President Clinton this week confronted the two most important environmental questions of the year, how to slow global warming and how to tighten the Clean Air Act. He satisfied the environmentalists in his constituency on the issue of smog and soot, but deeply disappointed many of them today on the question of climate change. It was a striking disparity, given that the two environmental problems have some important common roots. Soot and smog, like greenhouse gases, come mainly from burning fossil fuels. Imaginative new approaches to controlling both kinds of pollution are likely to involve plans for trading emissions credits and perfecting new technologies.

But there are important differences, and these illustrate why the President found himself seeming so torn on the overall question of what to do about air pollution.

For one thing, many ordinary people have immediate concerns about the smog they breathe and the way it and particles in the air can worsen the illnesses of their young children and aging parents. Not so many are worried about rising sea levels caused by melting glaciers, or about the scientific subtleties of global warming.

For another thing, President Clinton faced a pressing deadline for deciding on the new air pollution rules, which under a court order had to be issued by July. His ultimate decisions about how fast to move on global warming are not due until December, when international negotiations sponsored by the United Nations culminate in Kyoto, Japan.

And above all, fixing the global warming problem requires not just more powerful medicine, but a lot more of it, than controlling local smokestacks and tailpipes.

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To satisfy the critics demanding swift and decisive action from the United States to head off global warming, Mr. Clinton would have to pledge that a nation in the midst of an energy binge was about to break its addiction to cheap fossil fuels.

And that implies an act of extraordinary political will.

At the United Nations today, Mr. Clinton promised to mobilize his own country in such an effort, but he disappointed anybody who wanted a precise plan. In the President's own crucial word, any goals he will eventually accept must be ''realistic.''

The industry giants who lambasted the Administration for its decision on air quality on Wednesday praised Mr. Clinton's caution on global warming today.

''We are encouraged by the fact that President Clinton seems to be taking a slightly more cautious approach to climate issues than he appeared to be taking several months ago,'' said Gail McDonald, president of the Global Climate Coalition, an industry group. ''There are many vested interests pushing very hard for an agreement that would have dire consequences for the economy.''

Several policies favored by many environmentalists are seen by the Administration as unrealistic.

Energy taxes as a way to lower fuel consumption are pretty well ruled out in Washington's current political climate. Regulations to enforce fuel-efficiency on automakers are equally out of favor.

Federico Pena, Mr. Clinton's Energy Secretary, said in an interview today that the most important thing in the President's speech was his commitment to proselytizing the public on the urgency of the problem and then to persuading the country that the solution will not be unduly painful.

''We have not done a good job of educating the American people, first about the challenges we face on global warming, and secondly about the value of making the long-term investments that actually pay back in your pocketbook,'' he said.

It is impossible to say, of course, whether technology breakthroughs of the kind Mr. Clinton vowed to pursue are the most realistic way of weaning America from coal and oil. Despite the progress toward wider use of photovoltaic solar cells, electric cars, fuel cells that run on hydrogen, and so on, the technical hurdles are still formidable.

''Solar energy is an important part of the long-term solution, but it is no substitute for binding targets and timetables for reducing emissions soon,'' said Michael Oppenheimer, chief scientist at the Environmental Defense Fund.

America is, after all, a nation where energy waste is common enough that many readily available conservation measures could pay for themselves by reducing the energy costs of businesses and households. Still, market mechanisms do not yet exist to drive investments in those directions, at least not at the speed needed to dramatically slow emissions of carbon dioxide, the principal greenhouse gas.

The Administration is working hard to develop an approach toward holding down emissions that will create pollution permits that companies can buy and sell to each other. Known as the ''cap and trade'' approach, it is intended to steer investments in pollution control toward the cheapest possible solutions. It pleases some environmental groups, but others view it with the suspicion of a grandmother confronted with a newfangled gadget.

But the White House appears to be persuaded by its own economists that the economic pain of abruptly taking new paths on energy consumption might be too great to bear, and that a more moderate approach is called for. Such conclusions are amplified in dire warnings from powerful industries about the dangers of moving too quickly.

Fed by an economic boom that has overwhelmed steady improvements in industrial energy efficiency, its gas-guzzling vehicles slaked by affordable gasoline, the United States appears likely to increase emissions of carbon dioxide by 13 percent in this decade. This is just the reverse of what it promised to do five years ago in Rio de Janeiro, where the industrial countries aimed to reduce greenhouse gas output to the level of 1990 by the year 2000.

As Mr. Clinton said in his speech, only 1 out of every 25 people on the planet is an American, but the fuel they burn produces one out of every five tons of the carbon dioxide that is steadily building up in the atmosphere, threatening to seriously disrupt the climate 100 or 200 years from now.

To be sure, some countries have met their targets and others are willing to promise to obey a new, binding treaty that would actually reduce greenhouse gases further. The target the European Union proposes is to cut emissions by 15 percent from the 1990 level, achieving that goal by the year 2010.

The British cut emissions when they ended subsidies for an inefficient coal industry. The Germans, after unification, tore down the obsolete state-run factories of the east. Russia met the target of reducing emissions, but only because its economy collapsed along with Communism. And as a whole, Europe does not have the aversion to energy taxes -- or taxes in general -- that has been displayed in the United States, where the Clinton Administration's first big economic policy defeat was on the BTU tax, and where just last year Congress, egged on by Bob Dole, was busily trying to roll back gas taxes because of a blip in prices at the pump.

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