The NASA Program for Arctic Regional Climate Assessment (PARCA) has the prime objectives of measuring and understanding the mass balance of the Greenland ice sheet. It includes measurement of changes in ice sheet volume using different approaches, analysis of satellite and aircraft data to investigate various characteristics of the ice sheet, and in situ measurements aimed at understanding observed changes. The most important result from PARCA is the first accurate assessment of the mass balance of a polar ice sheet. Higher-elevation parts of the ice sheet are in overall balance, with local areas of quite rapid thickening or thinning, most of which could represent the effects of temporal variability in snow accumulation rates. Many coastal regions thinned considerably during the 1990s, with net losses from the ice sheet sufficient to raise global sea level by almost 10% of the total increase. There was coastal warming during this period, and a network of automatic weather stations (AWS) on the ice sheet measured a 2¿ warming for 1995--1999 compared to the 1950s. Nevertheless, observed ice-thinning rates of up to several meters per year cannot be explained by increased melting, indicating that discharge velocities must also have increased. In addition to the scientific results, major advances have been made in the use of satellite data for ice sheet research, particularly application of synthetic aperture radar (SAR) data to the measurement of ice velocity and identification and monitoring of glacier grounding lines. On the basis of these observations, PARCA focus has shifted to the coastal regions, with emphasis on surface ablation and its sensitivity to summer warming and possible albedo feedback, and on the dynamics of the glaciers that are observed to be changing rapidly. ¿ 2001 American Geophysical Union