Mountain regions play a critical role for downstream water supply in the arid and semiarid regions of the tropics and subtropics, affecting millions of people [Viviroli et al., 2007]. This is particularly true for the tropical Andes, which are relatively moist compared with the adjacent hyperarid coastal desert to the west. Much of the rain and snow falling in the tropical Andes is initially stored, either as ice in mountain glaciers or as water, retained in high-altitude tropical wetlands (“páramos”), before being gradually released over time [Buytaert et al., 2006; Juen et al., 2007; Vuille et al., 2008a]. Glaciers and páramos therefore act as critical buffers against highly seasonal precipitation and provide water for domestic, agricultural or industrial use during the dry season, when rainfall is low or absent. Yet climate change may soon lead to dramatic changes in Andean hydrology, with unknown consequences regarding the availability of water for human consumption, irrigation, mining, power generation and the social and cultural life of Aymara and Quechua cultures [Young and Lipton, 2006; Vergara et al., 2007]. While changes in climate are already being observed [Vuille and Bradley, 2000; Vuille et al., 2003] and glaciers are retreating throughout the tropical Andes [Ramirez et al., 2001; Francou et al., 2003, 2004; Jordan et al., 2005; Mark and Seltzer, 2005; Ceballos et al., 2006; Thompson et al., 2006; Mark and McKenzie, 2007; Seimon et al., 2007; Vuille et al., 2008a] detailed assessments of how climate will change in the 21st century and how these changes will impact the environmental service provided by glaciers and wetlands are sorely missing. Here we take a first step in this direction by simulating different scenarios of future climate change in the tropical Andes as provided by the Intergovernmental Panel on Climate Change–Special Report on Emission Scenarios (IPCC-SRES) [Nakicenovic and Swart, 2000], using a high-resolution Regional Climate Model (RCM). Such high-resolution models, run over a limited domain have been successfully applied in studies of weather forecasting, climate prediction and the study of specific mesoscale circulation systems in South America [e.g., Roads et al., 2003; Seth and Rojas, 2003; Rojas and Seth, 2003; Seth et al., 2004, 2007; Fernandez et al., 2006; Rauscher et al., 2006, 2007; Rojas, 2006] but with a few exceptions [Fuenzalida et al., 2007; Garreaud and Falvey, 2008; Cook and Vizy, 2008] they have not focused on future climate change scenarios and to our knowledge, none so far have focused specifically on the tropical Andes. This mountain region provides a particular challenge for climate modeling given the complex topography and the steep climatic gradients ranging from tropical rain forest on the eastern slopes to absolute desert along the Pacific coast. Since GCMs are not capable of adequately resolving these climatic gradients, some studies have tried to circumvent this problem by instead analyzing projected temperature changes in the free troposphere [Bradley et al., 2004, 2006], but this is not a suitable approach for surface variables such as precipitation. Regional climate models could yield the most significant improvement as they are better suited to resolve the geographic complexity of regional climate. Here we use the Hadley Centre Regional Climate Modeling System, PRECIS (Providing REgional Climate for Impact Studies), nested in the Hadley Centre Atmospheric Model version 3 (HadAM3) to assess how climate changes under two different IPCC-SRES scenarios, A2 and B2 between 1961 and 1990 and 2071–2100. We limit our discussion to variables known to affect glacier energy and mass balance, such as temperature and precipitation [e.g., Francou et al., 2003, 2004; Vuille et al., 2008b]. Our model domain includes entire tropical South America, but we will focus primarily on changes in the Andes as we are particularly interested in seeing whether certain changes in climate are elevation-dependent and whether this elevation dependence (e.g., temperature lapse rate) is a robust feature of the climate system or will change under varying greenhouse gas scenarios. Several analyses are performed separately for the eastern and western Andean slopes, as observational studies indicate significant differences in current climate trends [Vuille and Bradley, 2000]. A more detailed assessment of changes in variables other than temperature and precipitation, including the atmospheric circulation and of changes in the lowlands to the east of the Andes will be provided elsewhere.