Fire in the Amazon: El Nino's handiwork

The two scientists are Dr Sandy Tudhope, of the University of Edinburgh, UK, and Professor David Lea, of the University of California, Santa Barbara, US.

Writing in the journal Science, they describe their analysis of coral reef terraces on the Huon peninsula of New Guinea.

Analysis of isotopic and chemical variations in reef samples dating back 130,000 years yielded 14 different climatic "windows", each spanning 20 to 100 years. By analysing the temperature and salinity of the water, recorded in the coral, the scientists were able to reconstruct past climate.

They analysed cold periods 40,000 years ago and warm periods 125,000 years ago to evaluate the behaviour of El Nino during different climatic conditions. They found that El Nino had been strongest during the warmest periods, and about 50% weaker during ice ages.

Professor Lea said: "The samples indicated that El Nino was never more intense than the events of the last hundred years.

"Over the last century, we have very accurate records of El Nino, with 1982-83 and 1997-98 being the largest events on record.

"Everyone wants to know if the intensity of these large events is somehow related to global warming. Our data suggest that the behaviour of the tropical Pacific over the last 100 years is atypical, but it does not pinpoint which factors modulate El Nino."

Patchy record

Professor Lea acknowledged that the 14 climatic windows he and Dr Tudhope identified are a patchy record of the last 130,000 years, comparing their evidence with a book with missing pages.

He said: "That's just the way this science works. It's the same for other fields, like archaeology. You're not likely to get a continuous record."

The coral records past climate data

Dr Mike Hulme is director of the Tyndall Centre for Climate Change Research at the University of East Anglia, UK. He told BBC News Online: "The evidence from models that climate change alters El Nino remains equivocal.

"But it would be most remarkable if El Nino didn't change its behaviour as the world warms. I shouldn't like to say we've pinned down how El Nino will change.

"But this research is especially interesting because of the long timescale. It's quite provocative evidence that we've seen more extreme El Nino behaviour this last century.

"And it provides us with a new, rich source of evidence that will help us to narrow down the hypothesis that El Nino and global warming are linked."