May 15, 2000 AMHERST, Mass. - The New England region underwent El Niño-like climate changes during the Ice Age, a team of researchers has found. University of Massachusetts geoscientist Julie Brigham-Grette led the research, along with former UMass graduate student Tammy Rittenour (now a doctoral candidate at the University of Nebraska), and environmental scientist Michael Mann, now at the University of Virginia. The findings are detailed in the May 12 issue of Science. The project was funded by the National Science Foundation, National Geographic Society, and the University of Massachusetts.
Scientists define El Niño as a disruption of the ocean-atmosphere system in the tropical Pacific, having important consequences for weather around the globe. A weakening of the tradewinds allows unusually warm currents in the western Pacific to flow eastward across the equatorial Pacific to the western coast of South America. This exceptionally large area of warm ocean surface waters occurs cyclically, often causing significant changes in global weather patterns, including those affecting the northeastern United States. The team's findings show a strong three-to-five-year cycle of El Niño activity during the later part of the last Ice Age - the same frequency with which El Niño occurs today. Researchers focused on the era when the Laurentide Ice Sheet was slowly receding northward across New England, leaving Glacial Lake Hitchcock in its wake. At its peak, the glacial lake filled much of the Connecticut Valley, including the area that eventually became the UMass campus.
The report offers scientists a clearer understanding of El Niño's persistence at a time when climate conditions were fundamentally different from the climate conditions of today, according to the team. "El Niño-like climate change can happen under all sorts of conditions, even when the Northern Hemisphere is covered with large ice sheets," said Brigham-Grette. "It's not just a warm-weather phenomenon. Knowing this will help scientists in determining what drives El Niños."
"It was previously thought that El Niños were a warm-weather phenomenon that began only 5,000 years ago," said Rittenour. "Our findings have pushed the record of El Niño activity back to 17,500 years ago. This is remarkable because at that time, all of Canada and most of the northern United States was covered by a large ice sheet."
"This glacial evidence suggests that the El Niño phenomenon is even more robust than we previously imagined," added Mann. "This provides a new perspective on the changes in El Niño observed in recent decades, and their possible relationship with global climate change."
In September 1997, Rittenour and Brigham-Grette retrieved two sediment cores from the western edge of campus, at a site used for civil engineering research. One core was 105 feet (30 meters) long; the other was 25 feet (7.6 meters) long. Much of Brigham-Grette's work has focused on learning about past climate by studying sediment archives in Arctic coastal areas and lake beds as far-off as Siberia; in contrast the UMass core was taken from her professional backyard. Mann assisted the team by applying modern statistical methods to develop a climate history from these records, and analyze it for insights into the past behavior of El Niño.
The team split and examined the cores before conducting scientific measurements on the samples. The layers of sediment offer scientists a very detailed record of climate variability, scientists say. "The thickness of the annual sediment layers in the cores tells us about the amount of meltwater coming off the glacier," Brigham-Grette explained. "The warmth of El Niño-like events caused faster melting of the glacier front and perhaps a change in precipitation. This resulted in thicker annual layers, or varves, during those seasons." The cores provided not only an annual record of climate, but seasonal records as well. The spring and summer season contains most of the variability. The project enabled researchers to place the local sediment history into the regional geological framework and in the future will allow comparisons with long records of climate change in the Greenland Ice Sheet.
The team's findings bolster those made by a University of Stockholm researcher nearly a century ago. In 1922, Ernest Antevs produced a 4,000 year-long, regional climate record based on layered sediments, (which scientists call a "varved chronology"). Antevs physically measured hundreds of areas of exposed sediments that dated to the glacier's retreat. Although his work is still considered by many to be a gold standard in geoscience, the Antevs record had some small gaps. The team was able to complete one major section of the Antevs record, based on findings from the UMass cores.
The paper represents a portion of Rittenour's master's thesis at UMass. For her new research in the Mississippi River Valley, Rittenour has just won the J. Hoover Mackin Award for 2000 given by the Geological Society of America's Quaternary Division. This is the highest award given to a geology Ph.D. candidate in the country. Brigham-Grette won the same award 19 years ago.
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