IS THE FIGHT against global warming hopeless? It can seem so. The long-term threat to the climate comes from carbon dioxide, which lingers in the atmosphere for hundreds of years, locking in higher temperatures for generations. After decades of effort, only about one-tenth of America’s energy mix comes from renewable sources that don’t produce carbon dioxide.

But two policies can buy the world more time to allow carbon-free technologies to catch up. One is aimed at greenhouse substances that clear out of the atmosphere after a few years, months or even days. Cutting back the emission of soot and ozone gases such as methane would reduce the world’s warming by as much as a half degree Celsius over the next few decades, according to a study in last month’s Science. Adding hydrofluorocarbons — another class of short-lived pollutants — to the list would help even more to delay the approach of temperature thresholds beyond which global warming could be catastrophic.

Reducing these emissions is relatively cheap, especially when the benefits to health are factored in. For example, primitive cooking stoves in developing countries produce much of the world’s soot; using more efficient ones would prevent perhaps millions of deaths from respiratory illness. Methane, meanwhile, is the primary component of natural gas — a commodity that pipeline or coal-mine operators could sell if they kept it from escaping into the atmosphere. Researchers have even concluded that global crop yields would rise.

Coordinating an effective international effort to cut so-called short-lived climate forcers will be the hardest task. The Group of Eight and the Arctic Council have acknowledged the need but have moved slowly. On Feb. 16 Secretary of State Hillary Rodham Clinton announced a new international initiative, along with $12 million for the program.

It will take more than American money. Regulators in the developing world must enforce stronger air-pollution rules. Since many of the health benefits will be immediate, though, some may be more eager to do so than they have been to cut carbon dioxide emissions. Nations may also add hydrofluorocarbons to the substances regulated by the Montreal Protocol, the treaty responsible for slashing the use of a related class of chemicals.

Another development that promises to provide time for clean technology to scale up — America’s natural gas boom — faces a challenge of a very different sort: environmentalists. Innovative drilling techniques have made huge amounts of fuel deep below Americans’ feet retrievable at low cost. Most of it is methane, a greenhouse gas that produces only about half the carbon as coal after combustion. Environmentalists should be cheering: Cheap gas transported for the most part in existing pipelines can start the United States on a carbon-reducing path with minimal added cost.

That path, however, must end with phasing out most, if not all, energy based on fossil fuels. There is reason for hope over the coming years — but not for complacency.