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El Niño, the periodic warming of waters in the eastern Pacific that affects the weather worldwide, began about 5,000 years ago, according to a study of fish bones.

A group of US researchers led by C. Fred T. Andrus from the University of Georgia studied tiny bones called otoliths found in the inner ears of Peruvian sea catfish. These bones grow in alternating opaque and translucent bands that reflect biological and environmental growth conditions, such as sea surface temperature.

A comparison of the bones from modern catfish and fish bones found in two Peruvian archaeological sites suggests that sea temperatures about 5,000 years ago were 3 to 4 degrees Celsius warmer than today, and that there was little variation in temperature.

The findings — publishing in the 22 February issue of Science — support the idea that the upwelling of the cool Peru-Chile current in northern Peru was weaker prior to 5,000 years ago, and that ‘modern’ El Niño Southern Oscillation conditions set in shortly after.

The study may also have implications for understanding early Peruvian civilisation, whose economy increased in complexity around this time.

Reference: Science 295, 1508 (2002)