The Program for Arctic Regional Climate Assessment (PARCA) was formally initiated in 1995 by combining into one coordinated program various investigations associated with efforts, started in 1991, to assess whether airborne laser altimetry could be applied to measure ice-sheet thickness changes. It has the prime goal of measuring and understanding the mass balance of the Greenland ice sheet, with a view to assessing its present and possible future impact on sea level. It includes: · Airborne laser-altimetry surveys along precise repeat tracks across all major ice drainage basins, in order to measure changes in ice-surface elevation. · Ice thickness measurements along the same flight lines. · Shallow ice cores at many locations to infer snow-accumulation rates and their spatial and interannual variability, recent climate history, and atmospheric chemistry. · Estimating snow-accumulation rates from atmospheric model diagnosis of precipitation rates from winds and moisture amounts given by European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. · Surface-based measurements of ice motion at 30-km intervals approximately along the 2000-m contour completely around the ice sheet, in order to calculate total ice discharge for comparison with total snow accumulation, and thus to infer the mass balance of most of the ice sheet. · Local measurements of ice thickness changes in shallow drill holes ("dh/dt" sites in Figure 1). · Investigations of individual glaciers and ice streams responsible for much of the outflow from the ice sheet. · Monitoring of surface characteristics of the ice sheet using satellite radar altimetry, Synthetic Aperture Radar (SAR), passive-microwave, scatterometer and visible and infrared data. · Investigations of surface energy balance and factors affecting snow accumulation and surface ablation. · Continuous monitoring of crustal motion using global positioning system (GPS) receivers at coastal sites.