"El Nino operates within its own kind of 2,000-year rhythm, and because of that, we believe these periodic changes have had a major impact on global climate conditions over the past 10,000 years," Christopher Moy of Stanford University in California said.

El Nino is an abnormal warming of waters in the Pacific that distorts wind and rainfall patterns around the world. It has been linked to heavy rainfall in northern Peru and southern Ecuador and heat waves in the northeastern United States.

While studying at Syracuse University in New York, Moy and his colleagues discovered the first continuous records of weather events caused by El Nino going back thousands of years.

By studying sediment from Lake Pallcacocha in southern Ecuador, the scientists were able to track dramatic changes in weather systems across North and South America.

"About every 2,000 years, we see a lot of El Nino activity," said Moy, who reported his finding in the science journal Nature.

"El Nino is an important part of our modern-day climate system. Likewise, our study shows it was also an important part of the earth's climate system 7,000 years ago," he added.

The scientists hope that by studying earlier El Nino cycles they can improve understanding of future climate changes.

Weather experts predict the current mild El Nino, which scientists have blamed for extending the dry season in parts of Asia, may strengthen this winter, increasing storms in the southern United States and causing drier conditions in the Midwest.

But the experts at the National Oceanic and Atmospheric Administration in the United States said it will be weaker than the 1997-1998 El Nino that claimed thousands of lives and caused billions of dollars of damage worldwide.