Abstract. The aim of this paper is to provide the community with a comprehensive overview of the studies of glaciers in the tropical Andes conducted in recent decades leading to the current status of the glaciers in the context of climate change. In terms of changes in surface area and length, we show that the glacier retreat in the tropical Andes over the last three decades is unprecedented since the maximum extension of the Little Ice Age (LIA, mid-17th-early 18th century). In terms of changes in mass balance, although there have been some sporadic gains on several glaciers, we show that the trend has been quite negative over the past 50 yr, with a mean mass balance deficit for glaciers in the tropical Andes that is slightly more negative than the one computed on a global scale. A break point in the trend appeared in the late 1970s with mean annual mass balance per year decreasing from −0.2 m w.e. in the period 1964-1975 to −0.76 m w.e. in the period 1976-2010. In addition, even if glaciers are currently retreating everywhere in the tropical Andes, it should be noted that this is much more pronounced on small glaciers at low altitudes that do not have a permanent accumulation zone, and which could disappear in the coming years/decades. Monthly mass balance measurements performed in Bolivia, Ecuador and Colombia show that variability of the surface temperature of the Pacific Ocean is the main factor governing variability of the mass balance at the decadal timescale. Precipitation did not display a significant trend in the tropical Andes in the 20th century, and consequently cannot explain the glacier recession.Published by Copernicus Publications on behalf of the European Geosciences Union. A. Rabatel et al.: Current state of glaciers in the tropical AndesOn the other hand, temperature increased at a significant rate of 0.10 • C decade −1 in the last 70 yr. The higher frequency of El Niño events and changes in its spatial and temporal occurrence since the late 1970s together with a warming troposphere over the tropical Andes may thus explain much of the recent dramatic shrinkage of glaciers in this part of the world.

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[1] A distributed energy balance model was applied to Zongo Glacier, Bolivia (16°S, 6000-4900 m above sea level, 2.4 km 2 ), to investigate atmospheric forcing that controls seasonal variations in the mass balance and in meltwater discharge of glaciers in the outer tropics. Surface energy fluxes and melt rates were simulated for each 20 × 20 m 2 grid cell at an hourly resolution, for the hydrological year 1999-2000, using meteorological measurements in the ablation area. Model outputs were compared to measurements of meltwater discharge, snow cover extent, and albedo at two weather stations set up on the glacier. Changes in melt rate in three distinct seasons were related to snowfall and cloud radiative properties. During the dry season (May to August), the low melt rate was mainly caused by low long-wave emission of the cloudless thin atmosphere found at these high altitudes. From September to December, meltwater discharge increased to its annual maximum caused by an increase in solar radiation, which was close to its summer peak, as well as a decrease in glacier albedo. From January on, melt was reduced by snowfalls in the core wet season via the albedo effect but was maintained thanks to high long-wave emission from convective clouds. The frequent changes in snow cover throughout the long ablation season lead to large vertical mass balance gradients. Annual mass balance depends on the timing and length of the wet season, which interrupts the period of highest melt rates caused by solar radiation.Citation: Sicart, J. E., R. Hock, P. Ribstein, M. Litt, and E. Ramirez (2011), Analysis of seasonal variations in mass balance and meltwater discharge of the tropical Zongo Glacier by application of a distributed energy balance model,

We have compared the annual surface energy balance (SEB) of Zongo Glacier (16°S, Bolivia, outer tropics) and Antizana Glacier 15 (0°S, Ecuador, inner tropics). On annual time scale energy fluxes are very similar in the ablation zone: turbulent heat fluxes compensate each other and net short‐wave radiation dominates the SEB. Albedo is central in controlling the melting. Consequently solid precipitation occurrence manages the annual mass balance variability. In the outer tropics, the annual melting is directly related to the annual distribution of precipitation, the period December–February being crucial. However, in the inner tropics, liquid precipitation can occur on the ablation zone, and snowline altitude remains very sensitive to air temperature. Tropical glaciers react rapidly to El Niño events, mainly because of an induced precipitation deficit in the outer tropics and to a temperature increase in the inner tropics, both leading to a rise in snowline altitude.