Changes in glacier length and icethickness during the past 50 yr

Changes in frontal positions and surface areas of outlet glaciers during the past 50yr were clarified by analysing satellite data (Landsat, Spot, and others), aerial photographs and topographic maps with field survey data (Aniya et al., 1992, 1997). Variations

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Figure 46.1 The Northern Patagonia Icefield (NPI) and its outlet glaciers (Aniya, 1988; Naruse et al., 1995). Reproduced by permission of the International Glaciological Society.

in frontal positions of eight major calving glaciers in the SPI since 1945 are shown in Fig. 46.3. At first sight, a large retreat is evident at O'Higgins Glacier, a northeastern outlet glacier, which showed a 13km retreat during 41yr, the mean being 330myr-1. This rate is about one order of magnitude greater than the mean retreat rate in the NPI (about 30myr-1). Whereas Upsala and Tyndall Glaciers in the southern SPI have retreated at relatively high rates, the other three glaciers in the northern SPI, Jorge Montt, Occidental and Viedma, have been retreating gradually.

A second feature to note is the peculiar behaviour of the advancing Pío XI (or Brüggen) Glacier, the southern calving tongue of which has advanced 9km during 31yr from 1945 and the northern tongue is still advancing. A possibility of surges in 1976 and 1992-1994 is suggested (Rivera & Casassa, 1999). In contrast, Perito Moreno Glacier has been almost in a steady state.

In the NPI, most of 22 outlet glaciers retreated at an increasing rate from 1945 to 1990; however, the retreat has slowed down since then (Aniya, 1999).

Surface elevations of bare-ice in ablation areas were measured several times with the conventional survey method at Soler Glacier in the NPI, and Tyndall, Moreno and Upsala Glaciers in the SPI since 1983 (Naruse et al., 1995). From the elevation data measured in different years, changes in ice thickness were obtained (Fig. 46.4). Large thinning rates of ice, from 3 to 5myr-1, were measured at Soler and Tyndall Glaciers; especially, Upsala Glacier has thinned markedly with a rate of 11myr-1. Compared with the thickness change data at 42 glaciers compiled by IAHS-UNEP-UNESCO (1993), the rate of Upsala Glacier is amongst the largest and one order of magnitude greater than the mean rate of receding glaciers in the world.

Figure 46.2 The Southern Patagonia Icefield (SPI) and its outlet glaciers. Determined (or inferred) ice divides are shown by solid (or broken) lines (Aniya et al., 1996, 1997).

Occidental

Viedma

P. Moreno

Upsala

1990 2000

Figure 46.3 Fluctuations in frontal positions of eight major calving glaciers of the SPI. Positive gradients indicate advances and negative gradients indicate retreats. Jorge Montt, Occidental and Pío XI-south (southern tongue) glaciers calve into fjords, and the other five glaciers and Pío XI-north (northern tongue) glacier calve into lakes. (Compiled using various sources: Aniya et al., 1992, 2000; Aniya & Skvarca, 1992, Naruse et al., 1995; Naruse & Skvarca, 2000; Skvarca et al., 2002.) Reproduced by permission of the International Glacio-logical Society and the Instituto Antárctica Argentina.

1940 1950 1960

1970 1980 Year

1990 2000

Figure 46.3 Fluctuations in frontal positions of eight major calving glaciers of the SPI. Positive gradients indicate advances and negative gradients indicate retreats. Jorge Montt, Occidental and Pío XI-south (southern tongue) glaciers calve into fjords, and the other five glaciers and Pío XI-north (northern tongue) glacier calve into lakes. (Compiled using various sources: Aniya et al., 1992, 2000; Aniya & Skvarca, 1992, Naruse et al., 1995; Naruse & Skvarca, 2000; Skvarca et al., 2002.) Reproduced by permission of the International Glacio-logical Society and the Instituto Antárctica Argentina.

value of estimates, 0.3 mmyr-1, for the contribution of melting of mountain glaciers and ice caps to sea-level rise (IPCC, 2001a), Patagonian ice contributes to 13% of that value, although Patag-onian ice occupies only 3.2% of the total surface areas of mountain glaciers and ice caps. From this argument, we can recognize how extensively the Patagonian glaciers have been shrinking.

Figure 46.2 The Southern Patagonia Icefield (SPI) and its outlet glaciers. Determined (or inferred) ice divides are shown by solid (or broken) lines (Aniya et al., 1996, 1997).

In contrast, the ice thickness of Perito Moreno Glacier has been almost unchanged between 1990 and 2000. Considering this with the glacier-front positions, the glacier is regarded to be in a steady state at present, which is a rather peculiar behaviour in Patagonia.

Aniya (1999) roughly estimated the total ice-volume loss due to area reduction and ice thinning of Patagonian glaciers during the past 50 yr to be 825 ± 320 km3, which corresponds to a global mean sea-level rise rate of 0.038 mm yr-1. If we take a median

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