Archaeologists, ocean scientists team up on detailed study of historic climate cycles in Pacific Ocean
FRISCO — Today’s climate models may not do a very good job of predicting changes in the Pacific Ocean El Niño-La Niña cycle, an international team of scientists said after studying old seashells that display a distinct history of climate variations.
Understanding how El Niño responds to global warming is significant because the undulating rhythm of warming and cooling waters in the equatorial Pacific is a key driver of weather patterns around the world. Some modeling studies have suggested that ancient El Niños may have been weaker than today’s but the new research suggests they were as strong and as frequent as they are now, at least going back about 10,000 years.
“We thought we understood what influences the El Niño 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.
“Our data contradicts the hypothesis that El Niño activity was very reduced 10,000 years ago, and then slowly increased since then,” said first author Matthieu Carré, who did the research as a UW postdoctoral researcher and now holds a faculty position at the University of Montpellier in France.
Along with a team of archaeologists, Carré visited Peru and sampled 25-foot-tall piles of shells from Mesodesma donacium clams eaten and then discarded over centuries into piles that archaeologists call middens. Carbon dating the remains of charcoal from fires and measuring oxygen isotopes in the growth layer of the shells helped establish an accurate record of ocean temperatures.
That record suggests that, 10,000 years ago the El Niño cycles were strong, contradicting current leading interpretations. Roughly 7,000 years ago the shells show a shift to the central Pacific of the most severe El Niño impacts, followed by a lull in the strength and occurrence of El Niño from about 6,000 to 4,000 years ago.
According to Carré, melting polar ice sheets 10,000 years ago may have shifted ocean currents and intensified El Niño cycles, outweighing the effects of slight wobbles in the Earth’s orbit around the sun.
“Climate models and a variety of datasets had concluded that El Niños 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 Niño 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 Falcón at Peru’s Minister of Culture; and Michèle Julien and Danièle Lavallée at France’s René Ginouvès Institute for Archaeology and Anthropology.