Mass Balance and Accumulation Rates Along US ITASE Routes

Mass balance describes whether an ice sheet is growing or shrinking or staying the same size. For example, an ice sheet with a negative mass balance is getting smaller. Changes in mass balance occur over thousands of years because it takes that long for changes in snowfall, temperature or sea level to have an effect on these huge physical features. Measuring the mass balance of polar ice sheets is incredibly difficult because of their very large size and remote location.

Why is ice sheet mass balance important? As ice sheets get smaller, sea level rises. Scientists already know that sea level has risen about 4-10 inches (10-25 cm) over the last 100 years. We know that about half of this sea level rise has been caused by thermal expansion of the oceans, groundwater depletion and melting of small mountain glaciers. But what about the other half? Shrinking polar ice sheets are obvious potential contributors. Our objective as part of the ITASE program is to make measurements that will help us understand the role of ice sheets in affecting global sea level.

Over the last few years we have developed a technique for studying ice sheet mass balance by measuring their rate of thickening or thinning. If one were to follow the flow trajectory of a hypothetical snowflake that falls in the middle of the Antarctic Ice Sheet, one would see that it moves downward (as it is buried by newer snow) and outward towards the ocean (as the ice sheet flows by gravity down an inclined slope). For an ice sheet to maintain a constant thickness, the speed at which it moves downwards and outwards towards the ocean needs to be the same as the rate that new snow is added at the top. If one quantity is larger than the other, the ice sheet must be either thickening or thinning (so, for example, the ice sheet would be thinning if the vertical velocity is faster than the snow accumulation rate). In our experiment, we install special markers (see figure) just beneath the surface of the ice sheet and measure the vertical velocity of the upper layers over several years using very precise global positioning system (GPS) surveys. We compare these measured vertical velocities with the rate of snow accumulation, which comes from the study of layers in ice cores.

We call this experiment the coffee-can method because we used to use empty coffee cans (really! - we rummaged around in the camp trash to come up with something suitable and this was the best we could find) as the special markers. Nowadays we use wires instead of coffee cans but the name stuck. For the past three seasons, we have been installing 'coffee-can' sites (see figure) at the same places where the expedition stops to collect the long ice cores. At the end of each field season, we fly on a small ski-equipped aircraft (a Twin Otter) to remeasure the sites that were installed the season before. The vertical velocity of an ice sheet is very slow, so we have to wait at least a year for our markers to move far enough for us to be able to measure something. So far, results are available for sites that were established during the first and second ITASE seasons. A small amount of ice sheet thinning (about 10 cm/year) seems to be taking place at the sites near the head of Ice Stream D (a fast flowing outlet glacier that drains part of West Antarctica). At higher elevations on the ice sheet, close to the ice drainage divide, the ice sheet appears to be very close to balance, meaning that it is neither thickening nor thinning by any significant amount.

The coffee-can method is a great way of studying mass balance, but it only provides a local measurement where the markers are installed. Mass balance varies from location to location. There is no way we can make ground-based measurements over all of Antarctica, even by conducting ITASE traverses for decades! Earth-observing satellites, on the other hand, can collect measurements over Antarctica very quickly. NASA will be launching ICESat in December 2002 to study the mass balance of Greenland and Antarctica. This satellite will use a laser to measure the elevation of the ice sheet surfaces. By making repeat measurements of the same locations over the course of several years, the satellite will provide a record of ice sheet elevation changes.

Ice sheet elevation changes are almost the same as mass balance, but not quite. The problem is that ice sheet surface elevations change over short periods (from a few days to a few years) whereas mass balance changes over much longer time scales. There are two reasons for the short-term variability in ice sheet elevation. One is that the amount of snowfall varies a lot from year to year, although when averaged over a century it may be constant. If the satellite happens to conduct measurements during five particularly snowy years, this change might be interpreted as ice sheet thickening. Another reason for the variability is that the uppermost layers of an ice sheet are not actually ice but old snow (called firn) that compacts over a long period of time to become ice. We do not completely understand the rate at which this process occurs. If firn happens to compact unusually fast during the period of satellite measurements, we might mistakenly interpret this as ice sheet thinning.

We have designed an experiment to study these short-term processes of snowfall of firn compaction. The experiment is called RASCAL (for Remote Autonomous System for the Control of Altimetry) and involves installing a tower equipped with a variety of sensors, similar to what is pictured at the left, that will make continuous measurements of snow surface elevation changes (snowfall) and firn compaction. The results are stored in a data logger; we revisit the RASCAL sites by Twin Otter to download the data and service the equipment for another year of operation.

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