In contrast to what some scientists believe, global warming may not be to blame for strong El Niño events in 1982-83 and 1997-98 that caused severe floods, drought, and forest fires throughout the tropics. Instead, these El Niños appear to fall within the range of natural variability, a study in the 17 July issue of Nature suggests. The research shows that in the 17th century, long before humans started churning out greenhouse gases, El Niños were at least as severe and frequent as they are today.
El Niño, the striking rise in sea surface temperatures that occurs in the eastern tropical Pacific every 3 to 7 years, is only the most notorious phase in what climatologists call the El Niño/Southern Oscillation (ENSO) phenomenon. (La Niña is the "cold phase.") Because of its worldwide effect on weather, ENSO can have high economic, social, and ecological costs.
To better understand the variable strength and frequency of today's ENSO and predict what the future may have in store, paleoclimatologist Kim Cobb of the Scripps Institution of Oceanography in La Jolla, California, and colleagues traveled to Palmyra Island in the central tropical Pacific. There they collected close to 100 1- to 2-meter-long cores of fossil coral, as well as a 3-meter-long sample of living coral. They dated the samples using the so-called uranium-thorium isotope series and determined the ratio between oxygen isotopes 18O and 16O, which goes down as the weather gets warmer and wetter. This allowed them to reconstruct the climate in Palmyra during five intervals between A.D. 930 and 1998 (see figure).
The study confirms many others that have shown a dramatic warming trend since 1976. However, the data also indicate that ENSO activity was more vigorous in the mid-17th century than during the 20th century, and activity often changed abruptly and markedly through the millennium. This suggests that global warming is not necessarily the cause of modern-day severe El Niños, notes Cobb, who is now at the California Institute of Technology in Pasadena.
Paleoclimatologist Michael Evans of the University of Arizona in Tucson says the work is "quite an achievement." Based on the coral data, Evans agrees that "the last 30 years' worth of ENSO events may not be all so unusual."