Over the last decade, mass balance has been monitored on several glaciers of the tropical Andes by the Institute of Research for Development (IRD, France) in collaboration with South American partners. This network includes glaciers in the Cordillera Real of Bolivia, Zongo and Chacaltaya (16°S), glaciers in the Cordillera Blanca of Peru, Yanamarey and Artezonraju (9°S), and glaciers in the eastern and western cordilleras of Ecuador, Antizana (0°28’S) and Carihuayrazo (1°S) (Fig. 1). Some of these have been listed as benchmark glaciers by the Word Glacier Monitoring Service (WGMS 2001), and the data are accessible to the scientific community. This network is designed to capture the effects of climate change, and especially ENSO variability, both in the outer (Bolivia, Peru) and the inner (Ecuador) tropical Andes. Glaciers have been selected to be representative of the regional glacierization. Each monitoring programme includes two glaciers, a large one (1 km
or more) with a substantial accumulation zone, and a small one that is more directly sensitive to ablation processes. Information about the long-term evolution of some of these glaciers has been extracted from aerial photographs, available for the last five decades (Francou et al. 2000; Ramirez et al. 2001). The particular nature of climate in the Tropics allows ablation to occur at anytime throughout the year in the lowest part of glaciers. Thus, the ablation zone has been surveyed in monthly intervals at several sites, providing interesting details about the seasonal response of tropical glaciers (Francou et al. 2003).