Glacier evolution in the tropical Andes during the last decades of the 20th century: Chacaltaya, Bolivia, and Antizana, Ecuador

Mass balance has been continuously monitored on Chacaltaya Glacier (16 degr eesS, Cordillera Real, Bolivia) since 1991, and on the Antizana Glacier 15 (0 degrees, Ecuador) since 1995. In ablation areas, mass balance has been s urveyed on a monthly scale, providing interesting details about the seasona l pattern in 2 contrasting tropical environments. Intermittent information about ice recession exists in both regions for the last 4 decades. The data point to a clear acceleration in glacier decline during this decade; ablat ion rates have been 3-5 times higher than during the former decades. Fluctu ations measured before on 3 glaciers in northern Peru, allow the assumption that the rate at which the glaciers retreated in the tropical Andes increa sed in the late 1970s. The present situation is particularly dramatic for t he small-sized glaciers (< 1 km(2)) and many such as Chacaltaya, could disa ppear in the next 10 years. As evidenced by the data collected, ablation in creases significantly during the warm phases of ENSO (El Nino) and decrease s during the cold phases (La Nina). Warm events becoming more frequent and intense since the late 1970s, it can be assumed that they have played an im portant role in the recent glacier decline in the central Andes, together w ith the global warming.