ut here it is late 2004, and Ford has finally delivered, producing the first American-made hybrid car. The country's flirtation with this relatively new kind of vehicle will be tested in earnest as more models arrive in showrooms. Toyota has tweaked the Prius -- already the darling of what is still a niche market -- to be even faster, cleaner, larger, and more fuel-efficient than last year's model, and will further up the performance ante with a hybrid Lexus SUV that promises the acceleration of a V8. G.M. is offering a hybrid pickup and claims that by 2007 it will have seven more hybrid models, with the capacity to build a million of these vehicles a year -- if Americans want them. Toyota's Camry and Honda's Accord, the two best-selling passenger vehicles in America, will go hybrid over the next two years.

Still, hybrids -- 20,000 from Ford in the next year, 100,000 from Toyota -- represent an insignificant number in a market that sells 17 million vehicles a year. Nor is it likely these carmakers want to build many more. The Escape hybrid is priced thousands of dollars more than a comparably powered standard Escape; Ford refuses to say whether it's actually covering its costs. But certainly for Ford, and perhaps for Toyota as well, the answer is: not yet. It's true that new technologies often fall dramatically in price over time, but the consensus is that hybrids will always cost more to build than internal combustion engines for the simple reason that instead of one propulsion system you have two, an electric motor and a gasoline engine.

These price barriers must be surmounted for hybrids to become real players in the market and thereby have a measurable impact on slowing the disastrous pace of climate change. Joe Tomita, senior vice president of regulatory and technical affairs for Toyota North America, is clear on this: "The best technology can't accomplish a thing without mass-market acceptance and volume sales." It's a real Catch-22. There won't be mass-market acceptance unless automakers help create the market, and this is something they are clearly unwilling to do without the certainty of market demand.

High manufacturing costs and the instability of energy prices mean that the major carmakers will want to keep their options open. All are pursuing at least one other power train besides gasoline. In Europe, where gasoline prices have been high enough, long enough to shape the market, diesel has been reborn. With no more nasty soot and noise, and with efficiency approaching that of hybrids, this new "clean diesel" has captured nearly half of European car sales. Surprisingly, European air quality standards for 2008 allow five times as much particulate matter and eight times as much nitrous oxide as in the United States, where carmakers are also required to warranty their exhaust systems for two to three times as long as in Europe. So diesel would have to prove itself adaptable to the more stringent emissions standards in the United States.

But the Office for the Study of Automotive Transportation at the University of Michigan believes this will happen and predicts that by 2012, diesel will capture 15 percent of vehicle sales in the United States, hybrids 7.5 percent, and hydrogen fuel cells 1 percent. What about beyond 2012? Will we then come upon the hydrogen future?