Glacier regression in the tropical Andes has accelerated considerably over the past 30 years. This change is cause for great concern, insofar as many regions of the Andes depend on the Cordillera’s glaciers for their water supply (2).
In 1991 scientists from the IRD research unit Great Ice (UR032) set up an observation network, jointly with their Bolivian, Peruvian and Ecuadorian partners. This system takes in a dozen glaciers along the Andes between the Equator and longitude 16°S (Bolivia). In contrast to the Alpine glaciers which undergo a long accumulation period in winter and a short ablation season in summer, the glaciers of the tropical Andes experience an ablation regime in their lower part throughout the year, with a maximum during the Southern summer (October to April) in Bolivia and Peru. That is the season when the strongest insolation coincides with the maximum rainfall. The glaciers, which react strongly to oscillations in these two parameters and are therefore highly sensitive indicators of climate changes. The scientists focused on two representative glaciers from among those scattered over the Cordillera: Antizana (5760-4800 m) in Ecuador and Chacaltaya (5375-5125 m) in the North of Bolivia.
The glacier mass balance, which is an estimate of the difference between the accumulation of snow and ice and their ablation by melting and sublimation, appears to be strongly controlled by the ENSO (El Niño-Southern Oscillation). During the latter’s warm phases (El Niño), the balances are always negative. In the course of a year, the glaciers lose the equivalent of a sheet of water of 600 to 1200 mm. In the cooler and more humid La Niña phase, however, the glaciers return to equilibrium and sometimes show a small increase which temporarily checks their decline.
Marie Guillaume | alfa
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