OK, so how do we get several hundred million carbon-spewing vehicles off the road in time to significantly slow global warming? I don’t think there’s a single answer, but chances are the solution is going to require massive government intervention. Tax incentives don’t count as massive. Neither does the “Cash for Clunkers� bill introduced in the Senate last week. The Cash for Clunkers program, officially dubbed the Vehicle Scrappage Bill, would give a $2,500 to $4,500 credit to drivers who scrap a fuel-inefficient vehicle and replace it with a more fuel-efficient vehicle. The bill’s sponsors estimate it’ll help upgrade half a million to a million vehicles a year. Americans sold 42.5 million used cars in 2004, so we’re not talking about much of a dent in the used car population, though presumably it would cull many of the dirtier cars. The bill is a step in the right direction, but it’s helpful in the same way getting a smoker to cut out that last cigarette of the day is good for his health. In the end it only matters if it starts a process that gets us to kick the habit entirely. The ideal would be for the government to wave a magic wand and turn everyone’s car into an electric vehicle (and wave it again to produce all our electricity from renewables, but that’s another matter). So how do we get as close to that as possible in the real world? First, the government should bail out Detroit but mandate that the automakers’ product lines be entirely plug-in hybrid or electric within five years. As amazing as that would be, it wouldn’t be enough. A study (pdf) by a pair of MIT researchers shows that even if clean cars immediately captured 50 percent of new-car sales they’d replace only 3.5 percent of the installed base after a year and 18 percent after five years. So we also have to address existing cars, which leads us to… We also need to create a major industry around converting existing vehicles, from R&D to consumer incentives. And we should put the Vehicle Scrappage Bill on steroids by requiring that used internal combustion engine vehicles be converted or scrapped rather than resold, with incentives weighted toward conversion. (Building new cars, even clean ones, puts a burden on the environment, so rehabbing existing cars has an added green dimension.) This may strike many people as unrealistic fantasy (or perhaps socialist nightmare). After all, we’re still working out the battery technology; we’re facing limits to the raw materials for batteries; Detroit can’t keep up with the competition, let alone lead the way; converting combustion engine vehicles is expensive and many are simply too heavy; and even with sizable government assistance most consumers are going to be too strapped or too scared to do anything. But we have to recognize the magnitude of the crisis we face. Global warming has already set us up for very rough times, and our grandchildren will inhabit a different planet. Without unprecedented action it’ll be even worse. (For a particularly bleak outlook, check out this interview with James Lovelock.) While I doubt we’ll do what’s necessary, it’s not impossible. In fact, we’ve done it before. Depression-scarred Americans willingly accepted rationing, bought war bonds and contributed to scrap metal drives during World War II. At the same time, industry rapidly retooled and geared up to crank out mind-boggling numbers of ships, airplanes and tanks. The difference is that the threat then was in the tangible form of hostile armies sweeping the globe. If only we saw our cars’ tailpipes the same way. Eric Smalley is editor of Energy Research News. He has written about technology since 1987 and has freelanced for many publications including Discover, Scientific American, Wired News and The Boston Globe on topics ranging from quantum cryptography to global warming.