Part of northern Chile's Atacama Desert is so arid botanists call it the Absolute Desert. But just 3000 years ago, researchers have found, the land was surprisingly green. The discovery conflicts with earlier estimates that the region was bone dry at that time, and it suggests ancient climate changes may be more complex than scientists thought.

The Andes Mountains of Peru and northern Chile are a good place to look for clues to climate change. The high mountain plateau, including the Atacama Desert, is very dry; that's because most of the continent's rain falls on the Amazon basin, and clouds are usually depleted by the time they reach the Andes. Such a dry climate is likely to retain evidence of subtle changes in rainfall.

To investigate how the Atacama's climate has changed since the last ice age, about 16,000 years ago, a team led by Julio Betancourt of the U.S. Geological Survey in Tucson, Arizona, used two methods. First, they studied the kinds of vegetation that rodents had placed in their nests and carbon-dated these plant scraps. Then they carbon-dated shells of aquatic organisms called diatoms that lived in springs that have long since dried up. Combining the data, the researchers found evidence for two wet spells. About 16,000 years ago, grasses and shrubs began to sprout, peaking about 6000 years later, when up to five times more rain fell than today. Rainfall increased again from about 8000 to 3000 years ago, the team reports in the 1 September issue of Science. Betancourt thinks the wet spells occurred because El Niño, the climate oscillation that normally prevents moisture from reaching the Atacama Desert, was weaker than usual or perhaps nonexistent.

The finding doesn't fit with work by Geoffrey Seltzer, a geologist at Syracuse University who has studied Lake Titicaca in Peru, about 1000 kilometers to the north. His measurements of lake level changes and sediments indicate drier conditions. But he nevertheless praised Betancourt's research methods and the accuracy of his dating technique. "The challenge is how to integrate this with other climate data from the Andes," he says.

--JOHN MACNEIL

Related sites
Atacama Desert Information from NASA
USGS Site on Packrat Middens