单选题
El Nino is the term used for the period when sea surface temperatures are above normal off the South American coast along the equatorial Pacific, sometimes called the Earth's heartbeat, and is a dramatic but mysterious climate system that periodically rages across the Pacific. El Nino means "the little boy" or "the Christ child" in Spanish, and is so called because its warm current is felt along coastal Peru and Ecuador around Christmas. But the local warming is just part of an intricate set of changes in the ocean and atmosphere across the tropical Pacific, which covers a third of the Earth's circumference. Its intensity is such that it affects temperatures, storm tracks and rainfall around the world. Droughts in Africa and Australia tropical storms in the Pacific, torrential rains along the Californian coast and the Peruvian deserts have all been ascribed to the whim of El Nino. This at least is the theory, and it has worked pretty well over the past century, with El Nino occurring about every three to five years and La Nina in between. But there have been some baffling developments in recent years. For one thing, El Nino has returned three times in the past four years. For another, since 1976 El Nino has dominated relative to the cooler phase (La Nina). There has been only one significant La Nina, but five El Ninos, including an extremely severe one in 1982—1983 that caused damage costing 8 billion dollars. Moreover a huge pool of warm water has settled down near the dateline in the central Pacific. Yet it is important to understand the changes if scientists are to be able to forecast the climatic effects of El Ninos with any degree of accuracy. This is not just an academic task— accurate forecasts can spell out feast or famine in many tropical countries around the world. Forecasting efforts have focused on El Nino, whose effects are generally more severe than those of La Nina. A worrying possibility is that the changes maybe due to greenhouse warming. If so, the recent fluctuations may be an early glimpse of worse things to come.