OCEAN SCIENCES
| Home | Science and Society | Science for Everyone | Ocean |
What Is Happening to the West Antarctic Ice Sheet?
Monitoring the West Antarctic Ice Sheet is similar to monitoring the pulse rate of the Earth's oceans, atmosphere, sea-level, and other dynamic systems.
by R. A. Bindschadler, NASA Goddard Space Flight Center, Greenbelt, Md., and D R. B. Alley, J. Anderson, S. Shipp, H. Borns, J. Fastook, S. Jacobs, C. F. Raymond, and C. A. Shuman
The West Antarctic Ice Sheet (WAIS) program addresses the West Antarctic as an environmental system (see figure 1). WAIS strives to answer a number of key questions that have directed our research in recent years that are related to the ice-surface elevation history, the ice-stream margins, onset areas, ice-stream modeling, mass input to the ice sheet, mass budgets beneath the ice shelves, and how WAIS can better educate the public and the scientific community.
| Figure 1 |
 |
Fig. 1. West Antarctica is an interactive, dynamic environment. The size of the ice sheet depends on snow accumulation, wind-driven reduction of glacial snow and ice, iceberg splitting, and subglacial melting and freezing. Under the floating ice shelves, circulating waters can drive melt rates in excess of 10 meters per year and freeze on large volumes of marine ice. The shape of the ice sheet depends on ice-flow, which varies greatly from the slow interior to the rapidly sliding ice streams. Subglacial water and till properties strongly influence where faster motion occurs. Ice domes and divides are the most stable locations for deposition and glacial archiving of samples of past atmosphere. Records of past ice-sheet extents are found in isolated mountains that are high enough to emerge from the ice-sheet surface and on the floor of the seas surrounding the ice sheet. |
Significant headway has been made toward reconstructing the ice sheet in the Ross Sea (see figure 2) during the last glacial maximum. Marine geological and geophysical data provide compelling evidence that both the East and West Antarctic ice sheets advanced across the continental shelf during the last glacial maximum. Shortly after post-glacial sea level began to rise, due initially to the retreat of Northern Hemisphere ice sheets, the East Antarctic ice sheet began to retreat (about 17,000 years ago), followed much later by the retreat of West Antarctica (about 10,000 years ago).
| Figure 2 |
 |
Fig. 2. Locations of West Antarctic features discussed in the text. Letters A-F refer to ice streams. Hatched line is edge of continental shelf. Dashed line is interior ice divide |
Retreating West Antarctic ice exposed the floor of the Ross Sea to examination by marine geologists. The seafloor consists of sedimentary deposits that are thicker with distance from the present ice sheet. Large troughs, tens of kilometers wide and carved into the seafloor, also contain sheets of till. These features indicate that grounded ice streams deformed the till as the ice sheet advanced seaward. The ice sheet expanded to within 150 km of the continental shelf edge in the western Ross Sea and to the shelf break in the eastern Ross Sea. Banks between the broad seafloor troughs are eroded and covered with little or no till, and significant subglacial meltwater appears to have been absent in most areas. Glacial marine sediments draped on top of the seafloor are very thin, which indicates that retreat from the continental shelf edge was rapid.
The volcanic record suggests that a continuous West Antarctic ice sheet first appeared approximately 9 million years ago. Since then many fluctuations have occurred, and there were many periods during which it resembled today's ice sheet. However, expansions during the last 1 million years of ice up to 550 m thicker are recorded on volcanoes near the coast. In the Executive Committee Range (see figure 2), lateral moraines up to 100 m above the present ice-sheet surface verify several greater heights during the last four glacial periods--the most recent of which was 20,000 years ago. In the near-coastal Ford Ranges, evidence of occupation by the ice sheet is present 800 m above the present ice surface.
A comparison between oxygen-isotopic records from Taylor Dome (East Antarctica) and Byrd Station (West Antarctica) suggests that the West Antarctic thinned several hundred meters during deglaciation over the last million years and another 200-400 m during the last 10,000 years. Similar techniques must be applied elsewhere within West Antarctica to establish the ice sheet's contribution to post-glacial sea-level rise. Ice coring at Siple Dome, begun in late 1997, is expected to contribute a detailed historical record of atmospheric chemistry and surface elevation.
Ice-stream margins are the boundary layers between the fast-moving ice stream and the much slower inter-ice-stream ice. Due to "lubrication" beneath the ice streams, the margins support a significant fraction of the gravitational force that drives ice flow. Margins can shift, probably in response to changing thermal and hydrological conditions at the base (figure 2). These changes in ice-stream width alter the balance of forces and the rate of ice discharge into the Ross Ice Shelf. Tracks of ice-stream margins carried onto the Ross Ice Shelf show that large changes in ice-stream widths, and probably discharge, occurred during the last thousand years. However, poorly lubricated "sticky" zones at the bed that seem to be unrelated to topography have been inferred beneath the outermost inactive margins of both C and D, against Siple Dome. Radar-reflectors placed in layers within the ice indicate that fast-streaming motion was prevented in spite of substantial melting and lubrication. This suggests that Siple Dome strongly resists streaming flow, has never been overridden by ice streams, and is maintained by stable boundaries associated with its subglacial geology.
Onset regions are where ice flow switches from slow in the thick, inland reservoir to fast in the thinner, marginal ice streams. If the onset were to migrate inland, acceleration and thinning of reservoir ice would reduce ice-sheet volume and raise sea level. Analysis of global positioning system (GPS) data pinpoint the onset of ice stream D (figure 2) by demonstrating a clear transition between driving forces and ice speed. Near the head of ice stream C, ice-stream flow occurs over a sedimentary basin where the ice-stream and basin margins are very close. Further downflow, ice streams tend to occupy subglacial troughs.
Most precipitation reaches West Antarctica across the coasts of the Bellingshausen and Amundsen Seas and to a lesser extent over the Ross Sea. A persistent low-pressure center off the coast in the southeast Pacific facilitates inland moisture transport and causes a peak in ice accumulation along the West Antarctic coast and at the interior divide. Accumulation decreases substantially away from these areas. Air masses with reduced moisture content then diverge, flow along the Transantarctic Mountains, and return to the coast. Important, but poorly defined, variations in ice accumulation at local areas affect the ability to calculate the regional mass accumulation. This remains the single largest error in calculating ice-sheet mass balance.
Ice shelves and floating glacial tongues comprise more than 30% of the West Antarctic area and receive nearly 40% of its precipitation. Ice shelves on both sides of the Antarctic Peninsula have retreated over recent decades, in concert with regional atmospheric warming. Steady advances of the ice fronts punctuated by large iceberg calving events appear to characterize West Antarctic ice shelves, although the junction between the grounded Pine Island Glacier and its ice shelf in Pine Island Bay recently retreated. More difficult to monitor are the bottom surfaces of ice shelves, which interact with the underlying ocean. Current melt rates are lower than were estimated several years ago, and complex melting and freezing patterns are underway. Similar work in the Amundsen Sea's Pine Island Bay suggests that the rate of melting is much higher than previously assumed for that region. It appears that the net melting occurring beneath Antarctic ice shelves (an area of 1.5 million square kilometers) exceeds the net snowfall over the entire ice sheet. The fate of the ice shelves in a warmer climate may hinge upon changes in the strength of the cold shelf-water circulation that now dominates the major embayments and the relatively warm deep-water circulation that now pervades the southeast Pacific.
To accurately represent the climate in the West Antarctic region in longer-term simulations of the ice sheet requires that GCMs adequately incorporate the critical climate interactions. Detailed experiments of the Antarctic region with fully coupled atmosphere-ocean-ice GCMs are still in the planning stage. Initial results will be qualified by the current lack of data from much of the region. The South Pacific is a particularly important area for data collection, both for constraining and initiating any GCM intended to produce accurate results for West Antarctica. Work is needed to contribute Antarctic-specific parameterizations for GCM experiments. Interaction between clouds and radiation is a particularly important process that is incorrectly parameterized for the current models.
How can WAIS better educate the public and the scientific community? Since the inception of WAIS, its researchers have participated in National Science Foundation programs such as Research Experiences for Undergraduates and Teachers Experiencing Antarctica. Several WAIS- related educational projects have been undertaken involving strong researcher/educator collaboration over a variety of topics and levels. Antarctic Science and Policy: Interdisciplinary Research Education (ASPIRE) uses the structure and implementation of the Antarctic Treaty to investigate social-, political-, and natural-science issues in undergraduate classrooms. The middle-school curriculum materials of GLACIER (Web site http://www.glacier.rice.edu) explore Earth science concepts through thematic modules centered on interdisciplinary questions.
Common themes in these programs are the use of the unique Antarctic environment as a vehicle to engage students in the process of science and the integration of real data with relevant issues facing the Antarctic and global communities.
Source: Eos, June 2, 1998, p. 257.
|
||||
| Home | Science and Society | Science for Everyone | Climate |
