Coral Records Suggest that Recent El Nino Activity Rises Above Noisy Background

By examining a set of fossil corals that are as much as 7,000 years old, scientists have dramatically expanded the amount of information available on the El Nino-Southern Oscillation, a Pacific Ocean climate cycle that affects climate worldwide. The new information will help assess the accuracy of climate model projections for 21st century climate change in the tropical Pacific.

The new coral data show that 20th century El Nino Southern Oscillation (ENSO) climate cycles are significantly stronger than ENSO variations captured in the fossil corals. But the data also reveal large natural variations in past ENSO strength, making it difficult to attribute the 20th century intensification of ENSO to rising carbon dioxide levels. Such large natural fluctuations in ENSO activity are also apparent in multi-century climate model simulations.

''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 Kim Cobb, 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.''

The research, sponsored by the National Science Foundation (NSF), was reported on 3 January in the journal Science. Researchers from the Scripps Institution of Oceanography and the University of Minnesota also contributed to the work.

El Nino Southern Oscillation extremes drive changes in global temperature and precipitation patterns every two to seven years. The variations are particularly pronounced in the central tropical Pacific, where Cobb and her team collected the fossil corals used in this study.

By analysing the ratio of specific oxygen isotopes in the coral skeletons, the scientists obtained information about ENSO-related temperature and rainfall variations during the periods of time in which the corals grew.