Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeto atmosphere-ocean-ice interactions that mayaffect ice shelves and calving fronts of tidewaterglaciers.An equally important aspect of these marinebasedice sheets which has long been of interestis that the beds of ice sheets grounded belowsea level tend to deepen inland, either due tooverdeepening from glacial erosion or isostaticadjustment. The grounding line is the criticaljuncture that separates ice that is thick enoughto remain grounded from either an ice shelf or acalving front. In the absence of stabilizing factors,this configuration indicated that marine icesheets are inherently unstable, whereby smallchanges in climate could trigger irreversibleretreat of the grounding line (Hughes, 1973;Weertman, 1974; Thomas and Bentley, 1978).For a tidewater glacier, rapid retreat occursbecause calving rates increase with water depth(Brown et al., 1983). Where the grounding lineis fronted by an unconfined ice shelf, rapidretreat occurs because the extensional thinningrate of an ice shelf increases with thickness,such as would accompany grounding-lineretreat (Weertman, 1974).which ice becomes buoyant, then a rise in sealevel will cause grounding line retreat (andvice versa). Following some initial perturbation,this situation thus leads to the potentialfor a positive feedback to develop betweenice retreat and sea level rise. Recent studiesfrom West Antarctica, however, suggest thatfor some geological situations, the sensitivityof grounding line retreat to sea level rise maybe less important than previously considered.Anandakrishnan et al. (2007) documentedformation of a wedge of subglacial sediment atthe grounding line of the Whillans Ice Stream,resulting in ice to be substantially thicker thereThe amount of retreat clearly depends on howfar inland glaciers remain below sea level. Ofgreatest concern is West Antarctica, where allthe large ice streams are grounded well belowsea level, with deeper trenches lying well inlandof their grounding lines (Fig. 2.9). A similarsituation applies to the entire Wilkes Land sectorof East Antarctica. In Greenland, few outletglaciers remain below sea level very far inland,indicating that glacier retreat by this processwill eventually slow down or halt. A notableexception may be Greenland’s fastest glacier,Jakobshavn Isbræ, which appears to tap intothe central core of Greenland that is below sealevel (Fig. 2.10). Other regions in the northernpart of the ice sheet are the Humboldt Glacier,the Petermann Glacier, and the NioghalvfjerdsfjordenGlacier (Fig. 2.10).Several factors determine the position of thegrounding line, and thus the stability of marineice sheets. On time scales that may lead to rapidchanges, the two most important of these are thebackstress provided by ice-shelf buttressing andsea level (Thomas and Bentley, 1978). Giventhat a grounding line represents the point atFigure 2.10. Bedrock topography for Greenland; areas belowsea level are shown in blue. Note the three channels in thenorth (1: Humboldt Glacier; 2: Petermann Glacier; 3: 79-NorthGlacier or Nioghalvfjerdsfjorden Glacier) and at the west coast(4: Jakobshavn Isbræ) connecting the region below sea level withthe ocean (Russell Huff and Konrad Steffen, CIRES, Universityof Colorado at Boulder).65