Bolivian capital La Paz could be desert in 30 years

Twice in the recent geological past warmer periods have turned parts of Peru and Bolivia into wasteland. Researchers at the Florida Institute of Technology who looked at the past 370,000 years of climate and vegetation in the region found that a rise in temperature of as little as 3 degrees could turn these areas, now home to more than two million people, into an arid desert.

The region, which includes the Bolivian capital city of La Paz, is currently woody shrub and grassland. However climatologists who studied fossilized pollen in the sediments of Lake Titicaca on the border between Peru and Bolivia found that twice now the lake has shrunk by 85% and the grasslands were replaced by deserts.

The first period interglacial lasted from 330,000 to 320,000 years ago, the second from 130,000 to 115,000.

Both times, warming caused trees to migrate up hills, exactly as is happening today. However at a certain point in the warming, the area suddenly flipped from woodland to desert. The tipping point appears to have been caused by increased evaporation from Lake Titicaca. A large lake can almost double rainfall in a local area, so at a certain point when the lake got too small, rainfall would have decreased, creating a negative feedback loop.

The researchers say that based on the growth limits of Andean forests, the tipping point probably occurred when temperatures rose more than 3 to 5 degrees over current temperatures.

The research appears in the November issue of the journal Global Change Biology.

Based on current rates of warming in the Peruvian Andes, they say the necessary temperatures should be reached in between 30 and 40 years.

"The implications would be profound for some two million people," says Paul Filmer, program director in National Science Foundation's Division of Earth Sciences, which funded the research. Severe drought, and a loss of stored water in lakes in the region, would reduce yields from important agricultural regions and threaten drinking water supplies.

By Elizabeth Weise