A movie star appears on a talk show, lectures viewers about saving the environment, then boards a trans-Atlantic flight for a premiere in Milan, releasing untold amounts of greenhouse gases in a vapor trail of hypocrisy and excess.

It's OK, says the star's publicist — because the celebrity offset the trip by giving money to a forestry project. The trees that get planted will absorb the carbon released by the airplane, making the jaunt carbon-neutral.

But is that really the case? Or are forestry-based voluntary carbon offsets just a big scam?

• Click here to visit FOXNews.com's Natural Science Center.

"Does planting trees reduce global warming? Not in most places on the earth," Joseph Romm, senior fellow at the Washington, D.C.-based Center for American Progress and former acting assistant secretary of energy during the Clinton Administration, warned in a 2007 essay on the environmental news site Grist.

"If you are thinking about purchasing offsets, be wary of any company that says it plants trees."

Offsets work by appealing to consumers' sense of responsibility — or guilt.

Using an online calculator, a consumer can determine the amount of greenhouse gases he or she produces either over the course of a year or when going on a trip. The consumer can then pay someone to "offset" or neutralize the gases he has put into the atmosphere.

For trading purposes, greenhouse gases are measured in metric tons of carbon dioxide (CO2). Other greenhouse gases such as methane, which is 20 times as potent, are converted into CO2 equivalents.

Anyone who demonstrably neutralizes a ton of CO2 can then sell that cancellation to an offset provider, who resells it to consumers such as the hypothetical celebrity. The actual price to the consumer to offset his or her greenhouse gases varies by provider and project.

(The consumer market is entirely voluntary, which differs from the industrial market; in the industrial market, businesses that exceed greenhouse-gas targets can purchase credits called carbon financial instruments from other businesses with excess credits to spare.)

A number of different types of offsets exist for consumers. For example, wind farms create credits that can be traded. An amount of "clean" energy produced by a wind farm is thought of as displacing an equal amount of energy from a "dirty" source, such as a coal-burning electrical plant.

Another option is through "avoided deforestation," in which an endangered forest is purchased and allowed to stay as it is, thereby preserving the carbon already contained in its biomass and letting its trees continue to exchange carbon dioxide for oxygen.

But the third oft-used option is reforestation — planting new trees to absorb CO2 — and environmentalists aren't sure whether it really works.

Carbon Catalog, a Web site that lists and reviews offset providers, gives bonus points to those whose portfolios don't have a majority share of reforestation projects.

TerraPass, a San Francisco-based offset company, doesn't offer any forestry options for its customers. And the Kyoto Protocol's Clean Development Mechanism, which establishes standards for what constitutes credits, excludes forestry projects.

The Forests and the European Union Resource Network, a European environmental group with offices in Brussels and rural England, "does not believe tree-planting offsets should be granted any carbon credits," according to communications manager Richard Wainwright.

"Trees grow rather slowly. And particularly when they're small, they don't sequester much carbon," explains TerraPass co-founder Adam Stein on his company's Web site.

"The small print on tree-planting offsets typically indicate a 40-year maturity," Stein explains. "TerraPass's promise to our customers is that we fund greenhouse gas reductions that take place now."

To that end, TerraPass features investments in wind farms or landfill gas-captures that destroy methane before it escapes into the open air.

Stein also worries about the longevity of reforestation projects.

"Forests carry an inherent risk, because trees can die," Stein said, adding that decaying trees release the carbon they've absorbed during their lifetimes. "To manage that risk, you have to build in a lot of buffer."

Yet bigwig environmental groups such as the New York-based Rainforest Alliance and Conservation International and the Jane Goodall Institute, both based in Arlington, Va., have signed the Forests Now Declaration [http://www.forestsnow.org/declaration.php] petitioning carbon-credit markets to include reforestation and avoided deforestation projects.

"Tropical forests continue to be excluded from carbon markets," the declaration reads in part. "Instead, perverse incentives are in force, encouraging continued conversion and degradation of forests and discouraging their restoration and capacity to contribute to sustainable development."

The companies that sell reforestation offsets don't have any doubts about their effectiveness.

"Certain protection measures are built into the certification process," said Russell Simon, communications manager with Carbonfund.org, an offset seller based in Silver Spring, Md.

Carbonfund has three forestry projects: a 1,000-acre reforestation of former pastureland in Nicaragua [http://www.carboncatalog.org/projects/return-to-forest-reforestation/], a 1,100-acre reforestation of an old cotton field in Louisiana [http://www.carboncatalog.org/projects/tensas-river-valley-reforestation/] and a similarly-sized reforestation of abandoned cropland and grassland in southwestern China [http://www.carboncatalog.org/projects/protecting-biodiversity-in-tengchong/].

Simon explained that Carbonfund sets a deadline for its reforestation projects. The amount of CO2 sequestered by the Nicaragua project is measured only for the first 40 years.

"It becomes a closed loop at that point," said Simon. "Trees die and new ones are born," so that any carbon released by dead trees is reabsorbed by young growing trees.

Simon said the Nicaraguan project, orchestrated by local nonprofit Paso Pacifico, was modeled on existing areas of nearby rainforest. The project included multiple species of vegetation and was spread over 12 separate parcels of land as insurance against a catastrophe like a forest fire, which would release all of the sequestered carbon.

"That's just the nature of forestry — it's a long-term business," says Jeff Hayward, verification services manager with the Rainforest Alliance, a New York-based nonprofit. "You have a huge asset that's out there at risk of fire and pest and theft. Managing risk is part of the business."

Hayward explains that tree-planting projects sometimes withhold 20 percent of their credits for the duration of the project in case there's some kind of setback such as fire. Another mechanism, he said, is to insure those credits being sold by a planter to a provider.

The Alliance was hired by Carbonfund to verify that the Nicaragua project would sequester the amount of CO2 stated by Paso Pacifico.

Third-party verifiers check to see if a project lives up to a recognized standard. If so, they certify it with the certification then used as a marketing tool by the provider.

"We looked through all of their numbers," said Hayward, referring to Paso Pacifico.

And Simon believes that while renewable energy and methane capture are good because they don't add to greenhouse gases, they're useless at reducing existing excess carbon dioxide.

"Wind farms don't destroy CO2. They just reduce the need to produce electricity from dirty sources," he says. "Sequestration is the only kind of project out there that takes CO2 that's already been released and does something with it."