In 2002 Responsible Travel became one of the first travel companies to offer customers the option of buying so-called carbon offsets to counter the planet-warming emissions generated by their airline flights.

But in October, Responsible Travel canceled the program, saying that while it might help travelers feel virtuous, it was not helping to reduce global emissions. In fact, company officials said, it might even encourage some people to travel or consume more.

“;The carbon offset has become this magic pill, a kind of get-out-of-jail-free card,”; Justin Francis, the managing director of Responsible Travel, said in an interview. “;It's seductive to the consumer who says, 'It's $4 and I'm carbon-neutral, so I can fly all I want.”;'

Offsets, he argues, are distracting people from making more significant behavioral changes, like flying less.

In theory, the purchase of carbon offsets is supposed to cancel out the emissions generated by activities like flying or heating office buildings by directing money to programs that reduce emissions elsewhere, like tree-planting in Africa or a hydropower project in Brazil. An airline passenger might volunteer to pay $5 to $40 to offset his flight, with the price linked to distance.

Offsets have played a growing role in the greening of travel because carbon dioxide emissions from airplanes are growing so quickly and there is currently no technological fix that would drastically lower them.

In the United States, dozens of hotels and airlines have embraced such programs in the last year or two. United Airlines became the latest American airline to offer one this summer. Globally, offset programs have grown into a multimillion-dollar industry.

But it has proved difficult to monitor or quantify the emissions-reducing potential of the thousands of green projects financed by customers' payments, and there are no industrywide standards.

Responsible Travel is not the only organization that has changed its mind about the usefulness of offsets: Yahoo and the U.S. House of Representatives both ended trial offset-purchase programs this year, concluding that the money was better spent on improving their buildings' energy efficiency.

Some of the world's leading experts on the emissions issue have reviewed and rejected purchasing offsets for air travel.

“;We're always looking at it, but so far I've decided not to do it,”; said Paul Dickenson, chief executive of the Carbon Disclosure Project, a vast nonprofit consortium of companies that have pledged to report and reduce their emissions. For one thing, he said, offsetting the emissions of a flight from London to New York would probably require an extra fee of $200 to $300, far above what any airline is now charging.

And some experts say that emissions from airline travel are simply so large that it may be impossible to offset them.

“;Buying offsets is a nice idea, just like giving money to a soup kitchen is a nice idea, but that doesn't end world hunger,”; said Anja Kollmuss, a staff scientist for the Stockholm Environment Institute who is based at a branch at Tufts University.

“;Buying offsets won't solve the problem, because flying around the way we do is simply unsustainable,”; said Kollmuss, who has researched airline offsets.

A recent study in Britain concluded that one flight from London to Los Angeles produced more carbon dioxide per person than the average British commuter produces in a year by traveling by train, subway or car.

Airlines defend offsets, even while acknowledging that some projects have not lived up to their promises. For example, mango trees that were planted in India to offset a concert tour by the band Coldplay were found to have died a few years later.

EasyJet, one of Europe's largest low-cost airlines, did not offer offsets until 2007 — late for a European carrier — because it was trying to figure out how to ensure the money went to the right places, said Oliver Aust, a spokesman.

It now gives passengers the option of offsetting their flight emissions by investing directly in projects that have been approved by a U.N. certification program for reducing emissions.

EasyJet, which was founded in 1995 but has vastly expanded its fleet since 2003, also uses only the newest and most fuel-efficient jets, flies full planes and packs in extra rows of seats, making its estimated emissions per passenger 28 percent lower than more established carriers on the same routes. Some airlines are experimenting with innovations that may someday reduce emissions, like using fuels made with algae rather than crude oil.

Passenger offsets purport to cancel out carbon dioxide emissions ton for ton through investments in green projects. But critics say there is no transparency about how companies measure whether that happens.

For example, many airlines offer investments in tree-planting projects because trees absorb carbon dioxide. But experts say it takes decades for trees to start fully absorbing the gas, making them a questionable offset for airplanes, which emit carbon dioxide. Kollmuss said the quality of offsets depended on the project. “;But if it's very cheap, it does raise red flags,”; she said.

For Francis of Responsible Travel, the final straw came when he noticed that carbon offsets were being offered by private jet companies and helicopter tour operators, which generate very high emissions per passenger. “;The message was, 'Don't worry, you can offset the emissions,”;' he said. “;But you don't really need to see Sydney from the air, do you? And you can travel in a commercial airliner.”;

Francis said he was not advocating an end to flying — Responsible Travel offers tours to Jordan, the Galapagos Islands and China, among other far-flung destinations — but simply more reflection on the environmental impact of such journeys.

Responsible Travel had bought its offsets through one of the best-known offset companies, ClimateCare, which was purchased last year by JP Morgan.

While acknowledging that improved, universal standards for personal airline offsets are needed, Richard Folland, a climate change and energy adviser to JP Morgan, said the offset concept had played an important role in helping to direct money to otherwise unaffordable environmental projects in poorer countries.

“;The primary goal is to reduce emissions, but offsets are helpful”; in managing the costs, he said, adding, “;There has to be a balance struck.”;

Dickenson of the nonprofit Carbon Disclosure Project said that rather than buying offsets he had sharply scaled back on flying and was instead taking trains or conducting meetings by phone or teleconference. He said that if he owned an airline, he would now be diversifying into other modes of transport.

Referring to the recent purchase of a railroad by the investor Warren Buffett, he said, “;What does it tell you that the world's most successful investor is investing in trains?”;