Ancient coral reef record gives history of El Niño

Underwater photograph of a diver coring a massive living coral in Papua New Guinea. Large living coral colonies such as this one can yield continuous climate records going back up to a few hundred years. For this study, scientists collected and analyzed cores from living corals to provide a calibration between the coral record and the instrumental record of El Nino over the past century. They then collected, analyzed and dated cores from ancient 'fossil' corals of the same type that are now sub-aerially exposed on land (image 2). These ancient corals ranged in age from 2,000 to 130,000 years in age, and each coral provided a 20-100 year-long 'window' into El Nino climate variability during its lifespan. [Photo: Sandy Tudhope]

(Santa Barbara, Calif.) Using pieces of ancient coral reefs as windows on the history of climate, geologists have discovered that at no time in the past 130,000 years does the weather phenomenon known as El Niño appear to have been as intense as it has in the last century. In an article published in the January 26 issue of Science Magazine, the researchers reported that the strength of El Niño was diminished during ice ages. The findings are important in evaluating the hypothesis that the intensity of El Niño during the last century is related to greenhouse gas emissions and global warming.

In the Huon Peninsula of New Guinea, a series of coral reef terraces, part of an island that is being pushed up by plate movements, provided researchers with coral reef samples dating back to 130,000 years before present. Analysis of isotopic and chemical variations from these ancient corals yielded 14 different climatic windows, each spanning 20-100 years. Cold periods 40,000 years ago and warm periods of 125,000 years ago were analyzed to evaluate the behavior of El Niño during different climate regimes.

Uplifted coral reef terraces of the Huon Peninsula, Papua New Guinea. In this region, the land is moving upwards at a rate of approxomately two meters per 1000 years. Consequently, fringing coral reefs along the coast get uplifted, and become sub-aerially exposed. These ancient reefs now form a succession of 'steps', or terraces, in the coastal landscape, with the youngest reefs closest to the coast, and the oldest reefs at higher elevation further back from the coast. The oldest reefs in this image are about 250,000 years old and are seen as terraces towards the top-left of the photo. For this study, we collected ancient corals from reefs up to 130,000 years old, seen here as the terraces from the present-day coast up to the top of the first very distinctive set of cliffs near the middle of the photo. [Photo: Sandy Tudhope]

"The samples indicated that El Niño was never more intense than the events of the last hundred years," said David W. Lea, co-author and professor of geological sciences at the University of California, Santa Barbara. "Over the last 100 years we have very accurate records of El Nino, with 1982-83 and 1997-98 being the largest events on record. Of course, everyone wants to know if the intensity of these large events is somehow related to global warming. Our data suggest that the behavior of the tropical Pacific over the last 100 years is atypical, but it does not pinpoint which factors modulate El Niño."

Lea has collaborated for many years with first author Sandy Tudhope, who is based at the University of Edinburgh, Scotland. El Niño is generally considered to be the resulting world-wide weather changes that occur following an increase of warm water in the tropical eastern Pacific.

By analyzing the temperature and salinity of the water, as revealed by the ancient pieces of coral, the researchers were able to reconstruct past climate. Besides the discovery that past El Niño intensities were not as strong as recent ones, the researchers learned that during the ice ages the intensity of El Niño appears to be about 50 percent weaker. During the warmest times El Niño was the strongest.

Lea, who specializes in paleoceanography, the history of the oceans, hastens to add that the 14 samples do not represent a continuous time span, and that the evidence they found is rather like a book with pages missing. "But that's just the way this science works," said Lea. "It's the same for other fields, like archaeology. You're not likely to get a continuous record."