Digital elevation models of glaciated terrain produced by the NASA/Jet Propulsion Laboratory (JPL) airborne interferometric synthetic-aperture radar (InSAR) instrument in Greenland and Alaska at the C- (5.6 cm wave-length) and L-band (24-cm) frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth, or penetration depth, δp. On cold polar firn at Greenland summit, δp = 9±2m at C- and 14±4m at L-band. On the exposed ice surface of Jakobshavn Isbrae, west Greenland, δp = 1±2 m at C- and 3±3 m at L-band except on smooth, marginal ice where δp = 15±5 m. On colder marginal ice of northeast Greenland, δp reaches 60 to 120 m at L-band. On the temperate ice of Brady Glacier, Alaska, δp is 4±2 m at C- and 12±6 m at L-band, with little dependence on snow/ice conditions. The implications of the results on the scientific use of InSAR data over snow/ice terrain is discussed.

We report measurements of ice surface elevation, ice thickness and surface area for Glaciar Tyndall, Patagonia, made in 1999-2002. The measurements, together with previously published observations, show acceleration over the last few decades of the rates of thinning and retreat of the main calving front. The acceleration of shrinkage appears to be driven by a combination of climate and feedback processes, the dominant feedback being increased melting associated with lowering of the glacier surface (elevation feedback). The melting capacity in the main terminus lake is now too small to be a major factor accelerating the retreat. The glacier bed has low slope and remains below the elevation of the lake spillway for >14 km upstream from the 2000 calving front, indicating the potential for extensive retreat under the influence of strong elevation feedback and increasing interaction with the lake as it enlarges.