I have read the IPCC reports, I have also read the reports charging the IPCC with "cooking the intellectual books" the reports of the IPCC excluding any information contrary to their already determined opinion, and the reports of the IPCC only funding studies positive to their positions.
You have all of your eggs in one intellectually corrupt basket.
There's no point in beating around the bush, so I'll just come and and tell you that I think
you're lying. I don't believe you have read the IPCC reports, or even made any decent effort at trying to read them.
Just counting the 2007 report (which consisted of several sections), that would mean you read 2460 pages of dense, scientific material, mostly consisting of two-column pages. If you already have a strong grounding in math, biology, chemistry, physics, statistics, and economics, such that you don't have to stop to look up material as you go, you could probably average around 3 minutes per page, meaning it would take you 123 hours to work you way through it. If you're dedicated enough to spend two hours per day on it, which is exhausting for this kind of reading, that would be two months of daily reading. It took me about four months, and I admit I mostly just skimmed the 800 of those pages dealing with mitigation, since my primary interest was figuring out if anthropogenic climate change was happening and how bad it was likely to be -- I figure I can spend more time on understanding the solutions in the future. Anyway, I don't believe you're the type who'd put in that kind of concerted intellectual work. Are you HONESTLY asserting you read almost 2500 pages of scientific reports?
Even if you were that kind of glutton for punishment, like me, it's impossible for me to imagine you could have made it through the report and not realized that the IPCC wasn't projecting sea ice loss in Antarctica (as you last post so foolishly imagined). They're quite clear on the topic, even setting it out plainly in the summary for policymakers. They thought Antarctica would remain cold enough not to lose much ice, and that increased snowfall would actually increase the mass of ice there. How, if you read the report as you say, could you have missed that highlight-level information?
They have whole sub-chapters on the subject (11.8.2, 10.3, and 10.6). If you're interested, here you go:
-------------
11.8.2 Antarctic
11.8.2.1 Key Processes
Over Antarctica, there is special interest in changes in snow
accumulation expected to accompany global climate change
as well as the pattern of temperature change, particularly any
differences in warming over the peninsula and the interior of
the ice sheet. As in the Arctic, warming of the troposphere
is expected to increase precipitation. However, circulation
changes in both ocean and atmosphere can alter the pattern
of air masses, which would modify both precipitation and
temperature patterns substantially over the region.
The dominant patterns controlling the atmospheric seasonalto-
interannual variability of the SH extratropics are the Southern
Annular Mode (SAM) and ENSO (see Section 3.6). Signatures
of these patterns in the Antarctic have been revealed in many
studies (reviews by Carleton, 2003 and Turner, 2004). The
positive phase of the SAM is associated with cold anomalies
over most of Antarctica and warm anomalies over the Antarctic
Peninsula (Kwok and Comiso, 2002a). Over recent decades,
a drift towards the positive phase in the SAM is evident (see
Section 3.6). Observational studies have presented evidence
of pronounced warming over the Antarctic Peninsula, but little
change over the rest of the continent during the last half of
the 20th century (see Sections 3.6 and 4.6). The response of
the SAM in transient warming simulations is a robust positive
trend, but the response to the ozone hole in the late 20th century,
which is also a positive perturbation to the SAM, makes any
simple extrapolation of current trends into the future uncertain
(see Section 10.3).
Compared to the SAM, the Southern Oscillation (SO)
shows weaker association with surface temperature over
Antarctica but the correlation with SST and sea ice variability
in the Pacifi c sector of the Southern Ocean is signifi cant (e.g.,
Kwok and Comiso, 2002b; Renwick, 2002; Bertler et al., 2004;
Yuan, 2004). Correlation between the SO index and antarctic
precipitation and accumulation has also been studied but the
persistence of the signal is not clear (Bromwich et al., 2000,
2004a; Genthon and Cosme, 2003; Guo et al., 2004; Genthon et
al., 2005). Recent work suggests that this intermittence is due
to nonlinear interactions between ENSO and SAM that vary
on decadal time scales (Fogt and Bromwich, 2006; L’Heureux
and Thompson, 2006). The SO index has a negative trend over
recent decades (corresponding to a tendency towards more El
Niño-like conditions in the equatorial Pacifi c; see Section 3.6)
associated with sea ice cover anomalies in the Pacifi c sector,
namely negative (positive) anomalies in the Ross and Amundsen
Seas (Bellingshausen and Weddell Seas) (Kwok and Comiso,
2002a). However, a defi nitive assessment of ENSO amplitude
and frequency changes in the 21st century cannot be made (see
Chapter 10).
11.8.2.2 Skill of Models in Simulating Present Climate
Evaluating temperature and precipitation simulations over
Antarctica is diffi cult due to sparse observations and often
relies on numerical weather prediction (re)analyses. However,
signifi cant differences between those have been found, and
comparisons with station observations show that the surface
temperature can be subject to considerable biases (Connolley
and Harangozo, 2001; Bromwich and Fogt, 2004). Marked
improvement in the bias is seen after the satellite era (~1978)
(Simmons et al., 2004), and parts of the bias are explained
by the reanalyses’ smoothing of the sharp changes in the
terrain near coastal stations. Satellite-derived monthly surface
temperatures agree with antarctic station data with an accuracy
of 3°C (Comiso, 2000). Precipitation evaluation is even more
challenging and the different (re)analyses differ signifi cantly
(Connolley and Harangozo, 2001; Zou et al., 2004). Very few
direct precipitation gauge and detailed snow accumulation data
are available, and these are uncertain to varying degrees (see
Section 4.6).
Major challenges face the simulation of the atmospheric
conditions and precipitation patterns of the polar desert in the
high interior of East Antarctica (Guo et al., 2003; Bromwich
et al., 2004a; Pavolonis et al., 2004, Van de Berg et al., 2005).
Driven by analysed boundary conditions, RCMs tend to show
smaller temperature and precipitation biases in the Antarctic
compared to the GCMs (Bailey and Lynch, 2000; Van Lipzig et
al., 2002a,b; Van den Broeke and Van Lipzig, 2003; Bromwich
et al., 2004b; Monaghan et al., 2006). Krinner et al. (1997)
show the value of a stretched model grid with higher horizontal
resolution over the Antarctic as compared to standard GCM
formulations. Despite these promising developments, since
the TAR there has been no coordinated comparison of the
performance of GCMs, RCMs and other alternatives to global
GCMs over Antarctica.
Temperature
Compared to National Centers for Environmental Prediction
(NCEP) reanalyses, the MMD ensemble annual surface
temperatures are in general slightly warmer in the Southern
Ocean to the north of the sea ice region. The mean bias is
predominantly less than 2°C (Carril et al., 2005), which may
indicate a slight improvement compared to previous models
due to better simulation of the position and depth of the
Antarctic trough (Carril et al., 2005; Raphael and Holland,
2006). The temperature bias over sea ice is larger. Biases over
the continent are several degrees where the model topography
is erroneous (Turner et al., 2006). However, as emphasized
above, the biases have to be viewed in the context of the
uncertainty in the observations. Changes in cloud and radiation
parametrizations have been shown to change the temperature
simulation signifi cantly (Hines et al., 2004). A lateral nudging
of a stretched-grid GCM (imposing the correct synoptic
cyclones from 60°S and lower latitudes) brings the model in
better agreement with observations but signifi cant biases remain
(Genthon et al., 2002).
908
Regional Climate Projections Chapter 11
The spread in the individual MMD-simulated patterns
of surface temperature trends over the past 50 years is very
large, but in contrast to previous models, the multi-model
composite of the MMD models qualitatively captures the
observed enhanced warming trend over the Antarctic Peninsula
(Chapman and Walsh, 2006). The general improvements in
resolution, sea ice models and cloud-radiation packages have
evidently contributed to improved simulations. The ensemblemean
temperature trends show similarity to the observed spatial
pattern of the warming, for both annual and seasonal trends.
For the annual trend, this includes the warming of the peninsula
and near-coastal Antarctica and neutral or slight cooling over
the sea-ice covered regions of the Southern Ocean. While the
large spread among the models is not encouraging, this level of
agreement suggests that some confi dence in the ensemble mean
21st-century projection is appropriate.
Precipitation
The MMD models simulate the position of the storm tracks
reasonably well but nearly all show some defi ciency in the
distribution and level of cyclone activity compared to reanalyses
(see Section 8.3). Regional Climate Models generally capture
the cyclonic events affecting the coast and the associated
synoptic variability of precipitation with more fi delity (Adams,
2004; Bromwich et al., 2004a). Over the 20th century, the
MMD models simulate changes in storm track position that
are generally consistent with observed changes (i.e., poleward
displacement of the storm tracks; see Sections 9.5 and 10.3).
The precipitation simulations by both GCMs and RCMs
contain uncertainty, on all time scales (Covey et al., 2003;
Bromwich et al., 2004a,b; Van de Berg et al., 2005), as a result
of model physics limitations. All atmospheric models, including
the models underlying the reanalyses, have incomplete
parametrizations of polar cloud microphysics and ice crystal
precipitation. The simulated precipitation depends, among other
things, on the simulated sea ice concentrations, and is strongly
affected by biases in the sea ice simulations (Weatherly, 2004).
Recent RCM simulations driven by observed sea ice conditions
demonstrate good precipitation skill (Van de Berg et al., 2005;
Monaghan et al., 2006). However, as emphasized above, the
observational uncertainty contributes to uncertainty in the
differences between observations and simulations.
Sea Ice
The performance biases of SH sea ice conditions in presentday
MMD simulations are discussed in Section 8.3.
11.8.2.3 Climate Projections
Very little effort has been spent to model the future climate
of Antarctica at a spatial scale fi ner than that of GCMs.
Temperature
At the end of the 21st century, the annual warming over the
Antarctic continent is moderate but signifi cant (Figure 11.21;
Table 11.1; Chapman and Walsh, 2006). It is estimated to be
2.6°C by the median of the MMD-A1B models with a range
from 1.4°C to 5.0°C across the models (Table 11.1). Larger
(smaller) warming is found for the A2 (B1) scenario with mean
value of 3.1°C (1.8°C). These warming magnitudes are similar
to previous estimates (Covey et al., 2003). The annual mean
MMD model projections show a relatively uniform warming
over the entire continent (with a maximum in the Weddell Sea)
(Figure 11.21; Carril et al., 2005; Chapman and Walsh, 2006).
They do not show a local maximum warming over the Antarctic
Peninsula. This is a robust feature among the individual models
(Supplementary Material Figure S11.12). Thus, the pattern of
observed temperature trends in the last half of the 20th century
(warming over the Antarctic Peninsula, little change over the
rest of the continent) is not projected to continue throughout
the 21st century, despite a projected positive SAM trend (see
Figure 11.21. Annual surface temperature change between 1980 to 1999 and
2080 to 2099 in the Arctic and Antarctic from the MMD-A1B projections.
909
Chapter 11 Regional Climate Projections
Section 10.3). It has been argued that two distinct factors have
contributed to the observed SAM trend: greenhouse gas forcing
and the ozone hole formation (Stone et al., 2001; Shindell and
Schmidt, 2004). Their relative importance for the peninsular
warming is not readily understood (see Chapter 10).
The mean antarctic temperature change does not show a
strong seasonal dependency; the MMD-A1B ensemble mean
In 20th- and 21st-century simulations, antarctic sea ice
cover is projected to decrease more slowly than in the Arctic
(Figures 10.13c,d and 10.14), particularly in the vicinity of
the Ross Sea where most models predict a local minimum in
surface warming. This is commensurate with the region with
the greatest reduction in ocean heat loss, which results from
reduced vertical mixing in the ocean (Gregory, 2000). The
ocean stores much of its increased heat below 1 km depth in the
Southern Ocean. In contrast, horizontal heat transport poleward
of about 60°N increases in many models (Holland and Bitz,
2003), but much of this heat remains in the upper 1 km of the
northern subpolar seas and Arctic Ocean (Gregory, 2000; Bitz
et al., 2006). Bitz et al. (2006) argue that these differences in the
depth where heat is accumulating in the high-latitude oceans
have consequences for the relative rates of sea ice decay in the
Arctic and Antarctic.
While most climate models share these common
characteristics (peak surface warming in autumn and early
winter, sea ice rapidly becomes seasonal, arctic ice decays faster
than antarctic ice, and northward ocean heat transport increases
into the northern high latitudes), models have poor agreement on
the amount of thinning of sea ice (Flato and Participating CMIP
Modeling Groups, 2004; Arzel et al., 2006) and the overall
climate change in the polar regions (IPCC, 2001; Holland and
Bitz, 2003). Flato (2004) shows that the basic state of the sea
ice and the reduction in thickness and/or extent have little to
do with sea ice model physics among CMIP2 models. Holland
and Bitz (2003) and Arzel et al. (2006) fi nd serious biases in the
basic state of simulated sea ice thickness and extent. Further,
Rind et al. (1995), Holland and Bitz (2003) and Flato (2004)
show that the basic state of the sea ice thickness and extent
have a signifi cant infl uence on the projected change in sea ice
thickness in the Arctic and extent in the Antarctic.
10.6.3.3 Glaciers and Ice Caps on Greenland and
Antarctica
The G&IC on Greenland and Antarctica (apart from the
ice sheets) have been less studied and projections for them are
consequently more uncertain. A model estimate for the G&IC
on Greenland indicates an addition of about 6% to the G&IC
sea level contribution in the 21st century (van de Wal and Wild,
2001). Using a degree-day scheme, Vaughan (2006) estimates
that ablation of glaciers in the Antarctic Peninsula presently
amounts to 0.008 to 0.055 mm yr–1 of sea level, 1 to 9% of
the contribution from G&IC outside Greenland and Antarctica
(Table 4.4). Morris and Mulvaney (2004) fi nd that accumulation
increases on the Antarctic Peninsula were larger than ablation
increases during 1972 to 1998, giving a small net negative sea
816
Global Climate Projections Chapter 10
level contribution from the region. However, because ablation
increases nonlinearly with temperature, they estimate that for
future warming the contribution would become positive, with a
sensitivity of 0.07 ± 0.03 mm yr–1 °C–1 to uniform temperature
change in Antarctica, that is, about 10% of the global sensitivity
of G&IC outside Greenland and Antarctica (Section 10.6.3.1).
These results suggest that the Antarctic and Greenland
G&IC will together give 10 to 20% of the sea level contribution
of other G&IC in future decades. In recent decades, the G&IC
on Greenland and Antarctica have together made a contribution
of about 20% of the total of other G&IC (see Section 4.5.2).
On these grounds, the global G&IC sea level contribution
is increased by a factor of 1.2 to include those in Greenland
and Antarctica in projections for the 21st century (see Section
10.6.5 and Table 10.7). Dynamical acceleration of glaciers in
Greenland and Antarctica following removal of ice shelves,
as has recently happened on the Antarctic Peninsula (Sections
4.6.2.2 and 10.6.4.2), would add further to this, and is included
in projections of that effect (Section 10.6.4.3).
10.6.4 Ice Sheets
The mass of ice grounded on land in the Greenland and
Antarctic Ice Sheets (see also Section 4.6.1) can change
as a result of changes in surface mass balance (the sum of
accumulation and ablation; Section 10.6.4.1) or in the fl ux of
ice crossing the grounding line, which is determined by the
dynamics of the ice sheet (Section 10.6.4.2). Surface mass
balance and dynamics together both determine and are affected
by the change in surface topography.
10.6.4.1 Surface Mass Balance
Surface mass balance (SMB) is immediately infl uenced
by climate change. A good simulation of the ice sheet SMB
requires a resolution exceeding that of AGCMs used for long
climate experiments, because of the steep slopes at the margins
of the ice sheet, where the majority of the precipitation and all
of the ablation occur. Precipitation over ice sheets is typically
overestimated by AGCMs, because their smooth topography
does not present a suffi cient barrier to inland penetration
(Ohmura et al., 1996; Glover, 1999; Murphy et al., 2002).
Ablation also tends to be overestimated because the area at
low altitude around the margins of the ice sheet, where melting
preferentially occurs, is exaggerated (Glover, 1999; Wild
et al., 2003). In addition, AGCMs do not generally have a
representation of the refreezing of surface melt water within the
snowpack and may not include albedo variations dependent on
snow ageing and its conversion to ice.
To address these issues, several groups have computed SMB
at resolutions of tens of kilometres or less, with results that
compare acceptably well with observations (e.g., van Lipzig et
al., 2002; Wild et al., 2003). Ablation is calculated either by
schemes based on temperature (degree-day or other temperature
index methods) or by energy balance modelling. In the studies
listed in Table 10.6, changes in SMB have been calculated
from climate change simulations with high-resolution AGCMs
or by perturbing a high-resolution observational climatology
with climate model output, rather than by direct use of lowresolution
GCM results. The models used for projected SMB
changes are similar in kind to those used to study recent SMB
changes (Section 4.6.3.1).
All the models show an increase in accumulation, but there
is considerable uncertainty in its size (Table 10.6; van de Wal
et al., 2001; Huybrechts et al., 2004). Precipitation increase
could be determined by atmospheric radiative balance, increase
in saturation specifi c humidity with temperature, circulation
changes, retreat of sea ice permitting greater evaporation or a
combination of these (van Lipzig et al., 2002). Accumulation
also depends on change in local temperature, which strongly
affects whether precipitation is solid or liquid (Janssens and
Huybrechts, 2000), tending to make the accumulation increase
smaller than the precipitation increase for a given temperature
rise. For Antarctica, accumulation increases by 6 to 9% °C–1 in
the high-resolution AGCMs. Precipitation increases somewhat
less in AR4 AOGCMs (typically of lower resolution), by 3 to
8% °C–1. For Greenland, accumulation derived from the highresolution
AGCMs increases by 5 to 9% °C–1. Precipitation
increases by 4 to 7% °C–1 in the AR4 AOGCMs.
Kapsner et al. (1995) do not fi nd a relationship between
precipitation and temperature variability inferred from
Greenland ice cores for the Holocene, although both show
large changes from the Last Glacial Maximum (LGM) to the
Holocene. In the UKMO-HadCM3 AOGCM, the relationship
is strong for climate change forced by greenhouse gases and the
glacial-interglacial transition, but weaker for naturally forced
variability (Gregory et al., 2006). Increasing precipitation in
conjunction with warming has been observed in recent years in
Greenland (Section 4.6.3.1).
All studies for the 21st century project that antarctic SMB
changes will contribute negatively to sea level, owing to
increasing accumulation exceeding any ablation increase
(see Table 10.6). This tendency has not been observed in the
average over Antarctica in reanalysis products for the last two
decades (see Section 4.6.3.1), but during this period Antarctica
as a whole has not warmed; on the other hand, precipitation
has increased on the Antarctic Peninsula, where there has been
strong warming.
In projections for Greenland, ablation increase is important
but uncertain, being particularly sensitive to temperature change
around the margins. Climate models project less warming in
these low-altitude regions than the Greenland average, and less
warming in summer (when ablation occurs) than the annual
average, but greater warming in Greenland than the global
average (Church et al., 2001; Huybrechts et al., 2004; Chylek
and Lohmann, 2005; Gregory and Huybrechts, 2006). In most
studies, Greenland SMB changes represent a net positive
contribution to sea level in the 21st century (Table 10.6;
Kiilsholm et al., 2003) because the ablation increase is larger
than the precipitation increase. Only Wild et al. (2003) fi nd the
opposite, so that the net SMB change contributes negatively
to sea level in the 21st century. Wild et al. (2003) attribute this
817
Chapter 10 Global Climate Projections
difference to the reduced ablation area in their higher-resolution
grid. A positive SMB change is not consistent with analyses of
recent changes in Greenland SMB (see Section 4.6.3.1).
For an average temperature change of 3°C over each ice
sheet, a combination of four high-resolution AGCM simulations
and 18 AR4 AOGCMs (Huybrechts et al., 2004; Gregory and
Huybrechts, 2006) gives SMB changes of 0.3 ± 0.3 mm yr–1
for Greenland and –0.9 ± 0.5 mm yr–1 for Antarctica (sea level
equivalent), that is, sensitivities of 0.11 ± 0.09 mm yr–1 °C–1 for
Greenland and –0.29 ± 0.18 mm yr–1 °C–1 for Antarctica. These
results generally cover the range shown in Table 10.6, but tend
to give more positive (Greenland) or less negative (Antarctica)
sea level rise because of the smaller precipitation increases
projected by the AOGCMs than by the high-resolution AGCMs.
The uncertainties are from the spatial and seasonal patterns of
precipitation and temperature change over the ice sheets, and
from the ablation calculation. Projections under SRES scenarios
for the 21st century are shown in Table 10.7.
10.6.4.2 Dynamics
Ice sheet fl ow reacts to changes in topography produced
by SMB change. Projections for the 21st century are given in
Section 10.6.5 and Table 10.7, based on the discussion in this
section. In Antarctica, topographic change tends to increase
ice fl ow and discharge. In Greenland, lowering of the surface
tends to increase the ablation, while a steepening slope in the
ablation zone opposes the lowering, and thinning of outlet
glaciers reduces discharge. Topographic and dynamic changes
simulated by ice fl ow models (Huybrechts and De Wolde, 1999;
van de Wal et al., 2001; Huybrechts et al., 2002, 2004; Gregory
and Huybrechts, 2006) can be roughly represented as modifying
the sea level changes due to SMB change with fi xed topography
by –5% ± 5% from Antarctica, and 0% ±10% from Greenland
(± one standard deviation) during the 21st century.
The TAR concluded that accelerated sea level rise caused
by rapid dynamic response of the ice sheets to climate change
is very unlikely during the 21st century (Church et al., 2001).
However, new evidence of recent rapid changes in the Antarctic
Peninsula, West Antarctica and Greenland (see Section 4.6.3.3)
has again raised the possibility of larger dynamical changes
in the future than are projected by state-of-the-art continental
models, such as cited above, because these models do not
incorporate all the processes responsible for the rapid marginal
thinning currently taking place (Box 4.1; Alley et al., 2005a;
Vaughan, 2007).
The main uncertainty is the degree to which the presence of
ice shelves affects the fl ow of inland ice across the grounding line. A strong argument for enhanced fl ow when the ice shelf
is removed is yielded by the acceleration of Jakobshavn
Glacier (Greenland) following the loss of its fl oating tongue,
and of the glaciers supplying the Larsen B Ice Shelf (Antarctic
Peninsula) after it collapsed (see Section 4.6.3.3). The onset
of disintegration of the Larsen B Ice Shelf has been attributed
to enhanced fracturing by crevasses promoted by surface melt
water (Scambos et al., 2000). Large portions of the Ross and
Filchner-Ronne Ice Shelves (West Antarctica) currently have
mean summer surface temperatures of around –5°C (Comiso,
2000, updated). Four high-resolution GCMs (Gregory and
Huybrechts, 2006) project summer surface warming in these
major ice shelf regions of between 0.2 and 1.3 times the antarctic
annual average warming, which in turn will be a factor 1.1 ±
0.3 greater than global average warming according to AOGCM
simulations using SRES scenarios. These fi gures indicate that
a local mean summer warming of 5°C is unlikely for a global
warming of less than 5°C (see Appendix 10.A). This suggests
that ice shelf collapse due to surface melting is unlikely under
most SRES scenarios during the 21st century, but we have low
confi dence in the inference because there is evidently large
systematic uncertainty in the regional climate projections,
and it is not known whether episodic surface melting might
initiate disintegration in a warmer climate while mean summer
temperatures remain below freezing.
In the Amundsen Sea sector of West Antarctica, ice shelves
are not so extensive and the cause of ice shelf thinning is not
surface melting, but bottom melting at the grounding line (Rignot
and Jacobs, 2002). Shepherd et al. (2004) fi nd an average iceshelf
thinning rate of 1.5 ± 0.5 m yr–1. At the same time as the
basal melting, accelerated inland fl ow has been observed for
Pine Island, Thwaites and other glaciers in the sector (Rignot,
1998, 2001; Thomas et al., 2004). The synchronicity of these
changes strongly implies that their cause lies in oceanographic
change in the Amundsen Sea, but this has not been attributed
to anthropogenic climate change and could be connected with
variability in the SAM.
Because the acceleration took place in only a few years
(Rignot et al., 2002; Joughin et al., 2003) but appears up to
about 150 km inland, it implies that the dynamical response
to changes in the ice shelf can propagate rapidly up the ice
stream. This conclusion is supported by modelling studies of
Pine Island Glacier by Payne et al. (2004) and Dupont and
Alley (2005), in which a single and instantaneous reduction of
the basal or lateral drag at the ice front is imposed in idealised
ways, such as a step retreat of the grounding line. The simulated
acceleration and inland thinning are rapid but transient; the rate
of contribution to sea level declines as a new steady state is
reached over a few decades. In the study of Payne et al. (2004)
the imposed perturbations were designed to resemble loss of
drag in the ‘ice plain’, a partially grounded region near the
ice front, and produced a velocity increase of about 1 km yr–1
there. Thomas et al. (2005) suggest the ice plain will become
ungrounded during the next decade and obtain a similar velocity
increase using a simplifi ed approach.
Most of inland ice of West Antarctica is grounded below sea
level and so it could fl oat if it thinned suffi ciently; discharge
therefore promotes inland retreat of the grounding line, which
820
Global Climate Projections Chapter 10
represents a positive feedback by further reducing basal traction.
Unlike the one-time change in the idealised studies, this would
represent a sustained dynamical forcing that would prolong
the contribution to sea level rise. Grounding line retreat of the
ice streams has been observed recently at rates of up to about
1 km yr–1 (Rignot, 1998, 2001; Shepherd et al., 2002), but a
numerical model formulation is diffi cult to construct (Vieli and
Payne, 2005).
The majority of West Antarctic ice discharge is through the
ice streams that feed the Ross and Ronne-Filchner ice shelves,
but in these regions no accelerated fl ow causing thinning is
currently observed; on the contrary, they are thickening or near
balance (Zwally et al., 2005). Excluding these regions, and
likewise those parts of the East Antarctic Ice Sheet that drain
into the large Amery ice shelf, the total area of ice streams
(areas fl owing faster than 100 m yr–1) discharging directly into
the sea or via a small ice shelf is 270,000 km2. If all these areas
thinned at 2 m yr–1, the order of magnitude of the larger rates
observed in fast-fl owing areas of the Amundsen Sea sector
(Shepherd et al., 2001, 2002), the contribution to sea level rise
would be about 1.5 mm yr–1. This would require sustained
retreat simultaneously on many fronts, and should be taken as
an indicative upper limit for the 21st century (see also Section
10.6.5).
