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.
- compare existing models of Antarctica that focus on grounding line retreat;
- investigate stability issues by analytical methods and using models with simplified boundary conditions;
- develop models that can assimilate geological and dynamical data;
- encourage investigation that couple ice sheet models with atmosphere and ocean models.
- increase synthesis of data and communication between the field and satellite observation and modelling groups
The following needs were recognized:
- 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.
- 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.
- 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.
- 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
- ICESat satellite altimetry (plus radar altimetry from ERS, ENVISAT, Cryosat
- GRACE gravity/mass-change sensing for improvements in post-glacial rebound corrections
- GPS point calibrations on bedrock for post-glacial rebound models
- current surface temperature maps (monthly) for firn-compaction models
- continued/expanded network of AWS sonic measurements (short-term dH/dt) and GPS/coffee-can (long-term dH/dt)
- improved tide modelling
- coordination on ICESat data availability and analysis between NASA ICESat Project/NSIDC and SCAR nations' field-based research projects
- continuous (or near-continuous) satellite altimeter observations of surface elevation changes for 15-20 years
Modelling of Surface Elevation Changes
- state of results from large-scale models showing differences in sign of current change
- recommendation for detailed model inter-comparison
- need large-scale and regional models to help understand causes of observed changes
- improved parameterization/coupling of ice stream flow and ice-shelf interactions in models
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.
- What is the mean annual input to the Antarctic Ice Sheet and how is it distributed over drainage basins?
- Time variability shorter than one year must be assessed at selected sites. Observations of time variability of precipitation and accumulation of these (e.g. stakes farm, AWS, gauges), are needed to improve atmospheric models and the interpretation of satellite altimetry.
- Observations and models on inter-annual to centennial time scales (e.g. cores and meteorological time series) are important to detect current and predict future changes.
- Surface balance is known to vary strongly in space. Representative observations are important to estimate local (< 10 km) re-distribution processes, for interpolation of data and satellite ground truth. Precipitation distribution and redistribution process at scales of 10 km and more are required to build the high-resolution continental-scale surface mass balance map necessary for atmospheric and ice sheet modelling.
- To generate continent-wide surface accumulation values with spatial (10 km) and multi-annual (5-20 yr) resolution require different approaches (traverse Ground Penetrating Radar and Global Positioning Position, cores, satellites, numerical simulation, re-analysis of previous snow accumulation data). Trend of snow accumulation at the century scale (200-500 yr) at selected sites is required.
- To characterise spatial and temporal variability and covariance, comparisons are required over local (<10 km) and seasonal (<1 yr) scales at selected sites, of precipitation data from atmospheric models, field measurements (AWS + farm stakes) and remote sensing observations.
- Field observations and modelling of atmosphere/cryosphere processes are required to estimate snow redistribution and export to the ocean, sublimation, densification, and metamorphism processes.
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:
- Continental-scale mapping of balance velocity and topography.
- Observations of ice velocity on large outlet glaciers and Siple Coast ice streams.
- Ice-shelf disintegration and outlet glacier acceleration in Antarctic Peninsula.
- Flow acceleration of Pine Island Glacier and flow widening of Thwaites Glacier.
- Strongly negative mass balance of the Pine Island and Thwaites glacier basin.
- Prior-estimates of large positive mass balance of Pine Island Glacier and Lambert Glacier Basin not confirmed by recent studies, hence moving the current estimates of the mass balance of Antarctic Ice Sheet more firmly toward zero or negative.
- Large basal melting inferred near grounding zones.
What the near future holds:
- Continental-scale InSAR mapping of ice velocity with Radarsat AMM-2, and ERS-1/2.
- Observations of ice-shelf mass balance from ICESat and better definition of ice-shelf topography to improve grounding-line flux estimates.
- Improved understanding of ice dynamics from computer models that incorporate unprecedented, detailed remote sensing data.
What the major gaps are:
- Ice thickness along coast and over specific basins.
- Continuation of InSAR missions beyond ERS and Radarsat to map ice velocity changes and fill in gaps in past InSAR coverage.
- Direct measurements of basal melt, its spatial and temporal variability, underneath floating ice shelves.
- Collection of GPS control velocity for InSAR flow mapping.
- Characterization of bed conditions, ice temperature, fabrics, etc. to constrain ice flow models.
Focused research is required on the large outlet glaciers draining West and East Antarctica and the Antarctic Peninsula. Specific requirements are for:
- Measuring ice thickness in detail near the grounding zones that have now been identified with Interferometric Synthetic Aperture Radar (InSAR).
- Observing ice shelf bottom melting, especially in the proximity of grounding zones, and investigating if basal melting is changing with time in response to changes in ocean conditions.
- Measuring, using InSAR and ICESat, changes in ice velocity and elevation of the glacier surface.
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.
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.
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.
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.
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.
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.
The following activities will substantially improve the physical basis of models of past and future deglaciation:
- Validation of geodynamical parameters
- Mapping of basal interface character (warm/cold) till/rock
- Use of natural high frequency forcing (e.g. tidal forcing of water pressure) to test hypotheses about basal processes
- Measurement of internal temperature fields using drill holes or remote probes
- Continuing effort in laboratory ice mechanics and geotechnical experiments for temperatures close to pressure melting point, and for ice containing substantial impurities.
|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|
|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|