Global sea level is affected by numerous factors, including human contributions from groundwater pumping and dam building, and the growth and decay of mountain glaciers and polar ice sheets. Research into Antarctic ice mass balance addresses a key unknown in understanding the global sea level budget - is the Antarctic ice sheet growing, stable, or shrinking? Answering this is critical to understanding how sea level is presently changing and how it may change in the future.

An ice sheet that is gaining or losing mass would exert a changing force on the surface of the Earth, a force the Earth would respond to, in the same way that a spring scale responds to a weight. With colleagues from the Jet Propulsion Laboratory, we have been examining how modern geodetic techniques, such as the Global Positioning System (GPS), might be used to measure this response. The figure shows the elastic crustal response (vertical motion) to three differing, but realistic, scenarios of present day Antarctic mass balance (scenario 1, scenario 2, and J92 scenario) based on assessments of the available glaciological and oceanographic data. They give very different predictions of the vertical crustal response due to these changing surface loads, suggesting that GPS observations could help constrain present day mass balance.

Vertical crustal velocities for three scenarios of present Antarctic ice mass change and for a scenario of past ice mass change, the ICE-3G postglacial rebound model of Tushingham and Peltier (1991). Note the differing scale for ICE-3G. Scenario 1 contributes -0.1 mm/yr to sea level rise; scenario 2, -1.1 mm/yr; and J92 scenario, 0.45 mm/yr. This figure appeared in a recent article (Geophysical Research Letters, 22, 973-976, 1995). larger image [GIF, 70.0 kb, 595 X 696, notice]

Vertical crustal velocities for three scenarios of present Antarctic ice mass change and for a scenario of past ice mass change, the ICE-3G postglacial rebound model of Tushingham and Peltier (1991). Note the differing scale for ICE-3G. Scenario 1 contributes -0.1 mm/yr to sea level rise; scenario 2, -1.1 mm/yr; and J92 scenario, 0.45 mm/yr. This figure appeared in a recent article (Geophysical Research Letters, 22, 973-976, 1995).

However, postglacial rebound is also occurring in Antarctica and is potentially quite large, as shown in the figure for the ICE-3G postglacial rebound model. It will need to be considered when interpreting future GPS-based crustal motion observations in Antarctica. The size and location of postglacial uplift in Antarctica depends on the timing, magnitude, and location of past Antarctic ice mass changes, all of which are rather poorly known, although Antarctica likely contributed 20-30 m to sea level rise since about 20,000 years ago. Determining the past mass balance of Antarctica is critical to interpreting the history of sea level changes worldwide.