By Sid Perkins
Analyses of sediments from a South American lake suggest that the worldwide weather effects of El Nios occurred more frequently about 1,200 years ago, when Europeans were entering the Middle Ages, than they do today. El Nios are periodic, sustained warmings of the ocean in the equatorial Pacific.
Two 8-meter-long cores of layered mud from Laguna Pallcacocha, a lake 4,200 m high in the Andes, chronicle southern Ecuador’s climate for the past 12,000 years. But, those sediments probably don’t record all El Nios, says geologist Geoffrey O. Seltzer of Syracuse University in New York. That’s because weak El Nios probably wouldn’t send precipitation high enough in the Andes to wash thick layers of silt–an El Nio signature–into the lake.
The sediments suggest that between 12,000 and 7,000 years ago, no more than five strong El Nios occurred each century. Their frequency then increased, peaking in the 9th century A.D., when they occurred every 3 years or so. Seltzer and his colleagues report their findings in the Nov. 14 Nature.
Today, El Nios occur once every 2 to 8 years, says Seltzer. However, El Nios strong enough to be recorded in Laguna Pallcacocha’s sediments probably occur now only once every decade or so.
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