But is it addressable? Would any politically feasible policy changes by Congress and the president make a dent in this trend? There is good reason for skepticism. American emissions of greenhouse gases such as carbon dioxide are only part of a global problem. China and India are quickly building new coal-fired power plants to sustain massive economic growth. According to one estimate, China surpassed the United States in the production of greenhouse gases last year. We are only at the beginning of globalization -- and right now, that process is inseparable from the burning of fossil fuels. American restrictions on greenhouse gases, in isolation, would not be decisive.

This is not, however, an excuse for inaction. There is another, more compelling reason to consider a cap on the production of carbon dioxide. As in the case of fighting Los Angeles smog, this type of government regulation would create economic incentives for the development of new technologies -- incentives that do not exist in the free market. Capturing and storing carbon dioxide from power plants, by all accounts, is a difficult technical challenge. But the problem is much more likely to be solved if someone has a direct economic interest in solving it.

There are several proposals by members of Congress -- including a bipartisan bill from Sens. Joseph Lieberman and John Warner due next month -- that take a "cap-and-trade" approach to greenhouse gases. The government mandates overall reductions in emissions and lets companies decide how to implement them. If someone produces less carbon dioxide, for example, that reduction can be sold to another producer and yield some money in the process. This kind of market-based system has been used successfully to cut coal-powered plants' emissions of sulfur dioxide, the main cause of acid rain.

A cap-and-trade system isn't perfect. It is open to fraud -- companies in other countries have sometimes increased their production of pollutants to get benefits for cutting them later. A cap-and-trade bill could be used by Congress to push subsidies toward pet environmental projects of questionable value.

But the overall argument for a cap-and-trade system is strong. The answer to global warming will eventually be technological -- the production of energy without the production of heat-trapping gases. But only the government can create the incentives for Americans to work on this problem with urgency and seriousness. And there is hope to be found in the clearer skies of Los Angeles.