That is the main conclusion reached after studies conducted on the Chacaltaya glacier in Bolivia and the Antizana glacier in Ecuador.

Performed by French scientists from Institut de Recherche pour le Développement (IRD) in France and their Bolivian and Ecuadorian partners, mass balance assessments for these two glaciers indicate that their shrinkage rate has accelerated markedly over the past ten or so years.

If this ice recession continues at the same pace, their survival will be jeopardized even in the short term.

The researchers stress that the recent quickening of glacial melt in the Andes corresponds with the greater intensity and frequency of El Niño events observed over the same period.

Andean glaciers are particularly sensitive to climatic fluctuations owing to the position of most of them in the tropics and to the specific mechanisms by which they function.

(In contrast with the glaciers in the Alps, which undergo a long period of accumulation in winter, glaciers in the tropics are subject to a permanent regime of ablation in the lower half of their mass, which makes them sensitive to the slightest sudden variation in climate.)

How are tropical glaciers affected by climatic variations? At a time when an acceleration in global warming is being predicted, the question is especially vital in the Andes, where some regions are largely dependent on the Cordillera's glaciated mountain tops for their water supply.

In order to provide concrete information regarding such concerns, scientists from the IRD research unit "Great ice," the IHH (Institute of Hydraulics and Hydrology of Bolivia) and INAMHI (Ecuador National Institute for Meteorology and Hydrology) devoted particular attention to two small Andean glaciers: the Chacaltaya glacier located at a height of between 5125 and 5375 m in northern Bolivia and the Antizana glacier (4800-5760 m) in Ecuador.

In their size, which is less than 1 km2, and their altitude, these glaciers are representative of the many which are scattered over the Andean Cordillera. Since 1991 for the former and since 1995 for the latter, the researchers have been monitoring mass balance of these glaciers.

This investigation aims to estimate the difference between the quantity of water input in the form of solid precipitation and that lost by ice melting and sublimation. These mass balance determinations have been pursued on a monthly basis for about a decade in the lower zone of the glacier, where ablation processes are dominant.

Information concerning years previous to the assessments is gleaned from photogrammetric restitution and analysis of aerial photographs and from earlier documented records.

The results of these analyses show that the future of these small Andean glaciers is in jeopardy.

In the past decade, the two glaciers lost on average between 0.6 and 1.4 m of water per year. Chacaltaya has shed 40% of its thickness and two-thirds of its volume. Its surface area diminished by more than 40% between 1992 and 1998 and today is only 10% of what it was in 1940.

If this trend continues on these glaciers, whose reduced size makes them increasingly vulnerable to the present-day glacial retreat, Chacaltaya will have disappeared in 10 to 15 years' time and Antizana will have regressed enormously.

The question is whether or not the regression of these glaciers has intensified recently. Earlier, less detailed studies on other glaciers in the tropical Andes, such as in Peru, show that the process began to accelerate at the beginning of the 1980s.

The results obtained on Antizana and Chacaltaya confirm this. The latter lost 0.2 m of water per year between 1940 and 1963, 0.6 m/year from 1963 to 1983 and 0.9 m/year between 1956 and 1993.

Overall, the ablation rate of the two glaciers has been between three and five times higher over the past ten years than it was during the previous decades.

The researchers have moreover brought evidence that the more pronounced glacial shrinkage was largely associated with El Niño events (the warm phase of the El Niño Southern Oscillation ­ ENSO). The monthly mass balance determinations show that ablation peaks were reached during or immediately after El Niño events, notably in 1991-1992, 1994-1996 and 1997-1998.

In contrast, during the cold phase of the ENSO, La Niña, the ablation decreased sharply and sometimes, the glaciers stabilized or even advanced. Thus, the mass balance at Antizana has become positive again since July 1998, when La Niña conditions prevailed through the Pacific; this effect led to a 30 m advance of the glacial front between 1999 and 2000.

These data and the results of energy balance monitoring performed on the Zongo glacier -- a larger glacier in Bolivia studied by the same research team which reacts in a similar way to climatic oscillations of the Pacific -- suggest that the intensification and the greater frequency of El Niño events that has been observed for 20 years, which may be linked to global warming, go a long way in explaining the recent acceleration in the regression of glaciers in the tropical Andes.

(References: B. Francou, E. Ramirez, B. Caceres et J. Mendoza : Glacier evolution in the Tropical Andes during the last decades of the 20th Century : Chacaltaya, Bolivia, and Antizana, Ecuador. Ambio, vol.29, No 7, Nov. 2000. Also E. Ramirez, B. Francou, P. Ribstein, M. Descloitres, R. Guérin, J. Mendoza, R. Gallaire, B. Pouyaud et E. Jordan : Small glaciers disappearing in the Tropical Andes. A case study in Bolivia: the Chacaltaya glacier, 16°S. Journal of Glaciology, forthcoming.)

[Contact: Bernard Francou]

17-Jan-2001