El Niño and glacier melt in the tropical Andes

Glacier regression in the tropical Andes has accelerated considerably over the past 30 years. This change is cause for great concern, insofar as many regions of the Andes depend on the Cordillera’s glaciers for their water supply (2).

In 1991 scientists from the IRD research unit Great Ice (UR032) set up an observation network, jointly with their Bolivian, Peruvian and Ecuadorian partners. This system takes in a dozen glaciers along the Andes between the Equator and longitude 16°S (Bolivia). In contrast to the Alpine glaciers which undergo a long accumulation period in winter and a short ablation season in summer, the glaciers of the tropical Andes experience an ablation regime in their lower part throughout the year, with a maximum during the Southern summer (October to April) in Bolivia and Peru. That is the season when the strongest insolation coincides with the maximum rainfall. The glaciers, which react strongly to oscillations in these two parameters and are therefore highly sensitive indicators of climate changes. The scientists focused on two representative glaciers from among those scattered over the Cordillera: Antizana (5760-4800 m) in Ecuador and Chacaltaya (5375-5125 m) in the North of Bolivia.

The glacier mass balance, which is an estimate of the difference between the accumulation of snow and ice and their ablation by melting and sublimation, appears to be strongly controlled by the ENSO (El Niño-Southern Oscillation). During the latter’s warm phases (El Niño), the balances are always negative. In the course of a year, the glaciers lose the equivalent of a sheet of water of 600 to 1200 mm. In the cooler and more humid La Niña phase, however, the glaciers return to equilibrium and sometimes show a small increase which temporarily checks their decline.

It is vitally important to identify the physical processes responsible for the surface melting and sublimation occurring on the glacier. The research team therefore determined the energy-balance on Zongo Glacier (3) (16°S) in Bolivia, close to that of Chacaltaya, and in Antizana (0°28S) in Ecuador (4). They thus found the amount of energy received and absorbed by the glacier resulting from solar radiation and turbulent flow. Such quantities are calculated from meteorological data (including relative humidity, temperatures, wind direction and velocity) collected at the glacier surface over complete years. The energy-balance investigations show that glacier melt here is governed mainly by the net radiation, which represents the fraction of radiant energy the glacier absorbs. This emphasizes the determinant role of the glacial surface’s reflective power, the albedo (5). Melting is at its height in Bolivia during the Southern summer (October to April) and in Ecuador in the months close to the equinoxes (April-May and September), from the moment the energy input and atmospheric humidity are at their maximum.

During the El Niño periods rainfall declines by 10-30%. The rain-snow line on the glaciers climbs by 200-300 m consequent on atmospheric warming by 1-3°C. The albedo is thus kept low and melting accelerates. However, whereas in Bolivia the melt acceleration process stems from a decrease in rainfall, essentially in the form of snowfalls (therefore at high albedo), in Ecuador they are rather linked to increase in atmospheric temperature which transforms snowfalls into rain on the lower half of the glacier. La Niña periods, however, offer a cooler atmosphere and frequent and abundant precipitation, giving the glacier surface a continuous protective blanket of snow doted with a high albedo.

Analysis of mass balance determinations made over several decades at the scale of the central Andes shows that glaciers give a consistent response to the same climatic signal. Hence the periods of intense melting coincide with El Niño episodes in the Pacific, with a lag-time of 2 to 3 months. Moreover, the increase in the glacier regression rate since the end of the 1970s appears to be in synchrony with the Pacific shift of 1976, the date after which the El Niño event became more frequent and more intense. The average annual deficit of Chacaltaya Glacier in Bolivia thus increased from 0.6 m of water between 1963 and 1983 to more than 1.2 m between 1983 and 2003. At this rate it would disappear before 2015.

On the century scale, the oscillatory pattern of ENSO is superimposed on the tendency for regression which has been affecting the central Andean glaciers from at least 1880 (end of the Little Ice Age). In a process of climatic forcing, the warm phases would therefore accelerate deglaciation by reinforcing this tendency, attributed to global warming, which for between 20 and 30 years has been at work in the Cordillera at the faster rate of 0.3°C per decade.

The Great Ice scientists are currently working to model the response of these glaciers to possible climatic scenarios predicted for the XXIst Century. This they are doing by means of general circulation models elaborated for the tropical Andes. In this way they should offer populations whose living depends on the glaciers’ water resource a means of effective prediction of the amount of water that will be available in the future.

Isabelle Chaffaut / Marie Guillaume – IRD
Translation : Nicholas Flay

(1) This research is being conducted in partnership with the Bolivian Institute of Hydraulics and Hydrology (IHH) and the Ecuador National Institute of Meteorology and Hydrology (INAMHI).

(2) Ramirez, E., Francou, B., Ribstein, P., Descloîtres, M., Guérin, R., Mendoza, J., Gallaire, R., Pouyaud, B., & Jordan, E.: Small glaciers disappearing in the tropical Andes. A case study in Bolivia : Glaciar Chacaltaya (16°S). J. Glaciol., 2001, 47, 157 : 187-194

(3) Zongo Glacier, which is larger than Chacaltaya, reacts to climatic oscillations in the Pacific in a similar fashion.

(4) Favier, V., Wagnon, P. & Ribstein, P.: Glaciers of the outer tropics: a different behaviour but a common response to climatic forcing. Geophys. Res. Let., 2004, in press.
Wagnon, P., Ribstein, P., Francou, B. & Sicart, J.E.: Anomalous heat and mass budget of Zongo Glacier, Bolivia, during the 1997-98 El Niño year. J. Glaciol., 2001, 47, 156 : 21-28.

(5) In the tropics and at high altitude, solar radiation is intense and the albedo plays a dominant role. The higher this reflective power is, the greater the quantity of energy returned towards the atmosphere. This leads to a decrease in melting rate.