Eugene S. Takle
© 2003
Long-lived corals offer a continuous record of climatic conditions in certain ocean areas. Oxygen used for growth by corals becomes immobilized in the skeletal structure. Higher than normal sea-surface temperatures and above-normal rainfall both reduce the abundance of the oxygen isotope 18O that is taken up by the coral in that particular year. Analysis of 18O for each year's coral growth in El Nino regions then gives a year by year record of temperature and rainfall and hence El Nino conditions. Comparison of coral records with measured temperature and rainfall during recent years enables verification of this technique for reconstructing sea-surface temperature (SST) conditions. Cobb et al, (2003) use coral from Palmyra Island in the tropical Pacific Ocean to create 30-150 year windows of SSTs over the last 1,100 years. They find that the most intense El Nino period was in the mid-17th century. Cobb et al (2003) explore possible relationships between changes in El Nino frequency or intensity with changes in global mean temperature but find no definitive correlation. However, they note that the range of global mean temperature and rate of change of global mean temperature of the late 20th century is unprecedented in the coral record of the last 1,100 years.
Reference
Cobb, K, M., C. D. Charles, H. Cheng, and R. L Edwards, 2003: El Nino/Southern Oscillation and tropical Pacific climate during the last millennium. Nature 424, 271-276.