Converting every vehicle in the United States to hydrogen power would demand so much electricity that the country would need enough wind turbines to cover half of California or 1,000 extra nuclear power stations.

So concludes a British economist, whose calculation is intended to highlight the difficulties of achieving a truly green hydrogen economy.

"This calculation is useful to make people realize what an enormous problem we face," says Andrew Oswald, an economist from the University of Warwick.

The hydrogen economy has been touted as a replacement for fossil fuels, which release carbon dioxide when burnt, thus contributing to global warming. Burning hydrogen produces only water.

Most hydrogen is currently made from methane, in a process that releases carbon dioxide into the atmosphere. Splitting water molecules with electricity generates hydrogen - but the electricity is likely to have been generated from fossil fuels. Although this may shift urban pollution to out-of-town electricity plants, it makes little difference to greenhouse-gas output. "Today, hydrogen is not a clean, green fuel," says Oswald's brother Jim, an energy consultant who assisted with the calculation. "You've got to ask: where did the hydrogen come from?"

The only technology that can currently make large amounts of hydrogen without using fossil fuels relies on renewable power sources or nuclear energy, the Oswalds argue. Hydrogen will only mitigate global warming when a clean source of the gas becomes available, they say.

Unpopular options

The duo considered the United Kingdom and the United States. Transport accounts for about one third of each country's energy consumption.

UK transport uses only a tenth as much energy as the United States, but there is less land available: the hydrogen switch would require 100,000 wind turbines, enough to occupy an area greater than Wales.

It unlikely that enough turbines could ever be built, says Jim Oswald. On the other hand, public opposition to nuclear energy deters many politicians. "I suspect we will do nothing, because all the options are so unpopular."

"I don't think we'll ever have a true hydrogen economy. The outlook is extremely bleak," he adds. The brothers outline their calculation in the current issue of Accountancy magazine.

"Hydrogen is not a near-term prospect," agrees Paul Ekins, an energy economist at the Policy Studies Institute, London. "There will have to be a few fundamental breakthroughs in technology first," he says.

Politicians eager to promote their green credentials, yet unaware of the realities, have oversold the hydrogen dream, says Ekins. "I'm amazed by the number of politicians who think you can dig hydrogen out of the ground," he says.

However, he thinks that the Oswalds are too pessimistic about the possibilities of new technology. "An enormous amount of attention is being paid to generating hydrogen cleanly," he says.

If we could trap the carbon dioxide produced by fossil fuels underground, we could convert them to hydrogen, says Ekins. "It's not tried and tested, but it's a possibility." And it could become a reality by the time we have enough hydrogen-powered cars to make it necessary, he says.

So do the Oswalds have a more immediate answer to the hydrogen problem? "We could always use less energy, but that doesn't seem very likely," Jim Oswald says ruefully.