DAVID M. HOLLAND* (New York University, New York, NY 10012, U.S.A.), ROBERT H. THOMAS (EG&G Services, Wallops Flight Facility, Virginia 23337, U.S.A.), BRAD DE YOUNG (Memorial University, St. John’s A1B 3X7, Canada), MADS H. RIBERGAARD (Danish Meteorological Institute, Copenhagen DK-2100, Denmark), and BJARNE LYBERTH (Greenland Institute of Natural Resources, Nuuk 3900, Greenland)

*e-mail: holland@cims.nyu.edu

Observations over the past decades show a rapid acceleration of several outlet glaciers in Greenland and Antarctica[1]. One of the largest changes is a sudden switch of Jakobshavn Isbræ, a large outlet glacier feeding a deep-ocean fjord on Greenland’s west coast, from slow thickening to rapid thinning[2] in 1997, associated with a doubling in glacier velocity[3]. Suggested explanations for the speed-up of Jakobshavn Isbræ include increased lubrication of the ice–bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers[4] and weakening and break-up of the floating ice tongue that buttressed the glacier[5]. Here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of Greenland, suggesting that the changes in Jakobshavn Isbræ were instead triggered by the arrival of relatively warm water originating from the Irminger Sea near Iceland. We trace these oceanic changes back to changes in the atmospheric circulation in the North Atlantic region. We conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets.