They are the future, automakers say. But when that future will arrive depends on whom you ask.

Even if the government and consumers embraced the technology, more obstacles exist before a large number of vehicles can be made. One is the mass production of hydrogen.

Although it's one of most abundant gases in the universe, hydrogen usually bonds with other chemicals. Some of the extraction processes, like getting hydrogen from natural gas, still emit carbon dioxide, said Joseph Romm, a senior fellow at the Washington-based Center for American Progress.

"You don't solve the global warming problem if you're making hydrogen from natural gas," said Romm, who worked for the U.S. Department of Energy during the Clinton era.

Making hydrogen from water or renewable sources like wind power is better, he said, but more complex and possibly more expensive.

What's still unclear, though, is when — or if — widespread production of fuel cell vehicles will begin.

"I think that's unlikely in the lifetime of the majority of your readers," said Romm, who considers himself a clean-energy advocate but skeptical of hydrogen fuel cell vehicles.

A 2008 study by the Washington-based National Research Council was more optimistic. In a best-case scenario, it said, about 2 million fuel cell vehicles will be in use by 2020, growing to 220 million by 2050.