Book 2.indb - US Climate Change Science Program

Abrupt Climate Changemore than 10,000 meters per year, and glaciersurges, tidewater glacier flow, and ice streammotion are governed by basal flow dynamics(Clarke, 1987). Ice streams are responsible fordrainage of as much as 90% of West Antarctica(Paterson, 1994), leading to a low surface profileand a mobile, active ice mass that is poorlyrepresented by ice-sheet models that cannotportray these features.Glaciers and ice sheets that are susceptible tobasal flow can move quickly and erratically,making them intrinsically less predictable thanthose governed by internal deformation. Theyare more sensitive to climate change becauseof their high rates of ice turnover, which givesthem a shorter response time to climate (or icemarginal)perturbations. In addition, they maybe directly responsive to increased amounts ofsurface meltwater production associated withclimate warming.This latter process is crucial to predictingdynamic feedbacks to the expanding ablationarea, longer melt season, and higher rates ofsurface meltwater production that are predictedfor most ice masses.Although basal meltwater has traditionally beenthought to be the primary source of subglacialwater, models have shown that supraglacialstreams with discharges of over 0.15 m 3 s –1 canpenetrate down through 300 m of ice to reachbedrock, via self-propagation of water-filledcrevasses (Arnold and Sharp, 2002). Thereare several possible subglacial hydrologicalconfigurations: ice-walled conduits, bedrockconduits, water film, linked cavities, softsedimentchannels, porous sediment sheets, andordinary aquifers (Mair et al., 2001; Flowersand Clarke, 2002).Modern interest in water flow through glacierscan be dated from a pair of theoretical paperspublished in 1972. In one of these, Shreve(1972) discussed the influence of ice pressureon the direction of water flow through andunder glaciers, and in the other, Röthlisberger(1972) presented a theoretical model for calculatingwater pressures in subglacial conduits.Through a combination of these theoreticalconsiderations and field observations, it isconcluded that the englacial drainage systemprobably consists of an arborescent networkof passages. The millimeter-sized finger-tiptributaries of this network join downward intoever larger conduits. Locally, moulins providelarge direct connections between the glaciersurface and the bed. Beneath a valley glacier thesubglacial drainage is likely to be in a tortuoussystem of linked cavities transected by a fewrelatively large and comparatively straightconduits. The average flow direction in thecombined system is controlled by a combinationof ice-overburden pressure and bed topography,and in general is not normal to contours of equalelevation on the bed. Although theoretical studiesusually assume that subglacial conduits aresemicircular in cross section, there are reasonsfor believing that this ideal is rarely realizedin nature. Much of the progress in subglacialhydrology has been theoretical, as experimentaltechniques for studying the englacial hydraulicsystem are few, and as yet not fully exploited,and observational evidence is difficult to obtain.How directly and permanently do these effectsinfluence ice dynamics? It is not clear at thistime. This process is well known in valley glaciers,where surface meltwater that reaches thebed in the summer melt season induces seasonalor episodic speedups (Iken and Bindschadler,1986). Speedups have also been observed inresponse to large rainfall events (e.g., O’Neelet al., 2005).4.3.2 Flow Acceleration and MeltwaterSummer acceleration has also been observedin the ablation area of polar icefields (Copland61