The silt of long-vanished lakes in New England shows traces of El Niño as far back as 17,500 years ago. The findings, reported in the 12 May issue of Science, indicate that tropical Pacific warmings have appeared at roughly constant intervals, but their strength has waxed and waned for thousands of years at a time.
The new record is a byproduct of century-old geologic work. Early in the 20th century, Swedish geologist Ernst Antevs compiled records of the layers, or varves, of glacial sediment in lakes in New England. With these, he sorted out the history of the great ice sheets' retreat northward beginning 20,000 years ago. Geologists Tammy Rittenour and Julie Brigham-Grette of the University of Massachusetts, Amherst, shared Antevs's interest; they took a digitized version of his thickness records published in the 1920s, filled in a few gaps, and verified Antevs's chronology of the glacial retreat from 17,500 to 13,500 years ago.
This record offered tantalizing hints about climate as well. The duo teamed with statistical climatologist Michael Mann of the University of Virginia in Charlottesville to extract a history of climate variability over that same 4000-year period. The thicker a varve, the warmer the weather in New England that year. They found that New England climate varied with the same 2.5- to 5-year beat that marks the comings and goings of El Niño today. Apparently, then as now, El Niño's influence reached into New England. The researchers did find that strong El Niños were becoming less frequent toward the end of the ice age, 13,500 years ago.
That El Niño is such an old phenomenon came as a surprise; recently, a 12,000-year climate record retrieved from an Ecuadorian lake, close by the tropical Pacific, shows no sign of its torrential rains. But that record may contain only the strongest El Niños, the researchers say, while the New England record may contain both large and small ones. Perhaps El Niño never went away entirely, but waned so much that it temporarily disappeared from the Ecuadorian lake record, they argue; new climate models by two groups have recently shown how Earth's wobbling could have modulated the strength of El Niño by changing the timing of solar heating through the year.
The New England chronology is a "real opportunity" to look at El Niño at a time when the climate was different, says paleoclimate modeler John Kutzbach of the University of Wisconsin, Madison. By seeing how El Niño reacts when prodded by changes in the rest of the climate system, he notes, researchers should get a better idea of how El Niño will behave in the future--for instance, when the greenhouse effect warms Earth.
--RICHARD A. KERR