Evidence is mounting that the earth has entered the first stages of the long-forecast warming due to the "greenhouse effect," but it is premature to link it to the current drought in the Midwest, scientists said Friday.

Nonetheless, the drought and high temperatures plaguing the United States are consistent with the predictions evolving from the greenhouse theory, experts said, and may portend a permanent increase in world temperatures due to a buildup of carbon dioxide in the atmosphere caused by the burning of fossil fuels. Climatologists estimate that the Earth's surface temperature has already increased between 0.5 and 1.25 degrees Fahrenheit since 1850. That warming seems to be accelerating.

One major consequence of the greenhouse effect is that agricultural areas of the Midwest and the Southeast would have a steady decline in average annual rainfall and would require extensive irrigation if productivity were to be maintained at current levels.

Scientists agree that the 1980s have been unusually hot and that this year is the hottest and driest of all. Climatologist James E. Hansen of the NASA Goddard Institute for Space Studies in New York City told a Senate committee Thursday that the heat and drought mark the beginning of the greenhouse effect.

Four of the hottest years on record have occurred during the 1980s, according to Hansen, and this year may be worse. "The first five months of 1988 are so warm globally that we conclude that 1988 will be the warmest year on record unless there is a remarkable, improbable cooling in the remainder of the year," he told the Senate committee.

Other scientists disagreed, however. "You can't say that . . . ," said climatologist David Rind of the NASA institute. "Droughts have a multitude of causes."

"Climate is, by definition, the average over many years," said physicist John W. Firor of the National Center for Atmospheric Research in Boulder, Colo., "so there is no way you can tell if one year is part of a climate change or not."

"What the drought does underscore though is the real possibility that, in the future . . . we may see droughts more frequently and more intensively than we have in the past," said climatologist Alan Hecht of the National Oceanic and Atmospheric Administration, who is director of the National Climate Program. "Given that possibility, this drought is important as a case study of how we respond to it and how we would anticipate and respond to any future drought."

Scientists have debated the climatological effects of carbon dioxide since the late 1800s. The carbon dioxide allows the sun's light to reach the earth's surface and warm it, but absorbs heat that would otherwise be radiated into space _ thus trapping heat just like the glass panes of a greenhouse.

"We've always had a greenhouse effect _ that's the least controversial theory in meteorology," said climatologist Steven Schneider of the atmospheric research center. "If it didn't work, Mars wouldn't be a deep freeze, Venus wouldn't be a hothouse, and the Earth wouldn't be just right. The controversy is over how much added carbon dioxide will enhance the greenhouse effect."

The amount of carbon dioxide in the atmosphere has grown from about 270 parts per million in the mid-1800s to about 345 parts per million now. A 1983 National Academy of Sciences report predicted that the level could reach 600 parts per million within 50 to 100 years.

Such an increase, the academy predicted, would cause a global rise in temperature of as much as nine degrees Fahrenheit _ to temperatures that were last experienced during the age of the dinosaur.