 mm/yr) of global sea
level equivalent. A common feature of all methods is the insignificant role
of melting, even for summer temperature increases of a few degrees, so that
only accumulation changes need to be considered. The sensitivity for the case
that the change in accumulation is set proportional to the relative change in
saturation vapour pressure is at the lower end of the sensitivity range, suggesting
that in a warmer climate changes in atmospheric circulation and increased moisture
advection can become equally important, in particular close to the ice sheet
margin (Bromwich, 1995; Steig, 1997). Both ECHAM3 and ECHAM4/OPYC3 give a similar
specific balance change over the ice sheet for doubled versus present atmospheric
CO2 to that found by Thompson and Pollard (1997).
In summary, the static sensitivity values suggest a larger role for Antarctica
than for Greenland for an identical local temperature increase, meaning that
the polar ice sheets combined would produce a sea level lowering, but the spread
of the individual estimates includes the possibility that both ice sheets may
also balance one another for doubled atmospheric CO2 conditions (Ohmura
et al., 1996; Thompson and Pollard, 1997). For CO2 increasing according
to the IS92a scenario (without aerosol), studies by Van de Wal and Oerlemans
(1997) and Huybrechts and De Wolde (1999) calculated sea level contributions
for 1990 to 2100 of +80 to +100 mm from the Greenland ice sheet and about –80
mm from the Antarctic ice sheet. On this hundred year time-scale, ice-dynamics
on the Greenland ice sheet was found to counteract the mass-balance-only effect
by between 10 and 20%. Changes in both the area-elevation distribution and iceberg
discharge played a role, although the physics controlling the latter are poorly
known and therefore not well represented in the models. Because of its longer
response time-scales, the Antarctic ice sheet hardly exhibits any dynamic response
on a century time-scale, except when melting rates below the ice shelves were
prescribed to rise by in excess of 1 m/yr (O’Farrell et al., 1997; Warner
and Budd, 1998; Huybrechts and De Wolde, 1999; see also Section
11.5.4.3).
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