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HUMIDITY INDUCES MELTING IN TROPICAL-ZONE GLACIERS

October 28, 1999

On Zongo glacier in Bolivia, situated at between 6000 and 4900 m elevation on the Huayna Potosi massif, 30 km from La Paz, scientists have observed that the runoff stream, induced by ice melting, showed a discharge rate two or three times lower during the dry season (May to August at this latitude) than that in the wet season (November to February). This is the reverse of the events observed in the temperature and polar zones where periods of heavy snowfall in winter alternate with summer melting. How can these seasonal variations be explained whereas, in this tropical region, sunshine levels vary little from one season to another and seasonal temperature differences are low (2°C daily average)? An energy balance assessment, in other words the analysis of the amount of energy influx contributed by solar radiation on to the glacier surface, has given elements to answer this. The researchers were able to perform this balance assessment using recorded data (wind speed and direction, solar radiation, humidity, temperature, etc.) by an automatic weather station installed by IRD at a site 5000 m high. This is the first study of this type conducted on a tropical glacier.
During the dry season, most of the insolation energy, as a set of heat radiations of different wavelengths, is consumed by sublimation (the transformation of ice into water vapour) which requires eight times as much energy as melting (or ablation). Little energy is available for melting. The glacial stream discharges are therefore small. On the contrary, during the rainy season, the air humidity is at saturation point which in turn restricts sublimation. Owing to this fact, solar energy at the glacier surface is used up almost exclusively in melting snow or ice. The runoff discharge then increases. Thus it appears that humidity is the key meteorological parameter for explaining the seasonal hydrological regimes of runoff streams that operate in tropical glaciers like that of Zongo.
Moreover, since the beginning of the "Snow and glaciers in the tropics" programme, the researchers have observed that, during an El Nino event (seen in 1991-92 and 1997-98), ice melting increased noticeably on Zongo glacier. The energy balance determined for the glacier provides an explanation of the relationships between the variations observed and that climatic phenomenon. The passage of El Nino, which arises in the PacificOcean every three to five years, is reflected in the Bolivian Andes by a significant decrease in rainfall (around 25%) and a slight rise in temperature. Owing to this fact, the snow pack, which is anyway quite thin, disappears rapidly and the albedo (the fraction of incident solar radiation reflected by the surface) is reduced. The ice consequently absorbs more energy leading to increasd icemelt.
The results of these investigations improve understanding, in the present-day context of global warming, of why tropical glaciers are shrinking. The accelerated melting of this glacial ice fits in not only with the increase in temperature but also with a rise in humidity in the lower troposphere, which favours ablation. These temperature and humidity variations associated with global climatic change, on top of the effects of El Nino, can help explain why melting has been observed to be quickening in tropical-zone glaciers over the past two decades.


Institut de Recherche pour le Développement, Paris (IRD)