Thinning of the Patagonian Icefields From Recent Laser Altimetry, SRTM and Earlier Cartographic Data

The Northern and Southern Patagonia icefields of southern South America (NPI and SPI respectively), are one of the largest temperate glacier systems of mid-latitudes. The glaciers are fed by abundant precipitation of 1-11 m/yr water equivalent (w.e.) due to westerly air flow, and are affected by large ablation (>10 m/yr w.e.) and calving on fjords and freshwater lakes on the lower reaches, with an important east-west gradient. Outlet glaciers are known to be affected by generalized retreat and thinning over the past century, being explained by atmospheric warming and also decreased precipitation. Recently the 2000 Shuttle Radar Topography Mission (SRTM) data in combination with 1968, 1975 and 1995 cartographic data of Chile and Argentina and a few limited ground survey points have allowed the calculation of volume change for the largest 63 glaciers (Rignot et al., in press). Although SRTM data provide a complete coverage with a vertical precision of 7 m and a horizontal resolution of 90 m, the existing cartographic data do not map adequately the accumulation areas because of lack of stereoscopic coverage in the original aerial photographs used to compile the existing maps. In November/December 2002 two flights were made from Punta Arenas by the Centro de Estudios Cient¡ficos (CECS) aboard a Chilean Navy P-3 Orion aircraft, equipped with NASA sensors that included a scanning laser altimeter that yielded measurements of surface elevation to an accuracy of +/- 0.2 m along a total 4000 km flight track over the glaciers. Surveys covered most of NPI and provided a partial coverage of SPI as well. These data provide a unique opportunity for computing glacier thickness changes over the ablation and accumulation areas. Here we provide preliminary computations of ice thickness changes between the existing cartographic data, limited ground surveys, SRTM data and the laser altimetry data at selected glaciers over SPI and NPI. The data confirm the thinning observed in recent years, with larger values at lower elevations. Preliminary results from the 2002 laser altimetry and 1975 topographic data (19 m vertical error) at the ablation areas (between 700 and 50 m a.s.l.) of San Quint¡n and Benito glaciers (NPI) show a thinning rate of 1.3 +/- 0.7 m/yr and 3.6 +/- 0.7 m/yr respectively. The comparison between SRTM and laser altimetry data at the NPI glaciers, yielded ice elevation changes smaller than the errors due to the short (2-year) time period involved. At the ablation area of Tyndall glacier (between 800 and 50 m a.s.l.) a thinning rate of 3.0 +/- 0.7 /yr was obtained using the 2002 laser altimetry and the 1975 map data. We postulate that these large thinning rates cannot be explained only by warmer air temperature and decreased precipitation and suggest that ice dynamic effects are critically relevant.