Geophysicists are finding that explaining dramatic variations in climate is a tougher challenge than they first expected. They are warning decisionmakers not to wait for further scientific evidence before taking action to cope with possible man-made global warming.
Last year, for example, was one of the warmest on record and 1998 is fixing to be even hotter, says the US National Oceanic and Atmospheric Administration. While this may seem like evidence of a man-made climate change, don't bet on it.
Hindsight shows that much of last year's unusual warmth was due to the recent El Nio short-term climate shift, says climatologist Tim Barnett from Scripps Institution of Oceanography in La Jolla, Calif. Subtract the El Nio effect and "you'd probably say something different" about that nominally "warmest" year, he adds. Once again, natural weather changes muddled what seemed to be a clear global-warming signal.
To judge from a meeting here of the American Geophysical Union, climate researchers are resigned to the fact that they probably never will be completely certain of human effects on climate.
Nevertheless, there is a growing consensus that some degree of a man-made climate change may already be under way. That's why many experts say that scientific uncertainty is no longer a reason to delay action to reduce emissions of carbon dioxide and other climate-warming gases. The American Geophysical Union, after much debate, has prepared an official statement that makes these points.
Concern about the rising sea level illustrates the situation. The average global sea level has risen a few millimeters a year throughout this century. Man-made global warming, theoretically, could accelerate that rate. When the world's sea level suddenly rose by 0.8 inches between March and November last year, some environmentalists thought that man-made acceleration might have kicked in. It was a false alarm, as R. Steven Nerem explained at the meeting here.
Dr. Nerem is a member of the science team working with the French/American TOPEX/Poseidon satellite, which measures average global sea level every 10 days. Nerem says that, once again, hindsight shows this dramatic sea-level rise was a short-lived response to El Nio. So too was a 0.7 degree F. rise in sea-surface temperature between October 1996 and December 1997. Both sea-level and sea-surface temperature changes have returned to normal levels.
Nerem said that detecting sea-level variations, caused by long-term climate change, will be more difficult than scientists had anticipated. This is because such changes are significantly smaller than the dramatic variations than were seen during the short-term El Nio. He added that, while there are six years of satellite data on hand, "we really need a decade or more of continuous measurements before we can accurately detect any climate-induced change."
Nevertheless, the world's sea level continues to rise a few millimeters a year. Experts say that low-lying islands and coastal regions need to prepare for this rise, regardless of climate data uncertainties.
Not all of the experts are reticent in their conclusions. Jonathan Overpeck - who heads the National Oceanic and Atmospheric Administration's Paleoclimatology Program in Boulder, Colo. - looks back over 1,200 years. Because there's no instrumental record, he examines sea and lake sediments, glacial ice cores, tree rings, and, where available, historical documents that yield clues to past climates.
Dr. Overpeck calls global warming in this century "unprecedented" over that 1,200 year period. He says his research has failed to find any known natural mechanism that would propel this warming. "Twentieth-century global warming is a reality and should be taken seriously," Overpeck concluded at the San Francisco meeting here.
Few of his geophysical colleagues would put it that bluntly. But few of them also would want their caution to dissuade nations from treating man-made climate change as a serious possibility.