A comparison of data recently obtained on the “ice plain” of Ice Stream Β with data collected on an International Geophysical Year (IGY) traverse suggests that between 1958 and 1984 the ice-stream grounding line retreated at an average rate of 0.8 km/year.

An IGY traverse, led by A.P. Crary in January 1958, crossed what we (Reference Shabtaie and BentleyShabtaie and Bentley, 1987) now know to be the ice-stream grounding line near 83°23′S, 163°24′E. On that traverse, surface elevations were carefully measured by barometric altimetry (Reference Crary, Robinson, Bennett and BoydCrary and others, 1962). Comparison with elevations on the floating ice of the Ross Ice Shelf recently determined by satellite altimetry (Reference Shabtaie and BentleyShabtaie and Bentley, 1987) shows that the IGY traverse elevations were accurate within about ±3 m. Between traverse stations 28.11 and 29, Reference Crary, Robinson, Bennett and BoydCrary and others (1962) showed a pronounced surface-elevation drop of more than 30 m. AlthoughCReference Crary, Robinson, Bennett and Boyd rary and others (1962) believed, on the basis of seismic measurements, that all the ice was afloat, recent satellite determinations show that the higher-standing ice is grounded (Reference Shabtaie and BentleyShabtaie and Bentley, 1987).

The recent position of the grounding line can be determined within a few kilometers from data collected on radar-sounding flights in December 1984 and December 1985, specifically surface elevations compared with ice thicknesses, and changes in ice-bottom reflection characteristics. The position of the grounding line on Crary's traverse can then be estimated to within a few kilometers by comparison of both absolute elevations and surface slopes between the old and new profiles. That position for the 1958 grounding line is supported by the traverse log (Reference Crary, Robinson, Bennett and BoydCrary and others, 1962), which shows it to be the limit north of which crevasses were no longer encountered. The old and new positions of the grounding line are separated by 22 ± 5 km.

Changing surface elevations show a similar behavior. The ice surface is now lower by an amount that varies from no more than a few meters on the floating ice distant from the grounding line to 10–20 m on the grounded ice. In particular, absolute elevations at a station near the grounding line, where the ice was grounded throughout the 1958–85 period, show a drop of 15 ± 8 m. That decrease is consistent with the calculation by Reference Shabtaie and BentleyShabtaie and Bentley (1987) for the “ice plain” of Ice Stream B: 0.39 ± 0.29 m/ year. We conclude that there has been a regional thinning of the ice on the order of 0.6 m/year over the last three decades.