We have assembled an 18-year velocity record for Jakobshavn Isbrae, Greenland.
From a 1985 speed of -7000 dyr, the glacier had slowed by -1000 dyr in 1992,
which coincided with independently observed thickening in the early 1990s’. The
glacier then sped up by -4000 dyr between 1997 and 2000, during which time other
measurements show rapid thinning’. From 2000 to 2003, the glacier’s floating ice
tongue almost entirely disintegrated, as speed increased to 12,600 dyr. If the
retreat of the ice tongue caused the acceleration, then similar losses of floating ice
tongues since the Little Ice Age may explain the current rapid thinning observed
for many of Greenland’s outlet glaciers.

Draining -6.5% of the ice sheet’s area, Jakobshavn Isbrae is Greenland’s largest
outlet glacier. Because of its contribution to the Greenland Ice Sheet’s mass balance, an
airborne laser altimeter has surveyed this glacier repeatedly since 1991. When many
other glaciers were thinning, these surveys revealed that Jakobshavn Isbra: thickened
substantially from 1991 to 1997 at elevations less than 1100 m. After 1997, the glacier
began thinning rapidly with peak rates of - 15 dyr’.