The planet's largest and most powerful driver of climate changes from one year to the next, the El Nino Southern Oscillation in the tropical Pacific Ocean, was widely thought to have been weaker in ancient times because of a different configuration of the Earth's orbit. But scientists analyzing 25-foot piles of ancient shells have found that the El Ninos 10,000 years ago were as strong and frequent as the ones we experience today.

The results, from the University of Washington and University of Montpellier, question how well computer models can reproduce historical El Nino cycles, or predict how they could change under future climates. The paper is now online and will appear in an upcoming issue of Science.

"We thought we understood what influences the El Nino mode of climate variation, and we've been able to show that we actually don't understand it very well," said Julian Sachs, a UW professor of oceanography.

The ancient shellfish feasts also upend a widely held interpretation of past climate.

"Our data contradicts the hypothesis that El Nino activity was very reduced 10,000 years ago, and then slowly increased since then," said first author Matthieu Carre, who did the research as a UW postdoctoral researcher and now holds a faculty position at the University of Montpellier in France.

In 2007, while at the UW-based Joint Institute for the Study of the Atmosphere and Ocean, Carre accompanied archaeologists to seven sites in coastal Peru. Together they sampled 25-foot-tall piles of shells from Mesodesma donacium clams eaten and then discarded over centuries into piles that archaeologists call middens.

While in graduate school, Carre had developed a technique to analyze shell layers to get ocean temperatures, using carbon dating of charcoal from fires to get the year, and the ratio of oxygen isotopes in the growth layers to get the water temperatures as the shell was forming.

The shells provide 1- to 3-year-long records of monthly temperature of the Pacific Ocean along the coast of Peru. Combining layers of shells from each site gives water temperatures for intervals spanning 100 to 1,000 years during the past 10,000 years.

The new record shows that 10,000 years ago the El Nino cycles were strong, contradicting the current leading interpretations. Roughly 7,000 years ago the shells show a shift to the central Pacific of the most severe El Nino impacts, followed by a lull in the strength and occurrence of El Nino from about 6,000 to 4,000 years ago.

One possible explanation for the surprising finding of a strong El Nino 10,000 years ago was that some other factor was compensating for the dampening effect expected from cyclical changes in Earth's orbit around the sun during that period.

"The best candidate is the polar ice sheet, which was melting very fast in this period and may have increased El Nino activity by changing ocean currents," Carre said.

Around 6,000 years ago most of the ice age floes would have finished melting, so the effect of Earth's orbital geometry might have taken over then to cause the period of weak El Ninos.

In previous studies, warm-water shells and evidence of flooding in Andean lakes had been interpreted as signs of a much weaker El Nino around 10,000 years ago.

The new data is more reliable, Carre said, for three reasons: the Peruvian coast is strongly affected by El Nino; the shells record ocean temperature, which is the most important parameter for the El Nino cycles; and the ability to record seasonal changes, the timescale at which El Nino can be observed.

"Climate models and a variety of datasets had concluded that El Ninos were essentially nonexistent, did not occur, before 6,000 to 8,000 years ago," Sachs said. "Our results very clearly show that this is not the case, and suggest that current understanding of the El Nino system is incomplete."

The research was funded by the U.S. National Science Foundation, the U.S. National Oceanic and Atmospheric Administration and the French National Research Agency.

Other co-authors are Sara Purca at the Marine Institute of Peru; Andrew Schauer, a UW research scientist in Earth and space sciences; Pascale Braconnot at France's Climate and Environment Sciences Laboratory; Rommel Angeles Falcon at Peru's Minister of Culture; and Michele Julien and Daniele Lavallee at France's Rene Ginouves Institute for Archaeology and Anthropology.