The surface elevation data used for this study come from the Geoscience Laser Altimeter System (GLAS) aboard NASA’s ICESat. Processed ICESat data provide surface elevation profiles along ground tracks that repeatedly survey the same set of reference tracks two to three times per year, giving elevations for 30–70m footprints every 170m along track, with a spacing between adjacent reference tracks of ~30km in the northernmost parts of Antarctica to near zero at the 868 S turning latitude. The reference tracks are repeated with an accuracy of ~150 m, and the horizontal location of the ICESat footprints is estimated with an accuracy of ~10 m.

Over ice sheets, ICESat data are provided as GLA12 records (H.J. Zwally and others, http://nsidc.org/data/gla12.html), which include the latitude, longitude and elevation of each geolocated footprint, as well as a set of parameters that describe characteristics of the returned pulse and the processing used to derive the elevations. These data were acquired during 13 separate campaigns, between August 2003 and March 2008, each lasting approximately 1 month (Table 1). Low transmitted laser power degraded the data for the June 2004 (2c) campaign, and those data are not used in this study.

Most of our knowledge of subglacial lakes in Antarctica has come from ice-penetrating radio-echo sounding (RES) surveys since the 1950s (Robin and others, 1970). The most recent subglacial lake inventory reported 145 lakes (Siegert and others, 2005); however, more than 130 have been added since then (Siegert and others, 2005; Bell and others, 2007; Carter and others, 2007; Popov and Masolov, 2007), and at the time of writing there are ~280 subglacial lakes that have been identified using RES, the majority of these under the plateau of the East Antarctic ice sheet. It has long been known that the dynamics of much of the Antarctic ice sheet is largely controlled by meltwater at the ice-sheet bed (Budd and others, 1984).

The configuration of many large outlet glacier systems is known to depend on the presence of water-saturated sediments (Studinger and others, 2001), which can reduce the basal shear stress from frozen-bed values (~100 kPa) to values which permit rapid ice flow at low driving stresses (~20 kPa) (Kamb, 2001). Likewise, changes in water pressure and volume can lead to changes in basal lubrication. This has been observed in mountain glaciers, where the accumulation of large volumes of water drives surges during which surface velocities briefly increase by an order of magnitude or more (e.g. Kamb and others, 1985), and where episodic and seasonal water inputs can also modulate ice-flow rates (Harper and others, 2007).