Ancient clams on the shores of Peru have revealed a new history of Earth’s most important and unpredictable climate phenomenon. But the implications for the future are murky at best.

The phenomenon in question is the El Niño–Southern Oscillation, or ENSO, a global climatic cycle that affects both sides of the tropical Pacific Ocean and beyond. During its warm phase, called El Niño, warm water sloshes from the west side of the Pacific to the east side, where it brings warmth and rain to the Americas. (During La Niña, the warming of the water on the west side intensifies, providing moisture and heat to Indonesia and Australia.) ENSO shifts winds all over the world, can create devastating rains and flooding in South and North America, and has even been linked to social unrest in countries that require imported food. (An El Niño is probably imminent now.)

Given the stakes, scientists have sought for years to better understand what has driven ENSO in the past and to predict how variable it will be in the future. Previous studies have used a variety of computer models and data from fossils and flood events to argue that ENSO has become more exaggerated over the past 11,000 years, known as the Holocene period. (Due to changes in Earth’s orbit, the story goes, the way solar energy has struck the planet has changed over that period, shifting wind or ocean currents that affected ENSO.)

But a new study paints a different picture. Scientists set up what amounted to seven archaeological digs on giant piles of surf clams created by human inhabitants over the past 10,000 years at several coastal locations in Peru. Among the piles, known in archaeology as middens, were clam shells mixed with charcoal and other remains. Radiocarbon dating of the charcoal samples provided an age for the shells; measurements of oxygen isotopes found in the clam fossils gave sea surface temperatures every 2 to 4 weeks during the clam’s life. That way, says paleoclimatologist Julian Sachs of the University of Washington, Seattle, “we can establish very well the maximum and minimum temperature in each year the clam lived.”

In a paper published online today in Science, Sachs and colleagues show that ENSO was basically as active at the beginning of the Holocene as it has been during more recent periods. “It’s not as simple as those earlier modeling and data papers suggested,” Sachs says. For example, the new study cites data the scientists collected from about 7000 years ago suggesting a westward shift in the location of greatest El Niño impacts. That would have coincided with the tail end of melting of the ice sheets from the last ice age, says first author Matthieu Carré of the Institute of Evolutionary Sciences of Montpellier in France. Meanwhile, he notes, the fresh water from the ice sheets could have changed circulation patterns and affected ENSO’s sloshing.

The paper “is very well done and well argued,” says climate expert Mark Cane of Columbia University’s Lamont–Doherty Earth Observatory in Palisades, New York. But in terms of ENSO’s history, it “leaves me more confused than ever,” because the new data conflicts with previous studies. He says the work should be followed by new modeling studies to try to clarify the past and also to address another issue: how global warming might affect ENSO in this century and the next one. “We can’t say which way ENSO is going to change yet,” Carré says.