91̽»¨ oceanographers used clues from the Galapagos Islands — a dot in the middle of the Pacific Ocean — to trace El Niño patterns and seasonal tropical rains over the past 2,000 years. Evidence shows shifts that last for centuries, suggesting these tropical climate patterns have varied more radically and for longer durations than previously believed.

The is published the week of March 14 in the Proceedings of the National Academy of Sciences.

“This is one of the most representative records of precipitation changes in the eastern Pacific, and it goes a good way toward explaining seasonal tropical rain band and El Niño dynamics,” said first author , who did the research for his 91̽»¨doctorate in oceanography.

“We want to understand not just if precipitation has changed in the past, but why,” he added. “If we can understand the mechanisms that caused these changes, then we can better predict how precipitation might change in the future.”

Nelson collected sediments in June 2008 from three lakes on the edge of , the largest island in the Galapagos chain off Ecuador. The island sits directly on the equator, at sea level and far from any high elevation, so what happens there is a good indication of what’s happening in the wider tropical Pacific Ocean.

He studied remains of two organisms — a microscopic algae and the waxy coating of mangrove leaves — for clues to the amount of past rainfall. The form of hydrogen atoms preserved in molecules left behind by these organisms in the sediments tracks the rainfall intensity. (When tropical precipitation is more intense, the raindrops include more of the lighter form of hydrogen.)

Simply measuring the amount of rainfall could track either the strength of El Niño or the , a global atmospheric feature related to the trade winds that moves north and south each year, bringing heavy rains and seasonal flooding in its path.

So the researchers also looked for evidence of past sea level, which changes much more with El Niño than with the Intertropical Convergence Zone. Chemical clues in the mangroves and algae can also tell how salty the water was, which shows whether or not El Niño was raising the local sea level and causing more seawater to seep into the coastal wetlands.

“This new approach is the first quantitative reconstruction of these tropical climate parameters, rather than just saying it was ‘wetter’ or ‘drier,'” said co-author , a 91̽»¨professor of oceanography. “By providing historic climatological data we will be able to model future changes in tropical climate.

Results show that from the beginning of the record 2,000 years ago, until the year 1400, most rainfall changes on the island were indeed related to El Niño.

The record also shows long-term shifts. Heavier rainfall at the study sites from the year 0 to 400, and again during Europe’s , just before the Little Ice Age from about the year 800 to 1300, was probably caused by a centuries-long strengthening of El Niño.

“This record shows that there have been quite large changes in El Niño precipitation in this area in the past, and that we might expect large changes in the future,” Nelson said.

But during the , a period from roughly 1400 to 1850 when temperatures in Europe were cooler and many of Earth’s glaciers expanded, the biggest changes came from the Intertropical Convergence Zone shifting to the south.

Recent research has shown that the position of the Intertropical Convergence Zone, and the associated rainfall and drought, is tied to the between the Northern and Southern hemispheres.

“It’s consistent with what we understand about the changes in the ITCZ that it might have been positioned further south when the Northern Hemisphere was cooler,” Nelson said.

These samples augment other marine records such as coral and seashells, which provide detailed records over a short time period, and deep-ocean sediments, which preserve thousands of years of history but are harder to date precisely.

“These lake sediment records are a nice complement because they sort of span the gap,” said Nelson, who is now a postdoctoral researcher at the University of Basel in Switzerland.

This winter, Sachs got a from the National Science Foundation to measure the effect on Galápagos lakes of the first strong El Niño event in 18 years. In January, Nelson and , another former 91̽»¨doctoral student in Sachs’ group, installed sensors and collected samples. The observations will be used to allow more precise reconstructions of historic climate shifts.

The research was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration.

###

For more information, contact Nelson at daniel.nelson@unibas.ch or +41 61 267 29 88 and Sachs at jsachs@uw.edu or 206-221-5630.

Grants: NSF EAR-1348396 and ESH-0639640, NOAA NA08OAR4310685

The results, from the 91̽»¨ and University of Montpellier, question how well computer models can reproduce historical El Niño cycles, or predict how they could change under future climates. The paper is now and will appear in an upcoming issue of .

“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 , a 91̽»¨professor of oceanography.

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

“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 , who did the research as a 91̽»¨postdoctoral researcher and now holds a faculty position at the University of Montpellier in France.

In 2007, while at the UW-based , Carré 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, Carré 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 Niño 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 Niño impacts, followed by a lull in the strength and occurrence of El Niño from about 6,000 to 4,000 years ago.

One possible explanation for the surprising finding of a strong El Niño 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 Niño activity by changing ocean currents,” Carré 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 Niños.

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

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

“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 91̽»¨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.

###

For more information, contact Sachs at 206-221-5630 or jsachs@uw.edu and Carré at matthieu.carre@univ-montp2.fr. Carré is currently in Brazil and will have email access about once a day. Sachs leaves Aug. 14 for a month-long research trip during which he will have only intermittent email access.