Coral Record Puts Present-Day El Niño Trends in Context

Nearly 7,000 years of climate data stored in coral fossils shows that episodes of the climate phenomenon El Niño have intensified in the past 200 years, but it also indicates that there were numerous alternations between very weak and very strong El Niños well before the influence of industrial activity.

So concludes a new National Science Foundation-supported study by researchers at Georgia Tech University, Scripps Institution of Oceanography at UC San Diego, and the University of Minnesota. The result suggests that attributing the strength of the El Niño Southern Oscillation (ENSO) to rising levels of carbon dioxide in the atmosphere is not as simple as scientists had previously thought. The coral reconstruction shows that the range of El Niño variance is very large, and, according to the coral data, the period of greatest variance in El Niño strength actually occurred in the 17^th Century, well before widespread use of fossil fuels began. In order to assess the historical significance of recent El Niños, a dataset even longer than 7,000 years old is needed, the researchers said.

Nevertheless, the coral record holds promise for more detailed understanding of why the phenomenon is so variable.

“We looked at the long-term variability of ENSO in the climate models and asked how it compares to the long-term variability of ENSO in the real world,” said study lead author and former Scripps graduate student Kim Cobb, now an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. “We show that they actually match fairly well. This project sets the stage for conducting more detailed data-model comparisons from specific time intervals to test the accuracy of ENSO characteristics in the various models.”

El Niño is a climate phenomenon involving a shift in the distribution of warm water in the equatorial Pacific Ocean. El Niño years are characterized by unusual weather and storm activity globally. For example, the southwestern U.S. experiences warmer/wetter winters.

The researchers extracted cores of coral fossils from various locations in the Line Islands in the northern central tropical Pacific Ocean, the center of action for the El Niño disturbances in ocean salinity and water temperature.

“Fossil corals are the kings of El Nino reconstruction,” said Cobb. “Corals grow in the heart of the El Niño region, and with monthly-resolved records, they provide a very high level of detail.”

Cobb received her doctorate from Scripps in 2002 and began developing the analysis techniques used in the study as a student working with Scripps geoscientist Chris Charles. The team studied 17 such cores of varying lengths and ages recovered from beaches on Christmas and Fanning islands, which are part of the Line Island chain. Co-authors Larry Edwards and Hai Cheng at the University of Minnesota dated the cores by analyzing the ratio of uranium to thorium. Once the age of each core was determined, Cobb and her team chose a subset of the collection to be studied in detail.

They sawed each core in half then X-rayed the cross-sections to reveal the growth direction of each coral. The researchers then drilled out small samples of coral powder every millimeter down the core. The corals were analyzed in tandem at Georgia Tech and Scripps to determine the ratio of oxygen isotopes. Visiting Scripps student and study co-author Niko Westphal spearheaded the analytical effort at Scripps in Charles’ lab.

The isotope ratio of the coral skeleton changes with temperature and amount of rainfall. For instance, the abundance of the oxygen 18 isotope is lower at points in history when seawater was less salty and water temperatures were higher. The ratio of oxygen 18 to the more common oxygen 16 isotope provides detailed information about environmental conditions during each period of the coral’s growth. Corals add significant skeleton on a weekly basis, allowing monthly reconstructions of the local environment.

“We are able to count back in time, following the seasonal cycles locked in the coral skeleton, as long as the core will allow us,” Cobb said.

In all, Cobb’s team added 650 years of monthly-resolved information about ENSO variations across nearly 7,000 years. That required analyzing approximately 15,000 samples over the course of the study, which began in 2005.

Though the study shows that El Niño variance is particularly strong in the late 20^th Century, it does not automatically mean that human emissions of greenhouse gases are the reason.

“We heard a lot about the ‘new normal’ after Hurricane Sandy, but in order to say anything meaningful about the ‘new normal’ from a climatic point of view, you have to know what ‘normal’ is in the first place,” said Charles, a study co-author. “We're showing here that the variability of El Niño is fairly extreme on its own.” 

-- Robert Monroe

Georgia Tech University contributed to this story