Fall of Ancient Peruvian Societies Linked With El Niño

Hillary Mayell
for National Geographic News
July 24, 2001
A big shift in climate has been implicated in the fall of yet another civilization.

According to a report published in the July issue of the journal Geology, ancient civilizations of Peru collapsed at least in part because of a change in the weather pattern known as El Niño.

Indigenous people who lived on the coast of northern Peru began building large temple complexes about 5,800 years ago. The development of their culture, as seen in the elaborate temple building and public art discovered in the area, occurred even before the pyramids in Egypt were built.

The Peruvians continued building the complexes for nearly 3,000 years. But evidence indicates that around 2800 B.C., the sites were abandoned. Scientists working in Peru think they know why.

"We found that there was a change in the frequency of El Niño events about 3,000 years ago and that this correlates in time with cultural change," said Daniel Sandweiss, an archaeologist at the University of Maine who is a member of the team that has been conducting the research.

Other Examples

The idea that climate change can play a role in the rise and fall of civilizations is not new. Other studies have suggested that climatic events such as drought and flooding rains contributed to the downfall of early civilizations in Central and North America, Greenland, and the Middle East.

The climatic factor linked with the collapse of ancient settlements in Peru is the recurrent weather pattern known as El Niño. It occurs as a result of ocean and wind currents in the tropical Pacific, which lead to large-scale oceanic warming. The phenomenon induces major changes in weather patterns around the world, bringing drought, increased rainfall, and flooding in some areas.

In recent times El Niño has happened every two to seven years. Until the last quarter of this century, it occurred every seven to fifteen years.

But Sandweiss and his colleagues believe the pattern was quite different several thousand years ago.

Evidence from Shells

Evidence of El Niño's role in the collapse of ancient Peruvian settlements lies in mollusk shells found in middens—essentially trash piles—found at the sites. "We use the disappearance of some species and appearance of others in the middens as indicators of general frequency ranges," Sandweiss explained.

Mollusks are good indicators of climate because they are sensitive to temperature changes.

Two species of mollusk—Mesodesma donacium and Choromytilus chorus—were common in the middens of coastal Peru. But by 2,800 years ago, they had virtually disappeared.

The researchers attribute the rapid disappearance of these species to an increase in the frequency of strong El Niño events. Evidence in modern times shows that the first species has moved south as a result of recent episodes of El Niño. The second species is known to die more quickly when water temperatures rise.

"Looking at the mollusks, we are pretty certain that El Niño was absent or occurred very infrequently prior to 5,800 years ago, and that it was probably not more frequent than every 40 to 50 years between 5,800 and around 3,200 to 2,800 years ago," said Sandweiss.

Other researchers using lake cores and tree rings in the Galápagos Islands and the highlands of Ecuador have reached similar conclusions.

The temples and village sites in Peru were abandoned at about the same time that El Niño began occurring more frequently.

After building and growth continued for several thousand years, there was a hiatus several hundred years long, said Sandweiss. When settlements next arose, however, they had different characteristics than previous ones.

"When mounds are again constructed, not only are they larger, but they are part of a different political and religious system," he said.

Co-authors of the report include Kirk Maasch of the University of Maine; Richard L. Burger of Yale University; James B. Richardson III and Harold B. Rollins of the University of Pittsburgh; and Amy Clement of the University of Miami.

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