AbstractObservations between 1997 and 2001, of a 30% velocity increase and up to 60 m thinning of downstream parts of Jakobshavn Isbræ, Greenland, immediately following calving of about 4 km of its 15 km floating ice tongue, suggest that acceleration may have been initiated by the calving. Assuming that the force perturbation associated with such weakening is swiftly transmitted far up-glacier, I develop equations to estimate the perturbation. Initially, the observed changes are consistent with the comparatively small perturbation associated with the calving. Thereafter, it was probably sustained by thinning of the remaining ice tongue at rates of about 80 ma–1. Otherwise, the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned, with velocities dropping to their former values. The calculated force perturbation increases to a maximum about 10 km inland of the grounding line, consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards. Further inland, it progressively decreases, probably because marginal drag increased as the glacier accelerated. Both here and on the floating tongue, marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and/or fabric. More recent observations show continued acceleration and thinning, and most of the remaining ice tongue calved away in April 2003, so thinning is likely to continue.