Much of the warming of the seas -- especially in the Pacific regions -- is attributable to a phenomenon called El Nino, a pool of warm water which periodically surfaces in the tropical latitudes of the Eastern Pacific. The effects of El Nino are far reaching. They fuel violent storms in the Pacific, floods across California and the US Gulf Coast, and droughts in Australia and Africa. But while most El Nino events last only a year or two, scientists noted at the beginning of 1996 that the recent El Nino -- which had lasted for five years and eight months -- was a one-in-2000 year occurrence.
In the Jan. 1, 1996 issue of the journal, Geophysical Research Letters, researchers Kevin Trenberth and Timothy Hoar, of the National Center for Atmospheric Research, noted that the event -- coupled with the past two decades of sea surface temperature anomalies -- is "unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the...changes may be caused by the observed increases of greenhouse gases."
Historically, El Ninos have alternated every two or three years with La Ninas (upwelling pools of cool water in the eastern tropical Pacific) in what scientists call the Southern Oscillation. That oscillation is part of a large, complex interaction between the Pacific Ocean and the global atmosphere. But over the past twenty years, there has been only one significant La Nina.
By contrast, the strongest El Nino of the century occurred in 1982-83 and the most long-lived one concluded in 1995.
The authors of the study pointed out that the latest El Nino "occurred in the context of a tendency for more frequent El Nino events and fewer La Nina events since the late 1970s...These results raise questions about the role of climate change," Trenberth and Hoar wrote. "Is this pattern of change a manifestation of the global warming and related climate change associated with increases in greenhouse gases in the atmosphere? Or is this pattern a natural decadal-timescale variation? We have shown that the latter is highly unlikely."
Citation: "The 1990-1995 El Nino-Southern Oscillation event: Longest on Record," Geophysical Research Letters, Vol. 23, No. 1, January 1, 1996.
Moreover, the El Nino of 1997-1998 was the most severe on record, surpassing the previously strongest El Nino of 1982-83.
Citation: Press Release by the National Oceanic and Atmospheric Administration,