Did El Nino make societies bloom?

A tiny pond perched near the rim of the Andes has helped geologists decipher when the climatic trickster known as El Nino first started disrupting weather in South America. Their findings provide some support for the idea that El Nino's appearance helped stimulate the growth of prehistoric societies.

In its modern incarnation, El Nino is an irregular but frequent visitor to the Pacific basin. Every 2.5 to 7.5 years, it warms up the central and eastern equatorial Pacific, bringing rains to Ecuador and Peru. Up in the Andes, these storms wash loose sediment into mountain lakes, providing a means of tracking El Nino's history, says Donald T Rodbell of Union College in Schenectady, N.Y.

Rodbell and his colleagues drilled into the bottom of an Ecuadorian lake and pulled up a core of the sedimentary layers deposited over the past 15,000 years. The core has hundreds of light and dark bands, which the team dated using carbon-14 analysis. Dark bands represent sediments rich in the organic remains of vegetation that accumulated between storms. Light-colored layers, poorer in carbon, formed when rains swept freshly eroded rock into the pond.

The banding revealed a fundamental shift in the frequency of rains. Before 7,000 years ago, the storms came only once every 15 to 70 years--a pattern quite different from the modern El Nino cycle, the researchers report in the Jan. 22 SCIENCE. The El Nino pattern of storms every few years didn't start until 5,000 years ago, something hinted at in previous studies. "It supports the argument that prior to 5,000 years ago, there wasn't El Nino," says Rodbell.

That timing matches up with the growth of more complex societies in coastal Peru, says Daniel H. Sandweiss, an archeologist from the University of Maine in Orono. Agriculture spread, and people started building temple mounds along the central Peruvian coast. China, Japan, and other places also show cultural shifts at about the same time. The more frequent rains of El Nino, he suggests, triggered societal changes perhaps by spurring agricultural development or disrupting life in ways that allowed new political opportunities.

Don't give too much credit to El Nino, argues Lisa E. Wells, a geologist at Vanderbilt University in Nashville, Tenn. While the shift in storm frequency may have helped societies, she says, sea level also played an important role. Global sea level steadied about 6,000 years ago, after rising rapidly for thousands of years. Once the spreading seas stopped gobbling up land, river deltas started to form and silly soils covered flood plains. In coastal Peru, "there was no place for them to do agriculture before sea level stabilized," she says.

Rodbell's team plans to check its findings by getting equivalent records from other Andean lakes this year.