IT NOW SEEMS almost certain that humankind's brief reign on earth has guaranteed our great-grandchildren a warmer climate to live in than anything the planet has seen over the past 5,000 years. Not content with merely knowing this, however, scientists for the first time are considering the possibility that we might be able to do something about it.

A warner earth might not necessarily be a better place to live.Temperature variations from place to place could be extreme and the climate might actually get colder in some regions.Warmer weather is likely to bring searing drought to the U.S. Corn Belt but bumper crops to Russia. Snowfall could be much heavier during the shorter winters and weather in general might be more turbulent.

"Never in the history of mankind's affairs have planners and decision-makers been given such a forewarning - with the possible exception of the biblical story of Joseph's advice to the pharaoh about the seven years of plenty and the seven years of famine," wrote W. W. Kellogg of the National Center for Atmospheric Research at Boulder, Colo., in a recent World Meteorological Association bulletin.

Not only do we have a pretty good idea what is likely to happen, he added, but "action could be taken to avoid [the warming] if we really wanted to."

It has been a major achievement for climatologists even to reach a majority view on the warming trend, and there are still dissident voices. Talk of doing something about the trend or learning to manage it to avoid future ice ages is wildly premature in the eyes of most. Yet many admit their main goal these days is to build the pile of evidence high and strong enough to convince sluggish politicians and planners that the change is coming and that they ought to consider some kind of action.

"Chances are the decision-makers won't take much notice until the statistics show a dramatic warming curve, and by then it will be very difficult to turn off," said Murray Mitchell, a senior research climatologist at the National Oceanographic and Atmospheric Administration (NOAA).

"Turning it off" would involve somehow cutting back on the worldwide burning of fossil fuels. Oil, gas, coal and wood give off carbon dioxide, among other things, when burned, and it is the steady buildup of carbon dioxide (CO2) through the ages that is believed to have caused the warming trend. The gas prevents heat from radiating naturally out into space and the result is what is what has come to be called a "greenhouse effect."

As the world has industrialized, burning more fuel every day, CO2 concentrations have increased steadily. About half of what is emitted remains in the atmosphere; the other half is absorbed by plant life as part of photosynthesis or dissolves in the oceans.

Since Charles Keeling of the Scripps Institution of Oceanography began keeping accurate records of the buildup in 1958, CO2 has been tracked as increasing by about 4 percent per year. This translates into a northern hemisphere summertime average by the year 2000 - just 21 years away - almost 2 degrees higher than it is now: 69.1 degrees instead of the current 67.3 degrees, according to NOAA.

Revolutionary Changes

THAT MAY NOT seem like much, but in climatic history it is phenomenal. It is the warmest average of the last 1,000 years. It means a growing season roughly 10 days longer for the U.S. plains states, and one estimate puts that at $30 million more in wheat crops if wind and rain remain equal.

The problem is that wind and rain will probably not remain equal, but back to that in a moment. Since the temperature shifts will be more dramatic toward the polar regions, life at the 60 degree north latitude line will be 4 or 5 degrees warmer, affecting Leningrad, Oslo, Scotland, Hudson Bay and Anchorage.

Growing time could be 20 days more in those areas, and for the tiny flowers and scrub of today's summer north of the Arctic Circle (66.6 degrees north latitude), it could be 6 or 7 degrees warmer for 30 days, a truly revolutionary shift in potential agricultural patterns.

Rainfall is also likely to move. The more or less permanent polar air currents will probably shift further north, taking with them the reliable rain that grows wheat in our Midwest. That will be good news for Canada, combined with its warmer temperatures, but the soil is not as rich there and the net effect could be a drop in food production. Left behind will be a true desert in the area east of the Rockies.

The same trend could shift rain and warmth from some of the wheat fields in Russia, Australia and Argentina, the world's other major granaries, to areas that are now dry, chilly scrubland suited mainly for sheep. Other Russian grain areas would prosper under the change. When mini-ice ages and unexplained changes have created such shifts before, there has always been massive social dislocation at best. At worst there has been famine and political upheaval.

"We might decide that coal burning has such a tremendous negative effect on the climate that we can't have it . . . or maybe we'll find that the net cost is okay if we can adapt agricultural patterns in order to compensate for the shifts," said Edward S. Epstein, director of the Commerce Department's National Climate Program Office, signed into existence last September to coordinate study of the CO2 buildup and what it means.

"Either we will learn how to have some impact on the cause [of the warming trend] or how to live better with its effects, but at least we want to have that choice," he said.

All of the shifts, barely if at all visible yet, will intensify by the year 2050, when CO2 concentrations will have doubled if current trends continue. The northern hemisphere summer will be 3 degrees warmer than now; Arctic Circle rises may reach 10 degrees. The crust of ice over the Arctic Ocean will begin melting.

Although this crust is only about 39 feet thick at best, it has apparently never melted completely in the past million years. Even if it does now, there will be little visible change in the earth's sea levels. Only massive melting of the mile-thick ice of Greenland and Antarctica could really raise the tides, and although scientists are measuring the melt rate already, few if any of them expect a major change there within the next 2,000 or 3,000 years.

Staving Off the Ice Age?

THE LAST TIME the earth has been as warm as it is likely to be in 2050 was 5,000 years ago, at the very dawn of civilization. The great forests then were 60 miles further north here and in Europe. North Africa was abloom. There was more rain in Europe and Iran and less in Scandinavia than there is now. A belt of grassland swept across North America into much of what is now eastern forest.

There are so many variable elements in the equation that most of them have not yet been fitted into the computers making the projections. No one is sure just what the effect of cutting back on fossil fuel burning would be, since it is far from being the only change going on.

How, for example, does the CO2 buildup interact with the certainty of a coming ice age? Core samples from the sea bottom, fossil evidence and radiocarbon dating have revealed that the earth is prone to eons-long ice ages with periodic interglacial periods. These last a comparatively brief 10,000 to 12,000 years, and we are at the end of one of them now.

In fact, we have been sliding toward the cold end of the scale for the past 4,000 years. "We may actually be putting off the ice age with the CO2 buildup," said Eugene W. Bierly, director of the National Science Foundation's Climate Dynamics Research Program.

If we knew enough about how climate works, he continued, we might dare to think about controlling CO2 emission to stave off the icebergs permanently. "There really is no underlying theory as to why the climate changes as it does," he said.

Most current computer models have a strange construcition of the world weather system. They put a mathematical wall at the equator, dealing only with the northern hemisphere. They include temperature readings on a small part of the ocean surface, ignore the fact that seabottom water is continually welling to the surface, and ignore the fact that all the particles of dust and soot and pollen in the air are very different from one another. Some of the predictions that result are just the opposite of what one might expect.

For example, an early effect of the warming trend might be more snow. Not quite as much ice would freeze every year near the poles, so there would be more water surface available to evaporate in the sun.

That would make the air more humid, which would increase precipitation over the earth as a whole, which would mean deeper snow in the winters. Continuing accumulation at the poles might even reduce the level of the world's seas a bit for the first hundred years or so, just the opposite of what a warming trend suggests.

Some scientists continue to doubt there is enough proof there will even be a warming trend. Reid A. Bryson, climatologist at the University of Wisconsin, insists that carbon particles and other emissions that go along with burning fossil fuels tend to cool things off by reflecting heat out of the atmosphere, canceling out the warming effect of the CO2.

Bryson goes further and says, in fact, that things may actually be cooling off. Volcanoes, he says, are erupting and spewing cooling dust fast enough to overcome the CO2 greenhouse and speed us on our way to the next ice age if the trend continues.

His possible scenario could recreate northern hemisphere weather of the 1850s: more rain in the Rockies and the high plains, better grazing in the Midwest and a 2-degree drop in temperature by the year 2000. "The problem with saying all this for sure is that the best climate prediction [computer] model in the world - which is mine - isn't very good," he said. "We Have To Play It Safe"

CONCERNED about the possibility of future frostbite, some thinkers of the Victorian era suggested spreading carbon black on the Arctic ice to help it absorb the sun's rays and melt, or perhaps damming the Bering Strait to reduce the southward flow of colder currents.

While one group of eperts is trying to correlate the ice ages with the earth's magnetic field, the planet's slow wobble on its axis and a dozen other things, another group is trying to understand why the midwestern drought cycle seems to correspond with the 22-year so-called "double sunspot cycle."

Other research concerns the nature of the dust in the air and whether it is a cooling or a warming factor. Much attention is going to the oceans. Do they absorb CO 2 at a steady rate, and will they soon become saturated? How much is taken up by the sea and how much by the forests, and is the current rate of deforestation to crops and firewood - about 40,000 square miles of trees per year - being properly weighted?

A major effort to collect simultaneous temperature, humidity, pressure and wind data for the entire atmosphere, top to bottom and around the world for an entire year, gets underway for the very first time in December.

The World Weather Experiment, organized by the World Meteorological Association and the International Council of Scientific Unions, will involve 145 nations and a fleet of 50 ships, new satellites, weather stations, balloons and buoys, according to Rex Fleming, director of the U.S. effort.

Then there are the issues that affect climate but will surely be decided on other grounds. The rate of population growth, the pace of industrialization and the debate over nuclear power will all determine future carbon dioxide and other emission rates, and climatologists can only sit and watch. Certain kinds of fertilizers have a climatic impact in contributing to the number of heat-capturing particles in the atmosphere, as do the freon gases that are still used everywhere in refrigeration and in aerosol sprays outside the United States.

"We don't have the tools in hand yet to come to grips with all this, to say for sure what life will look like in the future," said Mitchell of NOAA. "But we have to play it safe. If we go ahead as we are with the belief it won't matter much, well, we're committing scores of future generations to live with the results."