BECAUSEOF THE greenhouse effect, this planet is a much warmer and pleasanter place to live than it would be if -- like Mars, for example -- it had a thinner atmosphere. The greenhouse principle is beyond dispute. It's equally clear that if this planet's inhabitants continue to pour carbon dioxide into the atmosphere, it will grow warmer. Carbon dioxide is the most common of the gases that trap heat, greenhouse-style.

But no one knows with any precision how fast the Earth will get warmer -- or whether that increase will make any significant difference in people's lives. A United Nations committee on climate change, in a careful report published a week ago in London, undertakes the useful service of sorting out what's certain from what's not. It's certain that the trend is toward higher temperatures. The rate of increase, on the other hand, is very much open to debate. The committee's best estimate is that the average temperature worldwide will probably rise about 1 degree Celsius by the year 2025, and 3 degrees by the end of the next century. There's a pretty wide consensus behind those figures, but they do not belong to the category of certainties. As these scientists warn, "the complexity of the system means that we cannot rule out surprises."

This committee's report, on the science of global warming, is the first of three. The others will address the consequences and, finally, what's to be done about it.

The first thing to do is plainly to ban CFCs -- chlorofluorocarbons -- since they are not only a threat to public health but powerful greenhouse gases as well. It's beyond explanation that President Bush refuses to support the international plan to get rid of them. He'll never have another chance to do as much for the environment at as little cost.

The much harder issue is carbon dioxide, the inescapable byproduct of any economy that runs on coal, oil and gas. It's not only the rich countries that generate it. The world leader in carbon dioxide generation is Brazil, thanks chiefly to the huge fires burning off its forests. To make any significant difference in the volumes of carbon dioxide being loaded into the atmosphere will require deep changes in the economies not only of rich countries that can afford it, but poor ones that have more urgent things to worry about.

For the present, it would be hard to justify an attempt to make dramatic reductions in carbon dioxide emissions. The costs would be enormous, and disproportionate to the danger. But it's certainly time to get more careful and to start looking for ways to hold emissions down over the years. It's time to begin planning the transition away from coal and oil to greater reliance on solar and nuclear energy. It's time to start talking with the developing countries.

It's not time for drastic action, and perhaps it never will be. But it's necessary to start thinking hard about humankind's oldest servant, fire, and the changes that it now promises to impose on the next century's climate.