This is the only margin of advance of the sheet. During
glaciation, the East Antarctic ice sheet is therefore slightly
thicker at the edges but this effect (confirmed by glaciation
tracks on the mountains) remains confined to the edges. The
central regions are thinner due to lower accumulation. It is
estimated that the Antarctic shelf was about 100 m lower
during the Last Glacial Maximum (Ritz et al. 2001). As for
volume, the impact of the central regions is stronger and
overall the volume of East Antarctica is less in glacial period
than in interglacial.
West Antarctica has much more room to spread out
(Fig. 24.8). Indeed, the two large embayments in which the
ice shelves of Ross and Ronne-Filchner are located are
shallow.
During glacial periods, these ice-shelves are grounded,
advancing the grounding line by about one thousand kilometers. Here the effect of the grounding line far outweighs
that of precipitation. This is why West Antarctica is more
voluminous during glacial periods. However, there is still
much debate about the exact volume. Estimates for the total
contribution of the Antarctic to sea-level between the glacial
maximum and now range from 7 m to over 20 m (sea level
equivalent). These extremes correspond to two possible
scenarios in West Antarctica during glaciation. For the same
grounding line position, it is possible to have relatively
stagnant ice streams and a very thick ice sheet at the edges,
similar to the edges of East Antarctica currently. However, it
could be that regions where there are ice shelves currently
were very active, formed from huge ice streams comparable
to the Sipple Coast in West Antarctica. These regions must
have been relatively flat, which had repercussions as far as
the center of West Antarctica. The first hypothesis was long
supported by the trimlines which indicated altitudes at least
1000 m above the current level, but recent datings of these
trimlines have in many places indicated that they are more
than one million years old and therefore do not concern the
Last Glacial Maximum (Bentley et al. 2010). Alternatively,
the second hypothesis is supported by the ice core analysis
from the Sipple Dome (the Ross ice-shelf slope) and Berkner
Island (middle of the Ronne-Filchner ice-shelf).
For the time being, numerical modeling is the only tool to
evaluate the evolution of Antarctic geometry during the
recent glacial-interglacial cycles. Only a few polar cap
models have tackled this problem because simulation of the
movements of the grounding line remains a major difficulty.
It should be noted that the mechanisms presented (which are
included in the models) accurately reproduce the evolutions
described above, that not all interglacials were similar (some
were less frozen, like stage 11 or Eemian, others were
intermediate between glacial and interglacial periods) and
that the freezing-up process is slow, the volume increasing
progressively throughout the glacial period. As for the last
deglaciation, it appears to have occurred late (15 ka BP), but
its speed depends on the model used (Huybrechts EPILOG
2002; Ritz et al. 2001), as well as on the volume of
Antarctica ice during the glacial period. The speed of
deglaciation depends on the model, but all agree that it has
only just finished, which is in accordance with observations
showing that the retreat of the grounding line in the Ross Sea
continued until 3000 years BP (Conway et al. 1999).
Greenland
In terms of process, Greenland is an intermediate ice sheet.
During the glacial periods, ablation was negligible there and
its expanse was limited by the movement of the grounding
line, in other words, it was limited by the continental slope.
During interglacials, ablation plays an important role, as
evidenced by the fact that its current edge is mostly terrestrial (there can be no coastline without ablation). This makes
Greenland sensitive to warming climates and explains why it
is assumed that this cap was significantly smaller during the
Eemian. In terms of shape, Greenland changed from domed
during interglacial (high and narrow) to flat and expanded
during glacial periods.
Conclusions
The climate system is complex and the long-term component
which includes sea level and the evolution of the polar ice
sheets is no exception to this rule. In this chapter, we have
seen that the mechanisms involved can be internal to the ice
sheets such as thermomechanical coupling and feedbacks
related to subglacial hydrology. However, most of the processes are related to interactions with other elements of the
system:
– the atmosphere, due to the altitude-surface temperature
link which causes the instability of small ice sheets. The
influence of the ice sheets on the general circulation is
also noted and we have seen that it could explain the
interactions between the ice sheets;
– the ocean, since the ice sheets determine sea level, but
also the oceanic general circulation. In the other direction,
the movement of the grounding line, which is the key
process of the Antarctic evolution, results from interaction between glacial dynamics and the local ocean;
– the solid Earth, through the mechanism of isostasy.
The data available for the past, in terms of sea level or
through glacio-geomorphology reconstructions, indicate that
all the mechanisms mentioned above are indeed active.
316
C. Ritz et al.
glaciation, the East Antarctic ice sheet is therefore slightly
thicker at the edges but this effect (confirmed by glaciation
tracks on the mountains) remains confined to the edges. The
central regions are thinner due to lower accumulation. It is
estimated that the Antarctic shelf was about 100 m lower
during the Last Glacial Maximum (Ritz et al. 2001). As for
volume, the impact of the central regions is stronger and
overall the volume of East Antarctica is less in glacial period
than in interglacial.
West Antarctica has much more room to spread out
(Fig. 24.8). Indeed, the two large embayments in which the
ice shelves of Ross and Ronne-Filchner are located are
shallow.
During glacial periods, these ice-shelves are grounded,
advancing the grounding line by about one thousand kilometers. Here the effect of the grounding line far outweighs
that of precipitation. This is why West Antarctica is more
voluminous during glacial periods. However, there is still
much debate about the exact volume. Estimates for the total
contribution of the Antarctic to sea-level between the glacial
maximum and now range from 7 m to over 20 m (sea level
equivalent). These extremes correspond to two possible
scenarios in West Antarctica during glaciation. For the same
grounding line position, it is possible to have relatively
stagnant ice streams and a very thick ice sheet at the edges,
similar to the edges of East Antarctica currently. However, it
could be that regions where there are ice shelves currently
were very active, formed from huge ice streams comparable
to the Sipple Coast in West Antarctica. These regions must
have been relatively flat, which had repercussions as far as
the center of West Antarctica. The first hypothesis was long
supported by the trimlines which indicated altitudes at least
1000 m above the current level, but recent datings of these
trimlines have in many places indicated that they are more
than one million years old and therefore do not concern the
Last Glacial Maximum (Bentley et al. 2010). Alternatively,
the second hypothesis is supported by the ice core analysis
from the Sipple Dome (the Ross ice-shelf slope) and Berkner
Island (middle of the Ronne-Filchner ice-shelf).
For the time being, numerical modeling is the only tool to
evaluate the evolution of Antarctic geometry during the
recent glacial-interglacial cycles. Only a few polar cap
models have tackled this problem because simulation of the
movements of the grounding line remains a major difficulty.
It should be noted that the mechanisms presented (which are
included in the models) accurately reproduce the evolutions
described above, that not all interglacials were similar (some
were less frozen, like stage 11 or Eemian, others were
intermediate between glacial and interglacial periods) and
that the freezing-up process is slow, the volume increasing
progressively throughout the glacial period. As for the last
deglaciation, it appears to have occurred late (15 ka BP), but
its speed depends on the model used (Huybrechts EPILOG
2002; Ritz et al. 2001), as well as on the volume of
Antarctica ice during the glacial period. The speed of
deglaciation depends on the model, but all agree that it has
only just finished, which is in accordance with observations
showing that the retreat of the grounding line in the Ross Sea
continued until 3000 years BP (Conway et al. 1999).
Greenland
In terms of process, Greenland is an intermediate ice sheet.
During the glacial periods, ablation was negligible there and
its expanse was limited by the movement of the grounding
line, in other words, it was limited by the continental slope.
During interglacials, ablation plays an important role, as
evidenced by the fact that its current edge is mostly terrestrial (there can be no coastline without ablation). This makes
Greenland sensitive to warming climates and explains why it
is assumed that this cap was significantly smaller during the
Eemian. In terms of shape, Greenland changed from domed
during interglacial (high and narrow) to flat and expanded
during glacial periods.
Conclusions
The climate system is complex and the long-term component
which includes sea level and the evolution of the polar ice
sheets is no exception to this rule. In this chapter, we have
seen that the mechanisms involved can be internal to the ice
sheets such as thermomechanical coupling and feedbacks
related to subglacial hydrology. However, most of the processes are related to interactions with other elements of the
system:
– the atmosphere, due to the altitude-surface temperature
link which causes the instability of small ice sheets. The
influence of the ice sheets on the general circulation is
also noted and we have seen that it could explain the
interactions between the ice sheets;
– the ocean, since the ice sheets determine sea level, but
also the oceanic general circulation. In the other direction,
the movement of the grounding line, which is the key
process of the Antarctic evolution, results from interaction between glacial dynamics and the local ocean;
– the solid Earth, through the mechanism of isostasy.
The data available for the past, in terms of sea level or
through glacio-geomorphology reconstructions, indicate that
all the mechanisms mentioned above are indeed active.
316
C. Ritz et al.