For the lowland areas the evidence shows mostly no significant trends, but interspersed with a few positive trends. For the purposes of the simulations in the next section, we will assume a slight warming trend of 0.05oC/decade in all lowland areas.

The general cooling of the highlands appears to be inconsistent with the rapid melting of several Bolivian glaciers (especially the Chacaltaya and the Zongo glaciers close to El Alto, see Francou et al _,_ 2000 and Ramirez et al _,_ 2001), but it is not. First of all, the glaciers have been melting continuously since the Little Ice Age (about 1550 to 1850), with only a brief slowdown during the relatively cool period of 1950-1980, and it is normal for melting to accelerate towards the end (just like a small ice cube melts faster than a big ice cube).

In addition, glaciers depend on other factors than temperature, notably precipitation, but also cloud cover, relative humidity and the intensity of solar irradiation (Ramirez, 2008). A study of oxygen isotope series generated from ice cores from two Bolivian glaciers suggests that precipitation has decreased steadily since about 1974 (Hoffman et al, 2003)<sup>2</sup> .

The reduction in precipitation is likely associated with the general reduction in cloud cover over the tropics since measurements began in the early 1980s<sup>3</sup> , and less clouds means more intense solar irradiation, which accelerates glacial melt. Decreased cloud cover at this altitude also works to amplify the diurnal temperature range, increasing daytime temperatures (which would cause increased melting), but reducing night-time temperatures even more (because of the missing cloud-blanket), which explains the reduction in average temperatures.

Moreover, upon closer inspection of the temperature data, it becomes clear that temperatures have not fallen equally throughout the year. At the El Alto station, for example, summer temperatures have been increasing and winter temperatures decreasing (see Figure 2). Since winter is the dry season in El Alto, the lower winter temperatures will provide little benefit for the glaciers, which cannot accumulate mass without snowfall. Instead, these glaciers are much more sensitive to changes in summer temperatures and precipitation. This explains why the ENSO (El Niño-Southern Oscillation) has such a strong effect on Bolivian glaciers. During ENSO’s warm and dry phase (El Niño), the mass balances are always negative, implying shrinking glaciers. In the cooler and more humid La Niña phase, the glaciers return to equilibrium and sometimes show a small increase. The increase in the glacier regression rate since the end of the 1970s appears to coincide with the Pacific shift of 1976, the date after which the El Niño event became more frequent and more intense (Ramirez et al, 2001). It was the unusually strong El Niño event of 1997/98, which caused the permanent closing of the World’s highest ski-resort on the Chacaltaya glacier.

> 2 However, this decrease is not considered unusual in a historical context and over the whole 20th century the isotope index is stable

> 3 According to NASA’s International Satellite Cloud Climatology Project – ISCCP

> (http://isccp.giss.nasa.gov/index.html ), average tropical cloud cover has decreased from about 66% in the 19080s to about 61% in the first 8 years of this century.

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