Iceberg production at Ilulissat (Isfjord), where the Jakobshavns ice stream meets the ocean. The 150-km-long ice stream drains a large part of southern Greenland. Between 1996 and 2005, the front receded ten km (six miles), added onto 25 km of recession since the first recorded observations in the 1850s. Meanwhile the glacier's speed doubled from six to twelve km per year (more than four feet per hour), discharging some 23 cubic km of ice each year. In following years the speed of this and some other Greenland ice streams dropped back to normal. Much remains mysterious about ice streams in Greenland — and even vaster ones in Antarctica, which have also surged ominously. Observations and calculations raise a possibility that global warming could cause the ice discharges to become much greater, gravely accelerating sea-level rise in this century and beyond.

Photo ©2005 Spencer Weart.  Numbers from Rignot and Kanagratnam, Science 311 (2006): 986.