Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2et al., 2003), where meltwater ponds drainthrough moulins and reach the bed through upto 200 m of cold ice (Boon and Sharp, 2003).The influx of surface meltwater triggers afourfold speedup in flow in the lower ablationarea each year. There is a clear link betweenthe surface hydrology, seasonal developmentof englacial drainage connections to the bed,and basal flow, at least at this site.It is uncertain whether surface meltwater canreach the bed through thick columns of cold ice.Cold ice is impermeable on the intergranularscale (Paterson, 1994). However, water flowinginto moulins may carry enough kinetic andpotential energy to penetrate to the bed andspread out over an area large enough to affectthe basal velocity. Zwally et al. (2002a) recordsummertime speedup events near the westernmargin of the Greenland Ice Sheet, associatedwith the drainage of large supraglacial lakes ina region where the ice sheet is several hundredmeters thick. It is unknown whether the meltwaterpenetrated all the way to the bed, but thisis interpreted to be the cause of the summerspeedups and is consistent with observationson valley glaciers.These observations are unequivocal, but thespeedups are modest (10%) and localized.Alternative interpretations of the Zwally etal. (2002a) data have also been proposed. Theregion may be influenced by seasonal accelerationat the downstream ice margin or throughaccelerated summer flow in nearby JakobshavnIsbræ, rather than local supraglacial lake drainage.Recent summer speedups in JakobshavnIsbræ are believed to be a response to marineconditions (summer calving, seasonal sea ice,and basal melting on the floating ice tongue).More studies like that of Zwally et al. (2002a)are needed to determine the extent to whichsupraglacial water actually reaches the bed andinfluences basal motion. At this time it is stillunclear how influential surface meltwater ison polar icefield dynamics, but it may prove tobe an extremely important feedback in icefieldresponse to climate change, as it provides adirect link between surface climate and ice dynamics.A modeling study by Parizek and Alley(2004) that assumes surface-meltwater-inducedspeedups similar to those observed by Zwallyet al. (2002a) found this effect to increase thesensitivity of the Greenland Ice Sheet to specifiedwarmings by 10–15%. This is speculative,as the actual physics of meltwater penetration tothe bed and its influence on basal flow are notexplicitly modeled or fully understood.4.4 Modeling4.4.1 Ice-Ocean ModelingThere has been substantial progress in thenumerical modeling of the ice-shelf–oceaninteraction over the last decade. A variety of62