The problem is that few tourists seem eager to write off their green guilt.

The idea is simple enough: "offsets" are schemes by which a tourist when paying his ticket can also buy into a project elsewhere that will compensate for the amount of carbon dioxide (CO2) resulting from his trip.

But calculating the size of that carbon footprint apparently is not so simple.

A Paris-New York return flight, for example, might emit between one and three tonnes of CO2, depending on the different calculators used by companies and environmental groups.

That means the price of an "offset" can vary by a factor of five, from 15 to 75 euros (25 to 125 dollars).

One of the first to introduce offsets in France, in January last year, was high-end tour operator Voyageurs du Monde. "Voluntary compensations have been a total failure," said company chairman Jean-Francois Rial.

"Only one percent of our clients really paid the cost of the CO2 emitted by his trip," he said.

Now his company simply taxes travellers without first asking their opinion, adding 10 euros to a ticket for a long-haul flight, tantamount to the price for a half-tonne of CO2.

-- Clients have to pay twice, for the tour and the offsets --

Rial attributed the failure of the system in part to the complications of putting it into place. "Clients have to pay twice," he said, first paying the tour operator for the holiday, then having to agree to pay for the "offset" on an environmental site on the Internet.

Low-cost air companies Easyjet and Atlas Blue believe they have the solution. "Customers just tick a box when they're buying the ticket, the same way they would opt for travel insurance or not, it's must simpler," said Matthieu Tiberghien, who is in charge of a programme called "Action Carbone".

According to a survey for French rail, 65 percent of travellers on the country's trains claimed to be ready to fork out five percent of the price of the trip to compensate for their carbon emissions.

But the percentage who actually put their hands in their pockets was far less.

A year after the national SNCF railways introduced "offsets", a mere 3,000 people have bought into the system out of a total 5.5 million travellers.

"It may be a modest result, but it does highlight a growing consciousness among the public," said Christophe Leon, marketing manager for the SNCF Internet site Voyages-sncf.com.

The site therefore has doubled the stakes by pledging to make its own matching donation to a good cause -- through partner association Goodplanet -- each time a traveller buys an "offset."

Air France, which initially blasted the none too air-friendly SNCF carbon emission calculator, has since last October also been urging its customers to "compensate" by sending donations to the same green group.

While Air France remains mum on the results of its green-friendly campaign, Goodplanet said that up until now, barely 1,000 Air France customers had sent in donations.

British Airways, the first airline to introduce carbon "offsets" in March 2006, is equally discreet about the outcome, as is German flag carrier Lufthansa, who launched its offsets in September 2007.

But TUIfly, a subsidiary of Europe's top tourism firm TUI, said seven percent of its clients had bought carbon "offsets" since last November, to the tune of 250,000 euros.

Though rail travel is less harmful to the environment than air travel, since last November Eurostar too has joined the green bandwagon.

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