46 years of environmental records from the Nevado Illimani glacier group, Bolivia, using digital photogrammetry Rafael da Rocha RIBEIRO, 1 Edson RAMIREZ, 2 Jefferson Cardia SIMO ˜ ES, 1 Abraham MACHACA 2 1 Centro Polar e Clima ´tico, Instituto de Geocie ˆncias, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil E-mail: r.ribeiro@ufrgs.br 2 Instituto de Hidra ´ulica e Hidrologı ´a, Universidad Mayor de San Andre ´s, La Paz, Bolı ´via ABSTRACT. This study determines variations in ice extent of Nevado Illimani, Bolivia (16838 0 S, 67844 0 W), from 1963 to 2009. The results are compared with net accumulation rate variations obtained from a local ice core. We then propose an interpretation of the recent environmental history (last 46 years) of the region based on a study of remotely sensed and ice-core data. From 1963 to 2009, Nevado Illimani lost a total ice area of 9.49  1.09 km 2 , a 35% reduction. Area variations generally followed variations in net accumulation rates during this period. Despite the current glacier area reduction trend, the Nevado Illimani glaciers will not completely disappear in the next few decades. 1. INTRODUCTION Tropical glaciers exist in South America (from Bolivia to Venezuela), Africa and Oceania (west Papua). The Andes mountain range is home to 99% of these tropical ice masses (Kaser and Osmaston, 2002). Of the 2500 km 2 of tropical glaciers in South America, 70% are found in Peru, 20% in Bolivia and 10% in Ecuador, Colombia and Venezuela. In the tropics, the 0.18C atmospheric isotherm remains practically at the same altitude throughout the year, allowing glacier front ablation at any time (Kaser and Osmaston, 2002), in contrast to glaciers at higher latitudes. Mountain glacier distribution is controlled, fundamen- tally, by two factors: precipitation and altitude. Mountain ranges ‘block’ air-mass humidity, forcing precipitation, and promoting glacier-forming conditions. The second factor controls the equilibrium-line altitude (ELA), as glaciers will only form where the ELA is below mountain summits (Clapperton, 1993). Some studies indicate that tropical glaciers are affected by regional climate variability. In the Andes, for example, glaciers exhibit a strong retraction during the positive phase of El Nin ˜o Southern Oscillation (ENSO) events (Francou and others, 2007). There is also evidence of strong glacial retraction for the past three decades along the full Andean mountain range due to atmospheric warming. This phenom- enon reflects a glacial retraction rate increase and glacier thickness reduction, leading to the disappearance of many tropical glaciers (GTNH, 2010). The volume losses of mountain glaciers may be the clearest indicator of the rapid (if not accelerated) nature of climate change on a global scale (Francou and others 2005; Lemke and others, 2007). There are many reasons to maintain interest in the study of Andean glaciers. They are important indicators of climate change. They also affect almost all the South American regional hydrologic regimes, particularly those that present dry seasons (e.g. southern Peru and Bolivia). In low- precipitation years, ice melt maintains minimum water flow levels, thus ensuring water supply to urban centres and hydroelectric power plants (Marengo and others, 2011). This study determines ice extent variations at Nevado Illimani, Bolivia, from 1963 to 2009 using digital photo- grammetry techniques. The results are compared with net accumulation rates obtained from ice cores extracted from the same ice mass (Ramirez and others, 2003). We then propose an interpretation of the recent environmental history (last 46 years) of the region based on the remotely sensed and ice-core data. 2. STUDY AREA There are two main mountain ranges in Bolivia that are home to glaciers: the Cordillera Ocidental (Western Cordillera), which is formed by extinct volcanoes with crater glaciers and small isolated ice-covered peaks, and the Cordillera Oriental (Eastern Cordillera), with 600 km 2 of glaciers. The Cordil- lera Oriental has four constituent parts: Apolobamba, Real, Mun ˜ecas and Tres Cruces/Nevado Santa Vera Cruz. Most types of glaciers are present, from ice caps and valley glaciers to small mountain glaciers (Jordan, 1998; Fig. 1). Our study site, Nevado Illimani, Bolivia (16838 0 S, 67844 0 W), is set within the mid-eastern sector of the Bolivian Andes, locally known as Cordillera Real. This mountain is 50 km south of La Paz and 180km from Lake Titicaca. It is one of the oldest tertiary plutonic bodies in the westernmost sector of the Andes and is the product of lava intrusions. It has been greatly eroded by the La Paz river and its tributaries (Jordan, 1998). Its dimensions are 10 km  4 km, with some peaks over 6000 m a.s.l. (e.g. Pico del Indio, Pico Layco Kkollu). Twenty-six glaciers were selected to represent a comprehen- sive range of size, aspect and elevation. Precipitation in this region occurs mainly during the austral summer (80% of annual precipitation), due to the humid air masses coming from the Amazon River basin (Vuille and others, 2003). In the dry season, corresponding to the austral winter (June–August), the prevailing wind directions are north and northwest. Extratropical precipi- tation events during the dry periods are attributed to the cold air masses flowing from Argentina, which originate from Antarctica. Vuille (1999) showed that in rainy seasons during El Nin ˜o events, the Bolivian Andes present negative precipitation anomalies. In La Nin ˜a years, the precipitation tends to be above average. The decrease in precipitation during El Nin ˜ o events delays snowpack formation. This, in turn, ‘exposes’ the glacial Annals of Glaciology 54(63) 2013 doi:10.3189/2013AoG63A494 272