The climate will undergo no dramatic change in the next 25 years except that the Earth might get a little warmer and droughts might continue to occur in the Upper Plains of the United States.
By the year 2000, enough carbon dioxide may have built up in the earth's atmosphere from worldwide burning of coal and oil that temperatures will begin to rise around the globe.
The increase in carbon dioxide, which might trigger a "greenhouse" effect where the Earth's heat is trapped in its atmosphere, could be enough to force a 2 percent increase in the average global temperature in the first 10 years of the 21st century.
These are the opinions of 24 climate experts in seven countries polled by the Pentagon's Advanced Research Projects which yesterday released the poll's results in a 100-page report published by the National Defense University.
The poll's result were discussed at an all-day meeting sponsored by the American Association for the Advancement of Science, whose annual convention was hold here this week. Involved in the study besides the 24 weather experts and the Pentagon were the Agriculture Department, the National Oceanic and Atmospheric Administration, and the Institute for the Future in Menlo Park, Calif.
"The most likely event (out to the resembles the average of the past 30 years," the report said, "arising primarily from a balancing of the warming effect of carbon dioxide with the cooling effect of a natural climate cycle."
The study pointed out that global temperatures varied little in the last 100 years, dropping four-tenths of a degree in the first 10 years, rising the same in the next 10, and then staying constant for the 25 years that ended in 1920. Worldwide temperatures then rose by almost one degree until 1948, when they began to cool slightly and remained somewhat cool until 1970.
Experts polled by the Pentagon said they foresee a "slight warming trend" in the 25 years ahead. By slight they said there was only one change in 10 that global temperatures would rise by more than one degree Fahrenheit out to the year 2000.
About the possibility of drought, the experts were more precise. They said they foresaw drought returning twice to the wheat-growing states of Nebraska, Kansas and North and South Dakota, once in the next decade and again in the decade starting in 1990.
One expert polled by the Pentagon said there is a "strong possibility" of frequent drought in the High Plains states in the next 25 years. This expert reasoned that 4,000 years ago "when the earth was a few degrees warmer than it is now . . . it was rainier in the subtropics but notable that in the central United States, in the lee of the Great Divide, it was generaly drier, and the prairie extended nearly to the Applalachians."
The experts did not agree on what might trigger drought in the Great Plains, but they all agreed that drought in this part of the United States has become cyclical. Some experts tied the drought cycle to the 22-year sunspot cycle, where the sun'smagnetic field changes poles at the end of two full 11-year cycles of sunspot activity.
"The 20-year periodicity seems to be a well-documented climate feature," one expert said. "Not only to miswestern U.S. droughts, but also in North American winter temperatures and polar mean temperatures. The expected warming toward the end of the century would seem to increase the probability of droungt.
Present at the meeting yesterday were panelists Helmut E. Landsberg of the University of Maryland, and Stephen H. Schneider of the National Center for Atmospheric Research, who felf that by the year 2000 the increased burning of fossil fuels like coal and oil would begin to force more than a slight global warming trend.
"By 2000 some of us think the carbon dioxide in the atmosphere jumps up out of the background noise," Schneider said. "It is very likely we could get a 2degree increase in temperature if nothing is done to curn our appetite for more and more energy."