Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2is important because calving has the capacityto destabilize an ice front. Acceleration ofJakobshavn Isbræ beginning in 2000 has beeninterpreted as a response to increased calving atthe ice front and collapse of the floating tonguefollowing very rapid thinning (Thomas, 2004;Joughin et al., 2004).The external variables that trigger such an eventare not well understood. Increased surface meltingdue to climatic warming can destabilize theice front and lead to rapid disintegration of anentire ice shelf (Scambos et al., 2004). Penetrationof surface meltwater into crevasses deepensthe fissures and creates areas of weakness thatcan fail under longitudinal extension.A number of small ice shelves on the AntarcticPeninsula collapsed in the last three decadesof the 20th century. Ice-shelf area declined bymore than 13,500 km 2 in this period, punctuatedby the collapse of the Larsen A and Larsen Bice shelves in 1995 and 2002 (Scambos et al.,2004). This was possibly related to atmosphericwarming in the region, estimated to be about3 °C over the second half of the 20th century.Vaughan and Doake (1996) suggest that iceshelfviability is compromised if mean annualair temperature exceeds −5 °C. Above thistemperature, meltwater production weakenssurface crevasses and rifts and may allow themto propagate through the ice thickness. It isalso likely that thinning of an ice shelf, causedby increased basal melting, preconditions itfor breakup. Consequently, warming of oceanwaters may also be important. The Weddell Seawarmed in the last part of the 20th century, andthe role that this ocean warming played in theice shelf collapses on the Antarctic Peninsula isunknown. Warmer ocean temperatures cause anincrease in basal melt rates and ice-shelf thinning.If this triggers enhanced extensional flow,it might cause increased crevassing, fracturepropagation, and calving.Similarly, the impacts of sea-ice and icebergcloggedfjords are not well understood. Thesecould damp tidal forcing and flexure of floatingice tongues, suppressing calving. Reeh et al.(1999) discuss the transition from tidewateroutlets with high calving rates in southernGreenland to extended, floating tongues of icein north Greenland, with limited calving fluxand basal melting representing the dominantablation mechanism. Permanent sea ice innortheast Greenland may be one of the factorsenabling the survival of floating ice tonguesin the north (Higgins, 1991). This is difficultto separate from the effects of colder air andocean temperatures.4.3 Ice Stream and Glacier ProcessesIce masses that are warm based (at the meltingpoint at the bed) can move via basal sliding orthrough deformation of subglacial sediments.Sliding at the bed involves decoupling of theice and the underlying till or bedrock, generallyas a result of high basal water pressures(Bindschadler, 1983). Glacier movement viasediment deformation involves viscous flowor plastic failure of a thin layer of sedimentsunderlying the ice (Kamb, 1991; Tulaczyk etal., 2001). Pervasive sediment deformationrequires large supplies of basal meltwaterto dilate and weaken sediments. Sliding andsediment deformation are therefore subjectto similar controls; both require warm-basedconditions and high basal water pressures, andboth processes are promoted by the low basalfriction associated with subglacial sediments.In the absence of direct measurements of theprevailing flow mechanism at the bed, basalsliding and subglacial sediment deformationcan be broadly combined and referred to asbasal flow.4.3.1 Basal FlowBasal flow can transport ice at velocities exceedingrates of internal deformation: 100s to60