When scientists affiliated with the Florida Institute of Technology analysed fossilised pollen trapped in sediments from Lake Titicaca, they found that during two of the past three interglacial periods, the lake shrank by as much as 85 per cent. In each case, steady warming caused trees to migrate upslope, as they are doing today, and then suddenly die out, with desert replacing the woodland. ‘The evidence is clear that there was a sudden change to a much drier state,’ said climatologist Mark Bush.

Further evidence from algae found in the lake and carbonate deposits also pointed to a sudden shallowing of the lake due to evaporative loss. An environmental reconstruction showed that evaporation of the lake was behind the sudden shift from woodland to desert. As the lake contracts, local climatic effects – notably a doubling of rainfall – that are the result of the presence of a large lake are lost.

The researchers used the growth limits of Andean forests to estimate that the sudden shift to desert would take place when temperatures were around 1.5–2°C warmer than modern conditions. As the Peruvian Andes are warming at around 0.3–0.5°C per decade, that suggests that the climate tipping point could be reached by 2050 or earlier. The resulting drought would be disastrous for the water supply and agricultural capacity of La Paz’s two million inhabitants, according to the scientists.

January 2011