The late 1980s will see the first unmistakable evidence that people are dangerously heating up the globe's atmosphere by massive coal burning, scientists predicted here yesterday.
The effect in this decade will be a change in average world temperature of several tenths of a degree, they said, but it will have measurable effects in some places and it will be a sure warning of cataclysmic changes to come.
Among these could be (some scientists say "will be") far greater warming in the future, which could cause a rise in the ocean level that would flood coastal cities, and changes in rainfall patterns that could turn the northern United States into a barren land unfit for most agriculture.
Coal burning--and to a far less extent oil and gas burning--puts carbon dioxide into the upper atmosphere. The carbon dioxide, with some other gases people are adding, add up to a greenhouse effect. They are like a lid on a cookpan that keeps in the heat.
"It now appears inevitable that the atmospheric carbon dioxide level will double some time in the next century," Dr. Roger Revelle of the University of California at San Diego said at the annual meeting of the American Association for the Advancement of Science here. Revelle is one of the world's leading geophysicists.
"We can expect a large climactic impact, very large changes," said James Hansen of the Goddard Institute for Space Studies in New York, a National Aeronautics and Space Administration studies center.
The signs are so ominous that "we must expect this . . . and take action to avoid it," said Prof. Herman Flohn, another geophysical leader, of the University of Bonn, Germany.
It is important to act soon, he said, since developing an alternative energy strategy for the world "would take 50 years or so," and once carbon dioxide settles in the upper atmosphere it may stay there for hundreds of years.
There are many uncertainties in this scenario, and some scientists do not agree that the dire effects are inevitable.
Yesterday's statements are a clear sign, however, that scientists are beginning to reach a consensus on this issue, and it may be only a short time before they begin to urge governments to reduce coal burning. Only a few years ago statements on the subject were more tentative, but continuing measurements are making the picture clearer.
Hansen is chief author of a Goddard Institute analysis predicting that the man-made temperature changes will begin, clearly, to rise above natural fluctuations in the 1980s. These will be more noticeable in some areas, he said, since a small worldwide change can mean sharp local ones.
The changes thereafter would be greater. They could include the increased drying of the Colorado River, for example, with "an end to agriculture in California."
The most drastic changes, speakers agreed, can be expected in the 21st century. If part of the Antarctic ice melts there would be the direst effect, a rise of 20 feet in ocean levels that would flood major coastal cities.
The most encouraging factor, he said, is that the matter might be solved if the three countries that produce most of the coal, the United States, the Soviet Union and China, "decide they do not want their agriculture ruined."