But critics say a rationing system, linked to personal financial rewards and penalties for millions of people, could be a costly slide towards Big Brother-style surveillance.

"It is easy to dismiss the idea as too complex administratively, too Utopian or too much of a burden for citizens," British Environment Secretary David Miliband said last month in a speech floating the idea of individual allowances.

"But ... in the long term, there may be potential to make a system work," he said.

Britain has come further than other nations, simply by airing the idea. Most other governments focus on education -- advising people to install energy-saving lightbulbs or turn down the thermostat in winter -- or on taxes as a way to spur lifestyle changes to cut energy use.

And many experts doubt that voters would accept cards, and billion-dollar administrative costs, especially in countries such as the United States which is not part of the United Nations' Kyoto Protocol on capping greenhouse gas emissions.

"The idea is too fraught with problems. It's overkill," said Raymond Kopp, a senior fellow at the Resources for the Future environmental think-tank in Washington. He noted that many people, even in the United States, do not even use credit cards.

He estimated personal rations for carbon dioxide, the main greenhouse gas, might only be worth a dollar or two a day -- a relatively small sum that would not encourage consumers to take part in the scheme by making efforts to cut their consumption.

"Americans wouldn't accept rationing for this -- it would cost less than a latte a day," he said.

IMPOSSIBLE

"In theory, these proposals look very nice but in practice it would be impossible to control," said Paal Prestrud, head of the Center for International Climate and Environmental Research in Oslo. "There must be 100 ways to get round it."

Many scientists say use of fossil fuels in power plants, factories and households will drive up temperatures and could spur floods, heatwaves, and rising sea levels by 2100. Most nations say it makes sense to cut fossil fuel use, especially with oil prices around US$75 a barrel.

"In my view there is no alternative to quotas in the long run," said David Fleming, a British policy analyst who suggested the idea a decade ago. "The point is that it opens up the way to achieve serious energy reductions."

Citizens' energy purchases -- mainly to buy petrol, heating fuel or electricity generated by fossil fuels -- make up about 30-40 percent of greenhouse gas emissions in developed nations.

Under rationing, all buyers of energy -- including citizens, companies and the government -- would take part with quotas.

Every adult would get an account with allowances for carbon dioxide emissions linked to an electronic card, perhaps with an emblem like a green planet or a factory belching fumes.

Points would be deducted every time energy was bought. Anyone exceeding their quota would have to pay a higher price for any more energy bought during the year while those below quota could cash in surplus allowances.

Those who felt the system involved too much hassle could simply cash in the allowances and pay higher energy prices year-round as a tax. And every year the allowances would be cut, to squeeze energy use lower and lower.

60 SECONDS

A report by the British Tyndall Centre for Climate Change Research estimated that queuing and transaction time at petrol stations could add an average 45-60 seconds to every person's visit, assuming a separate carbon card.

Fleming argues banks could add carbon allowances to existing credit cards, simplifying the system.

"People don't feel neutral about the idea," said Richard Starkey, an author of the Tyndall Centre study which broadly concluded quotas were feasible. "They either really like it or they don't. Either it's great or it's Big Brother."

Many governments reckon that education and carbon trading schemes limited to big emitters such as power plants and factories -- as in the European Union market since 2005 -- are better ways to cut back.

"Turn down. Switch off. Recycle. Walk," are the slogans of a European Commission campaign to encourage energy efficiency.

Americans, the world's top emitters of greenhouse gases, emit an average of about 20 tonnes of carbon dioxide a year, with perhaps 7 tonnes from transport and households. On average, the world's 6.5 billion people emit about 3.6 tonnes each.

In the EU market among major industrial emitters of carbon dioxide, prices are around 16 euros (US$20.45) a tonne. Assuming identical prices for individuals, seven tonnes would cost each American US$143 a year or 40 cents a day.

Starkey said uncertainties included whether some people -- those living in cold northern regions or people with large families -- might demand extra allowances.