CAUTIOUSLY but unequivocally, the United States has committed itself to negotiating a worldwide agreement limiting emissions of carbon dioxide. The nature of that limit is still unclear, but it will affect all combustion of fossil fuels. Most of the world's governments have become deeply concerned about the prospect that the Earth will grow steadily warmer as the density of carbon dioxide rises in the atmosphere. More than 100 countries were represented at the recent Chantilly conference that began work on the agreement. If the United States had refused to support it, the whole effort would have collapsed there. But as it turns out, the world is embarked on its first serious effort to get control of the greenhouse effect.

For itself, the United States has pledged to hold its emissions of the greenhouse gases in the year 2000 to the levels of 1987 or lower. That ought not be hard. Michael R. Deland, chairman of the White House Council on Environmental Quality, pointed out that this country's contributions of carbon dioxide have been stable over the past 15 years while the economy has grown by more than 50 percent.

But no one really knows what it would cost to make substantial reductions. That was one of the reasons the administration until now had shied away from all talk of carbon dioxide limits. Reputable estimates of costs run all the way from very high figures to claims of net benefits produced by rising fuel efficiency. And yet, among all the uncertainties there are two things that are clear beyond any reasonable doubt.

While it can't be proved that the greenhouse effect has already begun to change the Earth's climate, it will certainly do so if the world continues to pour into its atmosphere the present volume of greenhouse gases -- of which carbon dioxide is the most important.

Second, there can be very long lags between the causes and the effects of climate change. Ocean temperatures, for example, respond only slowly to changes in air temperature. Congress' Office of Technology Assessment recently observed that stabilizing the present output of greenhouse gases wouldn't go nearly far enough to stabilize the climate. That would require huge cuts, perhaps as much as 80 percent, in the current emissions of the industrial countries.

By finally deciding to support a world agreement, the Bush administration has taken a prudent and necessary first step to protect the climate. A great deal now depends on the next decision -- whether and how much to cut the present production of the greenhouse gases.