Ice with everything - Antarctica: Global Science from a Frozen Continent - page 100

Lake Vostok is an iconic image for such basal water; the analysis of the deepest
part of the Vostok ice core has demonstrated that the bottom part of the ice sheet
is locally formed by the refreezing of subglacial lake water. Most of the almost 200
subglacial lakes identi

ed to date are located in the interior of the ice sheet, within
100 km of the ice divides and ice domes, and most of them are less than 20 km in
length. Their volume is about a quarter of the total water in continental surface lakes.
Satellite imagery has been used to identify the distribution of Antarctic subglacial
water bodies, including dozens of lakes and rivers. Simultaneous rise and fall of
surface elevations at several places on the Antarctic ice sheet surface have revealed
that there is extensive water movement below the ice from one year to the next, and
that the ice sheet morphology reacts to the drainage and flow of subglacial water.

Such a large-scale drainage network is expected to modify the lubrication of ice sheet
beds and the ice flow. Catastrophic drainage of subglacial lakes could be closely

related to the onset of rapid ice stream flow. It is a challenge to characterise the

subglacial Antarctic water systems and their dynamics. Data suggest that some of
these lakes may have a turnover rate of several thousand years, up to hundreds of
thousands of years for large lakes such as Lake Vostok. Catastrophic drainage of
some lakes could have delivered large volumes of melt water to the Southern Oceans.
The amount of water is not signi cant in terms of global sea level, but it may play an

important role in triggering rapid ice flow and its associated contribution to sea level

rise. Ocean modelling suggests that the formation of deep water in the Southern
Ocean may be sensitive to large freshwater supplies, for instance in the Ross
Embayment.
Antarctica could be the sleeping giant of our climate system. Lessons from the
past have shown that high levels of atmospheric greenhouse gas composition were
associated, a few tens of million years ago, with a planet free of continental ice. They
have also revealed that northern hemisphere maritime ice sheets of the last
glaciations have undergone surge instabilities associated with abrupt ocean
circulation and climate changes. The most recent observations have, however,
revealed that the giant could wake up faster than expected. The presence of an active
hydraulic subglacial system suggests the potential for rapid changes in ice ow.

Recent measurements suggest that Antarctica is now losing mass, mostly due to
basal melt below West Antarctic ice shelves and increasing rates of ice ow down to

the sea. Glaciologists face numerous challenges to monitor this huge mass of ice, to
invent new methods to decipher the continental and marine subglacial processes,
and to produce a new generation of ice sheet models able to resolve the ice streams
and ice shelves.
Today, Antarctica has started to contribute to global sea-level rise. Have we
wakened the sleeping giant? How much of the observed reactions are caused by
anthropogenic interference with natural variability in global processes? What is