Introduction

Tide-gauge measurements indicate that sea level has risen by about 15 ± 5 cm over the past century, with perhaps 7.5 ± 5 cm of this rise caused by effects other than changes in the polar ice sheets (IPCC, 2001). The missing 7.5 ± 7 cm was most probably caused by net losses from the Greenland and Antarctic ice sheets at an average rate of about 300 ± 280 km3 of ice per year. This is equivalent to 12 ± 11% of their combined annual snow accumulation, and would represent a thinning rate of 2 ± 2 cm per year averaged over the entire area of both ice sheets. Although the uncertainty of this estimate is quite large, it is far lower than that resulting from many decades of glaciological observations, which have yet to yield even the sign of the collective mass balance of these two ice sheets, with an uncertainty equivalent to about 20% of their combined snow-accumulation rate. Until recently, this level of uncertainty applied equally to both ice sheets, and provided the prime motivation for NASA's Program for Arctic Regional Climate Assessment (PARCA) which had, as its initial goal, measurement of the mass balance of the Greenland ice sheet. To a large extent, this goal has been achieved, and PARCA has significantly improved our knowledge of many of the factors that determine the mass balance.