单项选择题

An image taken of the Pacific Ocean last September is astonishing. Made using data collected from satellites monitored by NASA’s Jet Propulsion Laboratory in Pasadena, California, the picture shows the surface level of the Pacific as clearly as a yardstick lying across a lumpy bed. One sample of water--with a volume 30 times that of all the Great Lakes--is white, indicating that it is as much as 13 inches higher than its normal level.
El Nino experts are still striving to tackle the really big question: what is causing the abnormal El Nino behavior of the past two decades Some said because global warming accelerates the pace of E1 Nino formation and reinforces each event. Supporters of global warming as El Nino instigator include Kevin Trenberth, a climate analyst with the National Center for Atmospheric Research in Boulder, Colorado. "There’s evidence that global warming didn’t have much impact until 1979, but now it’s beginning to break through," he says.
Two additional arguments are on the table. One theory is that the recent E1 Nino epidemic is simply one of nature’s climatic tiffs, but that we haven’t been heating the tune long enough to recognize the change in key. One problem is that historical data on El Nino is sketchy prior to the early 1970s and almost nonexistent going back more than 50 years. If one looks back further, however, fossil evidence suggests that something about E1 Nino has indeed changed. Fossil records of coral in the Galapagos Islands show that some 4,000 years ago, an El Nino occurred only every 60 years or so. Studies of tree rings and ice cores indicate a more recent cycle of seven years, still much less frequent than the present cycle of every three or four years.
Records are accurate enough to show that we’re in the second El Nino intensive era of the past 100 years. The first was during the 1920s and 30s, and probably was responsible for the Dust Bowl, when drought destroyed hundreds of farms in Oklahoma and north Texas. The second seems to have started around 1976. These shifts seem to be due to periods of natural warming in the Pacific. The warming does not necessarily cause El Nino but certainly amplifies it, creating the appearance of more and more severe El Ninos.
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