The reasons for the accelerated glacier retreat observed since the early 1980s in the tropical Andes are analyzed based on the well-documented Chacaltaya glacier (Bolivia). Monthly mass balance measurements available over the entire 1991--2001 decade are interpreted in the light of a recent energy balance study performed on nearby Zongo glacier and further put into a larger-scale context by analyzing the relationship with ocean-atmosphere dynamics over the tropical Pacific-South American domain. The strong interannual variability observed in the mass balance is mainly dependent on variations in ablation rates during the austral summer months, in particular during DJF. Since high humidity levels during the summer allow melting to be distinctly predominant over sublimation, net all-wave radiation, via albedo and incoming long-wave radiation, is the main factor that governs ablation. Albedo depends on snowfall and a deficit during the transition period and in the core of the wet season (DJF) maintains low albedo surfaces of bare ice, which in turn leads to enhanced absorption of solar radiation and thus to increased melt rates. On a larger spatial scale, interannual glacier evolution is predominantly controlled by sea surface temperature anomalies (SSTA) in the eastern equatorial Pacific (Ni¿o 1+2 region). The glacier mass balance is influenced by tropical Pacific SSTA primarily through changes in precipitation, which is significantly reduced during El Ni¿o events. The more frequent occurrence of El Ni¿o events and changes in the characteristics of its evolution, combined with an increase of near-surface temperature in the Andes, are identified as the main factors responsible for the accelerated retreat of Chacaltaya glacier.