In a report coinciding with a key UN conference on global warming, the IEA said that if conditions were right, hydrogen and hydrogen fuel cells could play key roles in weaning energy users away from oil, gas and coal.

"In the most favourable conditions, hydrogen fuel cell vehicles would enter the market (in mass numbers) around 2025 and power 30 percent of the global stock of vehicles by 2050 -- the equivalent of about 700 million vehicles," the IEA said.

"The oil saving would then be equivalent to some 13 percent of global oil demand, or five percent of the global energy demand."

If hydrogen power takes off and other emerging technologies are brought in, the annual global output of CO2 could be halved by the middle of the century, it said.

The fuel cell, invented in 1839 by British physicist William Grove, produces electricity thanks to a catalyst-assisted reaction between hydrogen and oxygen. Its only pollution is water.

The problem, though, is cost. At present, a fuel cell costs 2,000 dollars per kilowatt of energy produced, compared with 50 dollars per kilowatt for conventional car engines today.

Another big challenge is reducing the cost of obtaining hydrogen itself, which has to be extracted from fossil fuels, such as carbon, or from water.

"In the next few decades, hydrogen costs need to be reduced three to tenfold and fuel cell costs by 10 to 50-fold," the IEA said in a press release.

"Substantial improvements are also needed in hydrogen transportation and storage and fuel cell performance. At the same time, governments need to implement decisive policies and incentives to promote emission savings and diversify the energy supply."

The IEA steered away from specifying whether governments should take a tougher regulatory approach, as opposed to a voluntary one, to help the switch to the use of hydrogen.

"Governments should adopt strong policies to reduce emissions (and) we have to reduce drastically the cost of those technologies," said Giorgio Simbolotti, a co-author of the IEA study, Prospects for Hydrogen and Fuel Cells.

"If these two conditions are not met, it would be very difficult for hydrogen to reach the critical mass needed for market uptake."

Environmentalists and energy experts say that new energy sources can only succeed if there is a high but sustained price for fossil fuels, or if there are tough laws that, for instance, require greater fuel efficiency, lower carbon pollution or widen access to hydrogen at filling stations.

The IEA will present the report next Monday in Montreal at a conference of the UN Framework Convention on Climate Change (UNFCCC), which is focussing on ways to reduce pollution of "greenhouse" gases that trap heat from the Sun.

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