Bernard Francou

Annual cycle of energy balance of Zongo Glacier, Cordillera Real, Bolivia

An 18-month meteorological data set recorded at 5150 m above sea level (asl) on Zongo Glacier, in the tropical Andes of Bolivia, is used to derive the annual cycle of the local energy balance and to compare it to the local mass balance. The roughness parameters needed to calculate the turbulent fluxes over the surface are deduced from direct sublimation measurements performed regularly on the field site and serve as calibration parameters. For the hydrological year September 1996 to August 1997, net all-wave radiation (16.5 W m−2) is the main source of energy at the glacier surface and shows strong fluctuations in relation to the highly variable albedo. An important peculiarity of tropical glaciers is the negative latent heat flux (−17.7 W m−2) indicating strong sublimation, particularly during the dry season. The latent heat flux is reduced during the wet season because of a lower vertical gradient of humidity. The sensible heat flux (6.0 W m−2), continuously positive throughout the year, and the conductive heat flux in the snow/ice (2.8 W m−2) also bring energy to the surface. There is a good agreement between the monthly ablation calculated by the energy balance and the ablation evaluated from stake measurements. The seasonality of the proglacial stream runoff is controlled by the specific humidity, responsible for the sharing of the energy between sublimation and melting.

Economic impacts of rapid glacier retreat in the Andes

In the Andes, runoff from glacierized basins is an important element of water budgets, assuring year-round flows for agriculture, potable water, power generation, and ecosystem integrity. Thus, changes induced by tropical glacier retreat constitute an early case of the need for adaptation and the type and size of associated economic and social impacts caused by climate change.

Small glaciers disappearing in the tropical Andes: a case-study in Bolivia: Glaciar Chacaltaya (16o S)

Glaciar Chacaltaya is an easily accessible glacier located close to La Paz, Bolivia. Since 1991, information has been collected about the evolution of this glacier since the Little Ice Age, with a focus on the last six decades. The data considered in this study are monthly mass-balance measurements, yearly mappings of the surface topography and a map of the glacier bed given by ground-penetrating radar survey. A drastic shrinkage of ice has been observed since the early 1980s, with a mean deficit about 1 m a -1 w.e. From 1992 to 1998, the glacier lost 40% of its average thickness and two-thirds of its total volume, and the surface area was reduced by >40%. With a mean estimated equilibrium-line altitude lying above its upper reach, the glacier has been continuously exposed to a dominant ablation on the whole surface area. If the recent climatic conditions continue, a complete extinction of this glacier in the next 15 years can be expected. Glaciar Chacaltaya is representative of the glaciers of the Bolivian eastern cordilleras, 80% of which are small glaciers ( <0.5 km2). A probable extinction of these glaciers in the near future could seriously affect the hydrological regime and the water resources of the high-elevation basins.

Anomalous heat and mass budget of Glaciar Zongo, Bolivia, during the 1997/98 El Niño year

During El Nino-Southern Oscillation (ENSO) warm events, outer tropics glaciers usually experience a deficit of precipitation, an increase of air temperature and a strongly negative mass balance. At Glaciar Zongo, Bolivia, this was particularly striking during the vigorous 1997/98 El Nino event, one of the strongest of the century, and which resulted in an annual depth of runoff two-thirds higher than normal. We compare the energy balance on the glacier between two contrasting cycles, 1996/97 (La Nina year) and 1997/98 (El Nino year). Due to a 1.3°C increase of annual mean air temperature, the sensible-heat flux slightly increases from 6.1 to 9.8 W m 2 During the El Nino year, sublimation is reduced, leaving more energy for melting (LE = -18.1 W m -2 in 1996/97 and LE = - 11.6 W m - 2 in 1997/98). The main factor responsible for the dramatic increase in melting is the net all-wave radiation, which is three times higher in 1997/98 than in 1996/ 97 (48.7 and 15.8 W m -2 , respectively). This sharp increase of net all-wave radiation is related to the decrease of albedo due to the precipitation deficit.

A new Andean deep ice core from Nevado Illimani (6350 m), Bolivia

Abstract A new ice core record from the Nevado Illimani (16°S), Bolivia, covers approximately the last 18 000 years BP. A comparison with two published ice records, from Sajama (18°S), Bolivia [Thompson et al., Science 282 (1998) 1858–1864] and Huascaran (9°S), Peru [Thompson et al., Science 269 (1996) 46–50], documents a regionally coherent transition from glacial to modern climate conditions in South America north of 20°S. The strong resemblance between the Illimani and Huascaran water isotope records and their differences from the Sajama record, in particular during the period from 9000 years BP to 14 000 years BP, suggest that local water recycling or local circulation changes played a major role for Sajama. We interpret the common Illimani/Huascaran water isotope history in terms of a common change from wetter/cooler conditions during glacial times to drier/warmer conditions in the Early Holocene.

Monthly balance and water discharge of an inter-tropical glacier: Zongo Glacier, Cordillera Real, Bolivia, 16' S

Measurements of mass balance were performed every month on Zongo Glacier, Bolivia. Simultaneously, water-discharge, temperature and precipita- tion data were obtained. The first year of the survey, 1991-92, was marked by an ENSO (El Nino-Southern Oscillation) event with high temperature and low precipitation, whilst the following year, 1992-93, was normal. Results point to the early and late wet season (October-December and March-May) as playing a critical role in the determination of the annual mass balance. The wet season is the warmest period of the year and consequently the duration of the wet season is a highly relevant variable in determining mass balance. Both glaciological and hydrological methods for the determination of the mass balance provide similar results. Our study confirms that ENSO events have a major influence on the rapid glacier retreat currently affecting this part of the Andes.

Glacier Evolution in the Tropical Andes during the Last Decades of the 20th Century: Chacaltaya, Bolivia, and Antizana, Ecuador

Abstract Mass balance has been continuously monitored on Chacaltaya Glacier (16°S, Cordillera Real, Bolivia) since 1991, and on the Antizana Glacier 15 (0°, Ecuador) since 1995. In ablation areas, mass balance has been surveyed on a monthly scale, providing interesting details about the seasonal pattern in 2 contrasting tropical environments. Intermittent information about ice recession exists in both regions for the last 4 decades. The data point to a clear acceleration in glacier decline during this decade; ablation rates have been 3–5 times higher than during the former decades. Fluctuations measured before on 3 glaciers in northern Peru, allow the assumption that the rate at which the glaciers retreated in the tropical Andes increased in the late 1970s. The present situation is particularly dramatic for the small-sized glaciers (< 1 km2) and many such as Chacaltaya, could disappear in the next 10 years. As evidenced by the data collected, ablation increases significantly during the warm phases of ENSO (El Niño) and decreases during the cold phases (La Niña). Warm events becoming more frequent and intense since the late 1970s, it can be assumed that they have played an important role in the recent glacier decline in the central Andes, together with the global warming.

Atmospheric heavy metals in tropical South America during the past 22,000 years recorded in a high altitude ice core from Sajama, Bolivia

V, Co, Cu, Zn, As, Rb, Sr, Ag, Cd, Ba, Pb, Bi and U have been analysed by inductively coupled plasma sector field mass spectrometry in various sections of a dated snow/ice core drilled at an altitude of 6542 m on the Sajama ice cap in Bolivia. The analysed sections were dated from the Last Glacial Stage ( approximately 22,000 years ago), the Mid-Holocene and the last centuries. The observed variations of crustal enrichment factors (EFc) for the various metals show contrasting situations. For V, Co, Rb, Sr and U, EFc values close to unity are observed for all sections, then showing that these elements are mainly derived from rock and soil dust. For the other metals, clear time trends are observed, with a pronounced increase of EFc values during the 19th and 20th centuries. This increase shows evidence of metal pollution associated with human activity in South America. For Pb an important contribution was from gasoline additives. For metals such as Cu, Zn, Ag and Cd an important contribution was from metal production activities, with a continuous increase of production during the 20th century in countries such as Peru, Chile and Bolivia.

Mass balance of Glaciar Zongo, Bolivia, between 1956 and 2006, using glaciological, hydrological and geodetic methods

Abstract The longest continuous glaciological mass-balance time-series in the intertropical zone of South America goes back to 1991 on Glaciar Zongo, Bolivia. Photogrammetric and hydrological data have been used to (1) check the specific net balance over long periods and (2) extend the mass-balance time series over the last 50 years. These data reveal a bias in the glaciological mass balance which can be explained by the field-measurement sampling network. Our study shows a large temporal variability of the surface mass balances in the ablation area and reveals strong relationships between independent surface mass-balance data coming from selected ablation areas with numerous data. It demonstrates the very large contribution (80%) of low-elevation ranges (one-third of the surface) to the specific mass balance and, consequently, the importance of the reduction of the area of the tongue. With these new results, Glaciar Zongo offers the longest and most accurate mass-balance series in any Andean country. The dataset shows that Glaciar Zongo experienced a relatively steady state over the period 1956–75, with even a slight mass gain over 1963–75, and a rapid and continuous decrease since then.

Glacier recession on Cerro Charquini (16°s), Bolivia, since the maximum of the Little Ice Age (17th century)

Cerro Charquini, Bolivia (Cordillera Real, 5392 m a.s.l.) was selected as a site to reconstruct glacier recession since the maximum of the Little Ice Age (LIA) in the central Andes. Five glaciers, located on differently exposed slopes, present comprehensive and well-preserved morainic systems attributed to former centuries. The moraines were dated by lichenometry and show a consistent organization on the different slopes. The past geometry of the glaciers was reconstructed using ground topography and aerophotogrammetry. Lichenometric dating shows that the LIA maximum occurred in the second half of the 17th century, after which the glaciers have receded nearly continuously. Over the last decades of the 20th century (1983-97), recession rates increased by a factor of four. On the northern and western slopes, glaciers receded more than on the southern and eastern slopes (by 78% % and 65% % of their LIA maximum area, respectively). The mean equilibrium-line altitude (ELA) rose by about 160 m between the LIA maximum and 1997. Recession rates were analysed in terms of climatic signal, suggesting that glacier recession since the LIA maximum was mainly due to a change in precipitation and that the 19th century may have been drier. For the 20th century, a temperature rise of about 0.68C appears to be the main cause of glacier recession. Recent climatic conditions from 1983 to 1997 correspond to a mass deficit of about 1.36 m w.e. a -1 . If such conditions persist, the small glaciers below 5300 m a.s.l. in the Cordillera Real should disappear completely in the near future.