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Wednesday, 25 October, 2000, 17:58 GMT 18:58 UK
Coral record reveals secrets of El Nino
Coral
Coral documents large changes in the weather system El Nino over the past 155 years
New coral records suggest extreme weather events have become more prevalent since the mid 1970s, possibly because of global warming.

The warmer and wetter weather seen in the central Pacific over the last 25 years is unprecedented since 1840, say US climate researchers.

A core of 155-year-old tropical Pacific coral has enabled scientists to look back on climate change over the past two centuries.

The coral record has revealed large changes in the weather phenomenon El Nino, which brings hurricanes and severe droughts to parts of the world.

El Nino stepped up a gear in 1976, the researchers have found.

But another change in the El Nino weather system happened in the early 20th Century, before the burning of fossil fuels became widespread, according to the coral data.

This suggests that natural factors, as well as man's influence, can drive climate change.

Living record

Scientists at the University of Colorado, Boulder, studied a core of 155-year-old coral drilled at Maiana Atoll in the central Pacific.

Reef corals grow at a rate of about one centimetre (0.39 inches) a year. As they grow, ocean temperatures and salt levels leave their mark on the skeleton of the coral. This provides a record of past climate.

According to the coral data, in the late 19th Century, before carbon-based gases first started being released into the air on a massive scale by the burning of fossil fuels, El Nino returned about every 10-15 years.

In the early 20th Century, the frequency of El Nino events increased to about every three years.

The cycle changed again in 1976, but more dramatically. El Nino returned roughly every four years, along with a jump of about 0.6C (1.2F) in ocean warmth.

Other records

"Our record shows that the warming and (ocean) freshening in this region since 1976 is unprecedented since AD 1840," the authors wrote in the leading scientific journal Nature.

Commenting on the findings, Dr Robert Dunbar of Stanford University, California, said other coral records from the Indian and Pacific oceans support the idea that the recent warm and wet conditions in the central Pacific appear unique over the past 200 to 300 years.

"It seems likely that (El Nino and La Nina) will respond to further global warming," he warned.

El Nino occurs when a huge mass of warm water builds up in the western Pacific, with widespread effects on weather in many parts of the world.

When the system goes into reverse, it is called La Nina.

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See also:

08 Sep 00 | Sci/Tech
Ice records reveal warming trend
04 May 00 | Sci/Tech
Coral collapse in Caribbean
07 Aug 00 | Sci/Tech
The dangers of climate change
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