The State of the Greenland and Antarctic Ice Sheets: Observations Lead the Way (Core Science Keynote)

Rather than relying on the heroic efforts of the past to acquire a few dozen field-based measurements at some of the Earth’s most remote locations, satellites now downlink to computer servers that crunch massive volumes of data to spew out dense time series, providing a whole new perspective on how ice sheets function. In the 1990s it was not clear whether the ice sheets were gaining mass.  This uncertainty has vanished now that the present constellation of spaceborne altimeters, gravimeters, and radars clearly indicates that both Greenland and Antarctica are losing mass at accelerating rates. Prior to such observations, outlet glaciers and ice streams were thought to respond slowly to climate change, with little change at timescales of months to decades. Over the last two decades, however, new observations have shown that while some glaciers flow at steady rates, a remarkable number have responded to recent warming with large variations in speed over periods as short as a few days. A wide variety of behavior is observed with some glaciers steadily gaining speed, others accelerating and then leveling off, and a handful varying substantially with no clear trend. The wealth of new satellite observations provides important information with which to understand such change, particularly when used in conjunction with ice-sheet models. Satellites do have limitations, however, creating a strong need for airborne instruments. For example, accurate projection of sea level with ice-sheet models requires knowledge of the subglacial topography, which can only be measured efficiently at present with airborne radar. Despite the great progress made with these new data, the contribution that ice dynamics will make to future sea level remains poorly understood, and removing this uncertainty represents a grand challenge for glaciology. The ongoing record of spaceborne measurements and parallel development of ice sheet models are the key components in addressing this challenge.