THERE IS SOMETHING special Congress needs to worry about during its consideration of the president's new energy program. It is the rapid increase of carbon dioxide (CO 2) in the atmosphere, the so-called greenhouse effect.

The phenomenon itself is not new. It was, in fact, recognized more than 100 years ago. But only in the past few weeks has it headed toward the status of a genuine policy problem. That is because massive synthetic-fuels program will accelerate a CO2 buildup, already proceeding at a rapid rate from the burning of fossil fuels.

What exactly is the CO2 problem? There is uncertainty about its true dimensions. But scientists agree that CO2 in the earth's atmosphere is steadily increasing and that the increase will cause a warming of the earth's surface, because CO2 in the atmosphere absorbs heat radiated from the earth that would otherwise escape into space. Over the past 30 years, the rate of increase of CO2 has risen to about 4 percent a year. If present trends continue, carbon dioxide in the atmosphere is likely to double by about the middle of the next century. This will cause an increase in the average global temperature of 2*-3*c, and several times that near the poles.

Scientists don't know what the consequences of such a warming will be or how fast they will take place. It is generally thought likely, though that the expected average warming of 2*-3*c would cause the Antartic ice cap to melt (no one knows how fast), which would in turn raise the sea level by about 20 feet. Nearlt one-tenth of this country's population (and much more of its wealth) is found in areas that would then either be diked or under water. Other changes, such as massive shifts in agricultural patterns, are also possible. Areas that are now productive could become arid, while new areas could be farmed -- but the latter might be areas now paved over as metropolitan complexes. Some benefits from more CO2, such as increased photosynthesis by plants, are possible. Many of the results, whether positive or negative, are probably now beyond the reach of speculation.

Whatever the consequences of continued CO2 buildup, however, they are lively to be, if not absolutely irreversible, at least with us for a very long time, since there is no known way to remove vast amounts of CO2 from the atmosphere. So, given the magnitude of the possible results, the issue really should not be ignored.

Even with stepped-up research, it may take decades to get clear answers about CO2. But the need to reduce oil imports and to make long-range commitments to the development of various energy resources is upon us now. And while synthetic fuels accentuate the problem, all fossil fuels contribute -- gas the least, then oil coal more and synthetics the most. In this situation, the safest course may be to spread this country's energy bets more widely than the president has proposed.