In their search for convincing evidence of global warming, scientists have been puzzling over shifting tree lines in the Sierra Nevada, dying coral in the Caribbean, melting alpine glaciers, and seasonal temperatures so extreme that the 10 warmest years of the past century have occurred in the last 15 years.
When malaria-infected mosquitoes recently turned up in New Jersey and tropical microorganisms were discovered poisoning shellfish as far north as Monterey, climate experts were quick to wonder whether they had detected evidence of climate changes.
In each case, pests once confined to the world's hottest regions appeared to be moving into new territory--evidence, perhaps, of formerly cool zones warmed by greenhouse gases.
Now some researchers believe that they have detected the distinctive signature of global warming in the infamous Pacific Ocean current known as El Nino, a seasonal upwelling of warm seawater that has been implicated in disastrous droughts, torrential rains, killing heat waves and other distortions of the daily weather from Southern California to South Africa.
The El Nino current arises from the dance between order and chaos as the ocean and the atmosphere interact to balance the Earth's thermal energy. It is the heart of a complex system called the El Nino Southern Oscillation, which is so delicate that even a subtle alteration in temperatures can affect its seesaw, annual rhythms.
Global warming should make El Nino effects stronger and more frequent, said Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo. And, as if to prove his point, the most recent El Nino, which some scientists say lasted from 1990 through 1995, is the longest in 130 years of record-keeping.
Trenberth and his colleague Timothy Hoar raise the possibility that the unusually prolonged El Nino could be evidence of climate changes affecting the central and eastern Pacific Basin. They reported this year in Geophysical Research Letters that, if the climate were unchanging, a five-year El Nino event would be expected to occur only once in about 2,000 years.
Their controversial finding comes on the heels of a landmark report issued by the United Nations Intergovernmental Panel on Climate Change in December. Scientists on that panel, gathered from more than 30 nations, said global warming, triggered in part by human activity, will cause a 2- to 6-degree Fahrenheit rise in average surface temperature, melt a third of the Earth's glaciers and raise sea levels up to three feet in the next 100 years.
So far there is no conclusive proof of any climate change--only a growing number of suggestive effects, such as the prolonged El Nino identified by Trenberth and Hoar.
The unusual El Nino event "raises the question that something is changing," Trenberth said. "The obvious thing is the climate is changing, so we raise the question that global warming might be responsible. It is only about now we should be seeing the signs of things happening in the climate system that would go beyond the realm of natural variability."
The tropical Pacific is a wellspring of the world's weather. Along the equator, the western Pacific has some of the world's warmest ocean water, while in the eastern Pacific, cool water wells up, carrying nutrients that support large fish populations along the coast of South America.
In an El Nino year, strong westward-blowing trade winds subside and warm water slowly moves back eastward across the Pacific, interrupting the upwelling of cooler water. Peruvian fisherman first noticed the current almost 200 years ago because it affected the size of their catch, and they named it "El Nino" (the child), because it seemed to occur at Christmas.
Through most of this century, El Nino and its counterpart, a cooling of the eastern tropical Pacific called La Nina, have alternated every two to four years.
So far this year, a relatively weak La Nina cooling current has chilled global temperatures well below seasonal norms for the fifth month in a row, University of Alabama researchers say. Chilled by cold Pacific waters, a region of cooler than normal air now stretches from central Africa across the eastern Mediterranean, Russia, China, Japan, the north Pacific and into southern Canada as far east as Quebec.
Until recently, scientists thought that while much of the weather may change from day to day, the modern climate is relatively stable.
But new evidence from ocean sediments and polar ice cores suggests that during the past 10,000 years climate has shifted suddenly and frequently, causing decades of fiercer monsoon seasons in Africa, century-long droughts in California and the Midwest, as well as the kind of worldwide temperature changes some researchers associate today with global warming.
Some experts on the ancient climate suspect that shifts in prehistoric El Nino currents may have been linked to those dramatic changes in worldwide temperatures and rainfall.