Some mud stashed away in the back of a refrigerator at the College may begin to answer a number of questions that scientists are asking about El Niño.
Donald T. Rodbell, assistant professor of geology, reported in the January 22 issue of Science on a thirty-foot-long column of sediment he and a fellow researcher obtained in 1993 from a high-elevation lake in the southern Ecuadoran Andes.
Rodbell, the lead author of the Science article, wrote that this sample suggests that during the past 5,000 years, El Niño occurred every two to eight years — the same frequency we see in modern times. The sample also suggests that El Niño was weak or non-existent between 5,000 and 12,000 years ago.
Rodbell's co-authors are Jeremy Newman '97; Geoffrey Seltzer of Syracuse University; David Anderson of the National Oceanic and Atmospheric Administration; Mark Abbott of the University of Massachusetts, Amherst; and David Enfield of the Atlantic Oceanographic and Meteorological Laboratory.
Their study, while preliminary, has generated excitement among scientists worldwide who are trying to unlock the mysteries of El Niño. Previous studies of El Niño looked at coral reefs, showing changes in sea surface temperatures over centuries in places such as the Galapagos Islands in the Pacific Ocean. Other researchers looked at ice cores, tree rings, and flood deposits. But nothing to date provided as clear or as detailed a picture of El Niño's fluctuations as the lake sediments.
Rodbell and Seltzer obtained the sediment core sample from Lake Pallcacocha in southern Ecuador (El Niños typically begin off the coasts of Ecuador and Peru, and sea surface temperatures near the lake are among the first to warm during the onset of an El Niño). The lake was formed when the glaciers receded about 14,000 years ago, leaving moraines — piles of rock and sand formed by glacial motion. After initial analysis of the sample, the core went back into a laboratory refrigerator at Union, where it stayed for three years.
In 1996, Rodbell and his student, Jeremy Newman, began to take another look at the sample. Adapting medical imaging software to quantify shifts in the patterns of layers, they found a data-rich natural archive, the first continuous record of El Niño events dating back more than 5,000 years.
“We didn't know exactly what we had,” Rodbell says. “When we took this core, we thought the light-colored striations from landscape and flood events were very pronounced and unusual. We called them 'zebra stripes.' At the time, we were more interested in climate change associated with the last ice age, but in the back of my mind I wondered if these patterns were somehow connected to El Niño.”
The core sample contains hundreds of layers of sediment deposited over a period of about 15,000 years. About midway through the thirty-foot core (representing a period of time beginning about 5,000 years ago), there are a series of light-colored sediment bands that occur approximately every ten years or less. The bands contain the type of debris — mostly inorganic material washed from the slopes of nearby mountains — that would flow into the lake only during periods of heavy rainfall and flooding — conditions likely triggered by ancient El Niño events, Rodbell says.
El Niño researchers were excited by the findings.
Michael McPhaden, senior research scientist at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, said, “When you find a clear signal, it's pretty important.”
McPhaden said the findings are important “because El Niño is the strongest climate signal on the planet, other than the seasons. The key to forecasting the future is understanding the past. So these longer records open up new possibilities to us.”
Characterized by warm ocean currents that begin off of the western South American coast, El Niño can affect weather systems across both North and South America — from tumultuous rainfall in northern Peru and southern Ecuador to unusually warm and dry conditions in the northeastern United States and western tropical Pacific. Until now, scientists did not have a clear understanding of when these events first began. Written records and anecdotal observations of El Niño events go back only several centuries.
“The question many scientists are asking is, 'What will happen to El Niño as the global climate gets warmer,'” Rodbell says. “The computer models are good, but they are limited by our understanding of how El Niño works.”
Rodbell says his study poses a number of questions, among them what is driving El Niño's frequency.
“The question is what is causing that frequency to change,” he says. “Could it be global warming? We don't know the answer yet. The climate system is incredibly complicated. There are so many different variables that could be at work.”
Rodbell was recently awarded a two-year grant of $90,000 from the National Science Foundation to investigate the climatic record preserved in sediment cores from other high-elevation lakes in southern Ecuador. The initial core suggests that lakes in this area may provide a natural archive of El Niño events covering the last 15,000 years, the longest continuous record of El Niño activity ever discovered.
“Based on this work, we realized that this one lake, while interesting because it may have provided the longest El Niño record ever found, is only one sample,” Rodbell says. “We want to go in and compare it with samples from other lakes in the area.”
Rodbell has led ten research expeditions into the Andes, and he plans to return this summer for two more months of research.
Defining the terms
El Niño is the name given to the phenomenon that occurs when sea-surface temperatures in the equatorial Pacific Ocean off the South American coast become warmer than normal. Those persisting temperatures influence the atmospheric circulation and consequently change climate patterns globally.
El Niño is translated from the Spanish as “the boy child.” Peruvian anchovy fishermen traditionally used the term — a reference to the Christ child — to describe the appearance of a warm ocean current west coast of Peru and Ecuador around Christmas. Over the years the term has become synonymous with larger-scale, climatically-significant, warm events.
La Niña (“the girl”) is the opposite of El Niño; sea surface temperatures in the equatorial Pacific become cooler than normal.