Summary

The South American Altiplano is a high-level plateau in the central Andes between 15°S and 22°S, with a mean elevation of about 3,800 m above sea level, hosting more than 3 million inhabitants and diverse environmental conditions from the humid sector around Lake Titicaca to the arid sector around the Uyuni and Coipasa salt flats and the snowcapped mountains surrounding the plateau. The Andes at these latitudes separate a hyper-arid climate to the west from much more humid conditions to the east. Westerly winds in the middle troposphere bring dry air from March to November, hindering precipitation over the plateau. During December–January–February, however, the establishment of an upper-level anticyclonic circulation—the Bolivian High—allows the episodic transport of moist air from the interior of the continent, feeding convective storms over the high terrain. The frequent occurrence of clouds during the austral summer also favors the annual peak of the minimum (nocturnal) temperature and rather uniform values of the maximum (diurnal) temperature throughout the year. The chain of easterly winds aloft → high near-surface moisture → convective precipitation over the plateau (or, alternatively, westerly winds aloft → low near-surface moisture suppressed convection) also explains precipitation variations within the summer season and among years. In particular, the Altiplano tends to experience below (above) average precipitation during El Niño (La Niña) summers because of the stronger (weaker) westerly winds in those periods. Regarding local manifestation of climate change, there is consistent evidence that near-surface air temperature has increased across the Altiplanosince around 1980 to 2020 at about 0.25°C/decade. The output of global and regional climate models suggests that the observed warming is likely to continue and reach between 3°C and 5°C above the historic mean, depending on the emissions scenario. Trends in rainfall during the late 20th century and early 21st century are difficult to assess and vary in their sign and magnitude across the Altiplano. Likewise, future projections show little agreement in the fate of the precipitation during the rest of this century. Nonetheless, most climate models anticipate westerly wind anomalies at middle and upper levels over the Altiplano, suggesting that future rainfall would be significantly lower than those during the first decades of the 21st century.