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SCAR Bulletin 146, July 2002

Antarctic Ice Sheet Mass Balance and Sea Level (ISMASS)
Brief Report of a Workshop

A workshop on Antarctic Ice Sheet Mass Balance and Sea Level (ISMASS) was held in Annapolis, United States, 10 - 14 June 2001. The aim of the workshop was to review the current state of knowledge and identify those methods and techniques that might be used towards improving that knowledge and forecast possible changes in the future.

In the last 5 years, the picture of a slowly changing Antarctic ice sheet has radically altered. It is now realised that ice shelf basal melting may account for up to one third of the loss from the grounded ice; extensive, rapid thinning is occurring in one part of the West Antarctic ice sheet interior; and the collapse of the Antarctic Peninsula ice shelves is accelerating the grounded ice discharge. These discoveries inject a new sense of urgency into gaining a better understanding of the evolution of the ice sheet. Will the expected warming of the ocean adjacent to the ice shelves alter the basal melting rates, and what is the consequence for the marine based ice sheet? Is the thinning seen in the Pine Island and Thwaites regions of West Antarctica a sign of its continued deglaciation? If so, will the thinning accelerate? These questions also make a matter of present concern the sensitivity of numerical deglaciation models to the treatment of the grounding line. Assuming the present thinning in Pine Island is indeed a deglaciation signal, how confident can we be that today's numerical models can capture its future evolution?

While the Antarctic contribution to 20th century sea level rise of 1.8 mm yr-1 is uncertain by at least 1.2 mm yr-1, glaciological evidence has favoured a growing ice sheet. Atmospheric warming in the coming century may lock up a further 10 cm of ocean volume by 2100 due to increased precipitation. The new discoveries by satellite interferometry have shown that the ice previously thought to be accumulating in the interior of East Antarctica is probably being lost to the ocean through basal melting. This conforms with the radar altimeter observations that find no recent detectable change in the grounded East Antarctic ice sheet. Satellite interferometry has also demonstrated that the flow of numerous grounded glaciers of the Antarctic Peninsula has accelerated in response to the collapse of the Prince Gustav Channel and Larsen Ice Shelves. In summary, the 20th century Antarctic mass imbalance looks distinctly more negative than before and the newly recognised importance of ice shelf melting now emphasises mechanisms that may offset the 21st century expected growth due to global warming.

The determination of growth or shrinkage of the great ice sheets is the oldest scientific problem of Earth's polar regions. Today, the issue of ice mass balance has renewed urgency because of the role of grounded ice in sea level change. In fact, the significant variations in sea level over the past million years have been controlled by ice, and it is clear that the response of the ice sheets to climate change in the immediate future could significantly alter sea level. This issue is especially relevant at this time because the prediction of global sea level change is of practical concern. Recent observations of the ice sheet have discovered unexpected change in ice stream velocities as well as ice shelf collapse. Theoretical analysis of ice sheet response to climate change has indicated a wide range of outcomes on different time scales under different climate change scenarios. New technologies have resulted in a significant increase in the ability to observe and model ice sheet properties and processes. Recognizing the likelihood and potential of ice sheet change, SCAR-GLOCHANT has established the ISMASS project to examine and report on the study of the ice mass balance of Antarctica. This document addresses a strategy for ISMASS to result in a meaningful international scientific approach to understanding and predicting Antarctic ice sheet mass balance.

Model comparison

Recommendations

The following needs were recognized:

  1. the separation of short-term (< 30 yr) vs long-term (> 30 yr) surface elevation change; this requires knowledge of the accumulation rate, or, failing this, a statistical characterisation of its covariance function, over the last 10 years how that relates to the long-term trend.
  2. understanding how much of the fh/ft signal measured by satellite altimeters is related to the compaction of the firn, for annual and longer time-scales, through field experiments, particularly in West Antarctica.
  3. determining the extent to which the fh/ft signal measured by radar altimeters is affected by interactions of the microwave pulse with variations in the near surface layers, by a field experiment in central E. Antarctica that is continuously occupied.
  4. establishing a reasonably permanent site where surface elevation change is presently occurring, where firn conditions are reasonably typical, and where continuous measurements will allow the temporal variation to be examined on seasonal and longer time-scales.

Recommended Future Research

1. Surface Elevation Change Maps (dH/dt): Observation and Modelling of Current Changes

There are two basic approaches to measuring the mass balance of the ice sheet. One is an integrated approach, i.e. a measurement of its mass changes without separately determining the input and output fluxes. The other is a component (or flux) approach, in which the input and output fluxes are individually measured; this approach is particularly important when applied to individual drainage systems within the ice sheet. Both approaches are important for obtaining not only a simple measurement of mass change, but also an understanding of what is causing that change. Furthermore, the two approaches are largely independent and thus complement each other.

Observation of Surface Elevation Changes

Modelling of Surface Elevation Changes

2. Surface Mass Balance

The main input component of ice sheet mass balance is the net accumulation of snow at the surface. Large gaps in observations mean any estimate of the current mass input has a large error.

Recommendations

3. Mass output (ice dynamics, fluxes, melt/freeze, and calving)

New satellite remote sensing data have led to major advances in our current knowledge of ice flow dynamics, coastal fluxes, and inferred bottom melting underneath ice shelves. From these data, we learned that major changes are taking place at specific locations in the Antarctic on much shorter time scales than previously anticipated.

Major advances in recent years:

What the near future holds:

What the major gaps are:

Recommendations:

Focused research is required on the large outlet glaciers draining West and East Antarctica and the Antarctic Peninsula. Specific requirements are for:

4. Special Areas

Satellite remote sensing and the much-improved logistic support provided by the National Antarctic operators has allowed investigators to access much of the Antarctic continent during the last decade and this coverage should continue in the future to provide direct support of mass balance calculations. However, several areas require increased attention.

Amundsen Sea Embayment

We recommend a programme of fieldwork in the part of the West Antarctic ice sheet draining into the Amundsen Sea to include measurement of fluctuations in recent accumulation rates, measurement of ice-sheet thickness and a characterisation of sub-glacial conditions and ice flow velocities.

Antarctic Peninsula

We recommend field monitoring of climate, ice caps, glaciers and ice shelves, and especially an investigation of the grounded glaciers and ice caps affected by loss of ice shelves. We recommend continued monitoring by remote sensing and the initiation of modelling studies at a variety of spatial scales.

Coastal Antarctica

We recommend that ice cores covering late-glacial to decadal time-scales be collected in Antarctica, particularly from coastal sites, to provide Holocene histories of accumulation rates, constraints on deglaciation, and an envelope for future variability of accumulation and dynamic changes in these sensitive areas.

5. Deglaciation

Observational Basis

The observational basis of de-glaciation constitutes geological observations of former ice sheet extent, isostatic recovery and possibly information within isochrone layer architecture.

The glaciation signal comprises glacial sediments and glacially-moulded landforms that typically do not contain organic deposits and cannot be dated directly. The glaciation signal comprises material (sediments, animal remains, etc.) overlying glacial deposits that can yield radiocarbon dates and thus minimum ages of deglaciation. Other sources of information about the palaeogeometry and dynamics of ice sheets lie in the isochrone layer architecture, ice core records and in observations of current isostatic response.

Recommendations

We obtain further dated retreat sequences from the continental shelf (including under ice shelves) with priorities to areas with current rapid change. To improve dating we need more research on the physical basis of the radiocarbon reservoir effect. We encourage more systematic deployment of radars to obtain layer architecture.

Physical Basis

Model studies suggest that the present-day imbalance is very sensitive to the choice of bedrock parameters. This implies that it is crucial to validate bedrock properties, known to be different in the West and East Antarctic plates, by field measurements. Improved models of the geodynamical properties of the ice sheet will discriminate between deglaciation and short-term processes for the observed retreat in West Antarctica.

It is unlikely that the Antarctic ice sheet has reached thermal equilibrium following the warming at the end of the last glacial maximum. This means that the viscosity and areas where sliding is occurring are changing.

Recommendations

The following activities will substantially improve the physical basis of models of past and future deglaciation:

Acronyms

AWS Automatic Weather Station
Cryosat Cryosphere [observation] Satellite
ENVISAT Environmental [observation] Satellite
ERS Earth Resources Satellite
GLOCHANT Group of Specialists on Global Change and the Antarctic
GPS Global Positioning System
GRACE gravity/mass-change sensing
ICESat Ice, Cloud and Land Elevation Satellite
InSAR Interferometric Synthetic Aperture Radar
ISMASS Antarctic Ice Sheet Mass Balance and Sea Level
NASA National Aeronautical and Space Administration
NSIDC National Snow and Ice Data Center
Radarsat AMM Radarsat Antarctic Mapping Mission
SCAR Scientific Committee on Antarctic Research