THE rush to go on a carbon diet, even if by proxy, is in overdrive.In addition to the celebrities - Leo, Brad, George - politicians like John Edwards and Hillary Clinton are now running, at least part of the time, carbon-neutral campaigns. A lengthening list of big businesses - international banks, London's taxi fleet, luxury airlines - also claim "carbon neutrality." Silverjet, a plush new trans-Atlantic carrier, bills itself as the first fully carbon-neutral airline. It puts about $28 of each round-trip ticket into a fund for global projects that, in theory, squelch as much carbon dioxide as the airline generates - about 1.2 tons per passenger, the airline says.

Also, a largely unregulated carbon-cutting business has sprung up. In this market, consultants or companies estimate a person's or company's output of greenhouse gases. Then, these businesses sell "offsets," which pay for projects elsewhere that void or sop up an equal amount of emissions - say, by planting trees or, as one new company proposes, fertilizing the ocean so algae can pull the gas out of the air. Recent counts by Business Week magazine and several environmental watchdog groups tally the trade in offsets at more than $100 million a year and growing blazingly fast.

But is the carbon-neutral movement just a gimmick?

On this, environmentalists aren't neutral, and they don't agree. Some believe it helps build support, but others argue that these purchases don't accomplish anything meaningful - other than giving someone a slightly better feeling (or greener reputation) after buying a 6,000-square-foot house or passing the million-mile mark in a frequent-flier program. In fact, to many environmentalists, the carbon-neutral campaign is a sign of the times - easy on the sacrifice and big on the consumerism.

As long as the use of fossil fuels keeps climbing - which is happening relentlessly around the world - the emission of greenhouse gases will keep rising. The average American, by several estimates, generates more than 20 tons of carbon dioxide or related gases a year; the average resident of the planet about 4.5 tons.

At this rate, environmentalists say, buying someone else's squelched emissions is all but insignificant.

"The worst of the carbon-offset programs resemble the Catholic Church's sale of indulgences back before the Reformation," said Denis Hayes, the president of the Bullitt Foundation, an environmental grant-making group. "Instead of reducing their carbon footprints, people take private jets and stretch limos, and then think they can buy an indulgence to forgive their sins."

"This whole game is badly in need of a modern Martin Luther," Mr. Hayes added.

Some environmental campaigners defend this marketplace as a legitimate, if imperfect, way to support an environmental ethic and political movement, even if the numbers don't all add up.

"We can't stop global warming with voluntary offsets, but they offer an option for individuals looking for a way to contribute to the solution in addition to reducing their own emissions and urging their elected representatives to support good policy," said Daniel A. Lashof, the science director of the climate center at the Natural Resources Defense Council.

But he and others agree that more oversight is needed. Voluntary standards and codes of conduct are evolving in Europe and the United States to ensure that a ton of carbon dioxide purchased is actually a ton of carbon dioxide avoided.

The first attempt at an industry report card, commissioned by the environmental group Clean Air/Cool Planet (which has some involvement in the business), gave decidedly mixed reviews to the field, selecting eight sellers of carbon offsets that it concluded were reasonably reliable.

But the report, "A Consumer's Guide to Retail Carbon-Offset Providers," concluded that this market was no different than any other, saying, "if something sounds too good to be true, it probably is."

Prices vary widely for offsetting the carbon dioxide tonnage released by a long plane flight, S.U.V. commute or energy-hungry house. The report suggested that the cheapest offsets may not be legitimate.

For example, depending on where you shop for carbon credits, avoiding the ton of carbon dioxide released by driving a midsize car about 2,000 miles could cost $5 or $25, according to data in the report.

Mr. Hayes said there were legitimate companies and organizations that help people and companies measure their emissions and find ways to cut them, both directly and indirectly by purchasing certain kinds of credits. But overall, he said, an investment in such credits - given the questions about their reliability - should be looked at more as conventional charity (presuming you check to be sure the projects are real) and less as something like a license to binge on private jet travel.

In many ways, the carbon-neutral campaign mimics other efforts that use markets to save the environment. For nearly two decades, for example, forest protection groups have disputed the merits of "certified" tropical hardwood and other products that manufacturers claim are harvested in ways that don't imperil virgin forests.

Some environmentalists say it's better to offer some income to those who use forests in a renewable way. But others insist that instead of trying to police the trade by rooting our fraudulent planks, it's better to avoid the timber altogether. Only one of many forest certification programs, run by the Forest Stewardship Council, has been widely endorsed by environmental groups.

Michael R. Solomon, the author of "Consumer Behavior: Buying, Having and Being" and a professor at Auburn University, said he was not surprised by the allure of the carbon-offsetting market.

"Consumers are always going to gravitate toward a more parsimonious solution that requires less behavioral change," he said. "We know that new products or ideas are more likely to be adopted if they don't require us to alter our routines very much."

But he said there was danger ahead, "if we become trained to substitute dollars for deeds - kind of an 'I gave at the office' prescription for the environment."

Charles Komanoff, an energy economist in New York, said the commercial market in climate neutrality could have even more harmful effects.

It could, by suggesting there's an easy way out, blunt public support for what will really be needed in the long run, he said: a binding limit on emissions or a tax on the fuels that generate greenhouse gases.

"There isn't a single American household above the poverty line that couldn't cut their CO2 at least 25 percent in six months through a straightforward series of fairly simple and terrifically cost-effective measures," he said.

Jonathan Shopley, the chief executive of Britain's CarbonNeutral Company, which does only 5 percent of its offsetting directly for individuals and the rest for businesses, insisted that the voluntary markets fill a vital gap.

This is particularly true, he said, because laws or treaties, like the Kyoto Protocol, that have mandatory limits on greenhouse gases have so far failed to blunt the relentless global rise in such emissions.

"That isn't going to get us where we need to go," Mr. Shopley said.

Copyright 2007 The New York Times Company