Understanding ice-sheet mass balance: progress in satellite altimetry and gravimetry

Satellite remote sensing has come to dominate the measurement of glacier and ice-sheet change. Three independent methods now exist for assessing ice-sheet mass balance and we focus on progress in two: satellite altimetry (ICESat) and gravimetry (GRACE). With improved spatial and temporal sampling, and synergy with ice flow measurements, both the mechanisms and causes changing mass balance can be investigated. We present examples of mass losses due to widespread, intensifying glacier dynamic thinning in northwest Greenland, but local ablation rates in the northeast that are unchanged for decades. Advances in GRACE processing reveal Greenland net ice-sheet mass loss continuing into 2010, at 195 +/- 30 Gt a(-1). A similarly negative trend in the Gulf of Alaska has significant spatial and temporal variation, that highlights the importance of intense summer melting here. Strong summer melt on the Antarctic Peninsula also precipitated recent ice-shelf collapse and prompted rapid dynamic thinning of tributary glaciers at up to 70 m a(-1). Thinning continued for years to decades after collapse and propagated far inland. While understanding of the physical mechanisms of change continues to improve, estimates of future behaviour, and in particular the near-future glacial sea-level contribution, still rely on projections from such observations. We introduce the suite of new sensors that will monitor the ice sheets into the future.