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updated 4/21/2009 12:53:08 PM ET 2009-04-21T16:53:08

What's an offset?
More than 20 organizations have online calculators that estimate your personal share of the heat-trapping gases tied to global warming. Once they give you a ballpark figure, the organizations tell you how much you'd need to donate to eco-friendly projects to prevent or stop companies from emitting a comparable amount.

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How harmful is air travel?
No one knows for sure. Aviation may account for 2 percent of greenhouse-gas emissions. But most climate scientists believe that air travel has added warming effects because of jet contrails and nitrogen oxide emissions. And when considered by energy use per mile covered, planes eat up far more energy than buses and trains.

Are offsets affordable?
For the typical domestic trip, an offset costs less than $20 per person. For example, to offset a half-ton of carbon dioxide, which is approximately what would be produced by a round-trip flight between Chicago and Miami, you could buy an offset for $6 from TerraPass.

Do some organizations charge more than others to offset the same amount of greenhouse gases?
Yes, prices to offset one ton of emissions vary from $3 to $33, according to a survey of 24 organizations. The reason: Some eco-friendly projects cost more than others. For instance, a landfill project that captures and destroys methane, an especially potent greenhouse gas, may be cheaper than one that replaces soot-producing stoves in a developing nation.

How much does an offset accomplish?
Scientists can only estimate the emissions that will be prevented if an energy source like a wind farm is used as an alternative to fossil fuels. So if an offset provider says that a project will make a specific number of pounds of greenhouse gases vanish, be skeptical. No one knows the benefit of an eco-project that precisely. If the uncertainty bothers you, you might feel more confident investing instead in home improvement, such as installing energy-efficient windows in your house.

Who keeps these organizations honest?
There's no governmental oversight of carbon offsets. But roughly a dozen organizations make evaluations. The best watchdog groups verify that an offset exists for no other reason than to reduce carbon dioxide levels, that it's verifiable and measurable, and that the benefit to the Earth is long lasting. As one step, many organizations keep registries to ensure that offsets are not sold multiple times, says Anja Kollmuss, a staff scientist with the Stockholm Environment Institute and author of several carbon-offset studies.

Which watchdog groups are the most trustworthy?
Many are well regarded, but we'll single out two for special mention. The Gold Standard is backed by 60 nongovernmental organizations and was started in part by the World Wildlife Fund. It only approves energy-efficient projects, such as distributing power-saving appliances in developing nations, and renewable energy projects, such as building water power plants. It does not give its seal of approval to tree-planting projects out of concerns that reforestation may not directly reduce carbon dioxide concentrations. Offset providers Atmosfair, ClimateCare, and myclimate are among the companies with Gold Standard projects.

Another helpful resource is the Carbon Offset List. Unlike the Gold Standard, this is a Web site created by a single organization, the Environmental Defense Fund (EDF), and vetted with the help of independent experts. The Carbon Offset List highlights a dozen projects it deems trustworthy, from providers like Carbonfund.org, e-BlueHorizons, and TerraPass.

Are offsets a way to justify a wasteful lifestyle?
Like most things, an offset is what you make of it. An offset may serve as a last resort for reducing your carbon footprint after you've already made your trips as environmentally friendly as possible by taking direct flights, packing lighter, and renting a fuel-efficient car.

How can I buy a reliable offset?
Though offset companies may not let you put money toward a specific project, you can usually pick the type of program to subsidize, such as renewable energy creation, says Tom Murray, the managing director of corporate partnerships for the EDF. Some offset sellers, like myclimate, allow you to donate to only to projects certified by the Gold Standard.

Copyright © 2012 Newsweek Budget Travel, Inc.

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