SCIENTISTS HAVE AGREED for some time that the amount of carbon dioxide in the atmosphere has increased over the last century, and there is a general consensus that the current concentration is likely to double sometime during the next century. What has been uncertain is whether this increase, caused largely by the burning of fossil fuels, would cause the global warming known as the greenhouse effect, and how large the effect would be.

An important new contribution, by a team of NASA atmospheric physicists, now concludes that the carbon dioxide is causing a warming, one large enough to have very unfavorable consequences in the next century.

The prediction is that the mean global temperature will increase by 1 to 4.5 degrees centigrade by the end of the coming century. Which end of this range would be reached will depend largely on future energy growth. The low figure is premised on zero energy growth with depleted oil and gas resources being replaced by non-fossil fuels (including nuclear energy, hydroelectric power and others that do not add carbon dioxide to the atmosphere). A more likely, middle-range estimate--a 2.5-degree warming--is based on the prospect of slow energy growth, with oil and gas being replaced equally by synthetic fossil fuels and non-fossil fuels.

What would the ill effects be? Two and a half degrees (equivalent to 4.5 degrees Fahrenheit) doesn't sound like very much. But these are among the possible--though still uncertain--results. Precipitation patterns would shift, creating a hot, dry climate in the western two-thirds of the United States and Canada, now the world's bread basket. If the model is correct, the drought of last summer "may be typical" of next century's weather. The West Antarctic ice sheet is likely to melt, causing a sea level rise of about 16 feet, flooding 25 percent of Louisiana and Florida and 10 percent of New Jersey. Melting polar ice would open up the long-sought Northwest passage, the target of explorers since Verrazano.

Astonishingly, this 2.5-degree increase in the space of a century is a warming of "almost unprecedented magnitude" in the planet's long history. It would mean a world temperature that would "approach the warmth of the Mesozoic, the age of dinosaurs."

There are still many hedges to these findings, and much research that must be done before they are confirmed. The authors predict, however, that the warming will be clearly measurable--over and above normal climate variations--by the 1990s, and perhaps even in this decade. Since the amount of warming depends most strongly on energy growth and the type of fuels used, and since it takes decades and billions of dollars to make major shifts in energy use, it is not too soon to begin thinking seriously about how carbon dioxide should affect U.S. energy choices now.