Ice thickness changes in Antarctica and their
consequences for globalsea level
Duncan Wingham and Andrew Shepherd
Antarctica: Growing or Shrinking?
It
has been remarked that the oldest scientific question in connection with
the Antarctic ice sheet is whether it is growing or shrinking. For decades
the question has been regarded as open. As recently as 1992 a systematic
accounting of the known mass fluxes concluded that the direct glaciological
evidence would allow the ice sheet to range from a 600 Gt/yr source to a
600 Gt/yr sink of oc
ean
water mass. Indirect evidence is also ambiguous: the 20th century riseglobal
sea level is easier to explain with a 360 Gt/yr contribution from the Antarctic
ice sheet; on the otherhand a body of relative sea level and geodetic evidence
prefers the notion that the mass exchange with the ocean is close to zero,
at least on the millenial scale, and in particular suggests that the Holocene
deglaciation was substantially complete several thousand years ago. The
limited observations are also unable to confirm or dismiss theoretical speculation
that small changes in the West Antarctic Ice Sheet may lead to its widespread
'collapse'
The vast majority of ice discharge from the Antarctic Ice Sheet is drained through a handful of fast moving ice-streams and outlet glaciers that are fed by relatively inactive catchment areas. A snapshot of ice-sheet elevation data derived from the ERS radar altimeter reveals the intricate pattern of these ice flows (right).This map of Antarctic balance velocities shows giant rivers of ice, up to 1000 kilometres in length, flowing from the interior of the ice sheet to the sea.
ERS: A decade of measurements
The
10-year series of observations
by the ERS satellites is beginning to shed new light on the state of the
Antarctic Ice Sheet, and the consequent impact upon global sea levels.
The precision with which we can now make satellite measurements allows
us to see the internal changes for the first time.
Long time-series of
ERS radar altimeter measurements (left) show that the average elevation
of the Antarctic Ice Sheet interior fell by 0.9 +/- 0.5 centimeters per
year from 1992 to 1996 (Science, October 1998). Allowing for natural variations
in snowfall, the imbalance of the ice sheet interior this century is only
0.06 +/- 0.08 of the mean accumulation rate. At most, Antarctica has had
only a modest (+/- 1-2 mm) impact upon global sea levels this century.
Extending this time-series of elevation change using future generations
of satellite altimeters (e.g. ENVISAT, CryoSat) will reveal more subtle
flow features and changes in the ice-sheet morphology.
Western Antarctica: Dynamic thinning
We
have shown, through close examination of altimeter measureme
nts
(e.g. Science, February 2001 )
that widespread inland thinning of the ice is occurring in the Amundsen
Sea sector of the WAIS (right), and correlation of its spatial pattern with
the flow of ice (b/w contours) determined using satellite radar interferometry
(SRI) leads to the view that the thinning has a time scale associated with
ice dynamics. Close examination of the point of flotation of the ice and
its velocity using SRI has shown that the ~ 100 Gt yr-1 of ice hitherto
thought on the basis of glaciological measurements to be accumulating in
the E. Antarctic interior is in fact being lost to the ocean through basal
melting, in keeping with the altimeter observations that find no detectable
change in the grounded East Antarctic ice sheet. In summary, the 20th century
Antarctic mass imbalance looks distinctly more negative than was previously
thought, easing the problem of closing the 20th century sea level budget
and the newly recognised importance of ice shelf melting emphasises mechanisms
that may allow a warming ocean to offset the expected 21st century growth
due to increased precipitation from the warming atmosphere.
Antarctica: The future
The presumably long-lived thinning observed in West Antarctica (below) raises the important question as to the past and future evolution of theice sheet. Is the thinning seen in West Antarctica a sign of its continued deglaciation (below left) or the result of more recent event such as the collapse of an ice-shelf (below centre) or a glacier surge (below right)? Whatever the case, will the thinning accelerate? A feature of the newly observed changes is that they are not captured by present-day computer models of the ice sheet evolution, almost certainly because the thinning depends on poorly known 'mesoscale' processes and geometry at the bed. Given that the Amundsen Sea sector of WAIS alone contains a water mass equivalent to 1.1 m of eustatic sea level, and that there are at least theoretical arguments as to why the thinning may accelerate, this present inablity to capture the largest ongoing changes in the inland Antarctic ice sheet is perhaps a matter of concern.
