With a total of 723 km 2 of glaciers (1970) the Cordillera Blanca includes the largest glacier-covered area in the tropics. The climate is characterized by relatively large daily and small seasonal temperature variations as well as by a distinct succession between a dry (May-September) and a wet season (October-April). Since the early 1970s an ablation stake network has been installed on the tongues of the glaciers Uruashraju and Yanamarey. The determination of the equilibrium-line altitude at each end of a wet season was possible, showing a fair correlation with temperature, but not with the precipitation records of the nearby climatological station Querococha. Mean ablation rates at the lowest parts of the glacier tongues are markedly higher during the wet season than during the dry season. Reasons are presumably to be found in the seasonal variation of cloudiness and air moisture rates. Terminus variations of four glaciers in the Cordillera Blanca have been monitored since the early seventies, earlier positions are reconstructed back to 1948 by vertical air photographs. For the glaciers Uruashraju and · Yanamarey the terminus positions of 1939 are known from an early map. The general retreat of glaciers in the Cordillera Blanca during the last five decades correlates with the global attitude of glaciers and especially with the attitude of glaciers in other tropical areas. Decreased recession rates with minor advances (I974-79 and 1985-86) are accompanied by lower annual temperatures and preceded and accompanied by years with relatively high annual precipitation sums.

A detailed glaciological observation program was conducted on the Yanamarey Glacier in the Cordillera Blanca of Peru, including the monitoring of net balance and ice flow velocity during 1977-1988 and mappings of the surface topography in 1973, 1982, and 1988. These observations are here evaluated to combine net balance, surface lowering, and ice flow into a consistent picture of the mode of operation of a tropical glacier on the scale of a decade. The glacier extends between 5100 and 4500 m with a total area of 9 x 105 m 2 and length of about 1.3 km. Maximum flow velocity is 17.4 m yr -• and maximum volume flux 336 x 10 3 m 3 yr -• . In the ablation area, net 1 1 balance is about -6 m yr-and surface lowering 3 m yr-. About half of the mean annual water discharge from the glacier of 80 L s-is not renewed by precipitation but supplied by the ice thinning. The rate of surface lowering of 1.5 m yr-'liquid water equivalent translates to a glacier average departure heat supply for melting of 16 W m -2. Sensitivity analyses indicate that this could be produced by a cloudiness increase of less than one tenth, an air temperature decrease of 2øC, an increment in specific humidity of less than 1 g kg -1 or some combination of heat budget processes Such changes in the atmospheric environment would be required to stabilize the glacier at its recent volume. As another indication of the recent imbalance, the maximum volume flux is found some 100 m below the equilibrium line altitude. Under continuation of the recent climatic conditions, the glacier may survive for more than half a century. HASTENRATH AND AMES: YANAMAREY GLACIER, PERU 5107

For Yanamarey Glacier in Cordillera Blanca, Peru, mostly situated between about 5000 and 4600 m, maps of the surface topography at a scale of 1: 5000 obtained by terrestrial triangulation for 1973, 1982 and 1988 and by aerial photogrammetry for 1948 and 1962 are compared with the glacier boundaries from a 1939 map and an undated maximum extent inferred from moraine morphology. The glacier length decreased from the maximum, 2800 m to 1600 m in 1948. and to 1250 m in 1988, with an accompanying decrease in area from 17 × 105to 10 × 105and thence to 8 × 105m2. The shrinkage of ice volume was 35 × 106m3from the maximum to 1948. and 29 × 106m3from 1948 to 1988. compared to a total remaining ice volume of about 25 × 106m3in 1988. This quantitative assessment of mass-loss rates creates the observational basis for sensitivity studies of the climatic forcing.

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With a total of 723 km 2 of glaciers (1970) the Cordillera Blanca includes the largest glacier-covered area in the tropics. The climate is characterized by relatively large daily and small seasonal temperature variations as well as by a distinct succession between a dry (May-September) and a wet season (October-April). Since the early 1970s an ablation stake network has been installed on the tongues of the glaciers Uruashraju and Yanamarey. The determination of the equilibrium-line altitude at each end of a wet season was possible, showing a fair correlation with temperature, but not with the precipitation records of the nearby climatological station Querococha. Mean ablation rates at the lowest parts of the glacier tongues are markedly higher during the wet season than during the dry season. Reasons are presumably to be found in the seasonal variation of cloudiness and air moisture rates. Terminus variations of four glaciers in the Cordillera Blanca have been monitored since the early seventies, earlier positions are reconstructed back to 1948 by vertical air photographs. For the glaciers Uruashraju and · Yanamarey the terminus positions of 1939 are known from an early map. The general retreat of glaciers in the Cordillera Blanca during the last five decades correlates with the global attitude of glaciers and especially with the attitude of glaciers in other tropical areas. Decreased recession rates with minor advances (I974-79 and 1985-86) are accompanied by lower annual temperatures and preceded and accompanied by years with relatively high annual precipitation sums.