“It’s sending this message that somehow it’s this get-out-of-jail free card,” said Charlie Kronick, head of the climate and energy campaign for Greenpeace U.K.

That’s especially troubling if the offset itself doesn’t actually prove effective, something even the experts concede can be hard to judge.

If you do choose to use carbon offsets, here are a few things to look for:

• Transparency. Many say offset providers should clearly spell out what projects they are contributing to and offer tangible proof those projects are making a positive impact. Be wary of groups that provide only vague information on how your money will be used or that sell offsets in bundles or “pooled carbon offsets” that don’t clearly state all the projects involved.
• Additionality. By far the most contentious issue about carbon offsets involves whether the contributions are actually prompting an organization to do something that they wouldn’t have otherwise done.
  For example, if a farmer was looking for a way to dispose of methane gas produced by his business but couldn’t afford to build a system for using the gas as a renewable energy source, then contributing carbon offsets to that project would be considered additional. But if the farmer had been planning to build a system for using methane to produce energy anyway and had the funds for it, then accepting carbon offsets wouldn’t appear to add anything additional (except perhaps a bit of padding to everyone’s bottom line).
• Verification. Experts recommend that you look for carbon offsets that have been independently verified by a reputable third party. While there is no clear standard for validating carbon offset projects, many place faith in the Gold Standard, which was developed based on certain Kyoto Protocol criteria.
• Project type. In general, environmentalists favor offsets that work toward developing more widespread renewable energy, such as wind turbines or solar energy, with the goal of eventually replacing more pollution-heavy energy producers.
• Approach. The Clean Air–Cool Planet study faulted many offset providers for not providing enough education about global warming. Also, not all offset providers offer tips for how people can reduce their energy use before buying carbon offsets.

Many groups also are skeptical about a popular form of carbon offsets: reforestation. People may like the idea of having a tree planted in their honor, but it can take years for plantings to grow large enough to pay off with real environmental rewards. In the meantime, the trees are vulnerable to disease, fire and other forces that could eradicate them.

Reforestation also does little toward creating more sustainable, renewable energy sources —which many experts believe will be key to any solution.

Bill Stanley, director of the global climate change initiative at the Nature Conservancy, notes that small changes, such as adding insulation to your home, using public transportation or buying locally grown products, can reduce carbon emissions and potentially save money.

Still, Stanley said carbon offsets can be an effective tool. He said the Nature Conservancy plans to add an offset option to the portion of its Web site that lets people calculate how much carbon they are responsible for, although the group is still working to design one that meets its integrity standards.

Stanley also said he wouldn’t necessarily criticize someone for just buying carbon offsets, since a high-quality purchase could ultimately help spur more renewable energy projects.

“It’s a step in the right direction," Stanley said.

MORE FROM GREEN

The gift of knowledge: 6 green reads

The gift of knowledge: 6 green reads Obamas to green the White House? Earth from Above Make small changes for a big eco-impact Tap local resources, not bottled water How to save water — and $170 a year TODAY heads back to Ends of the Earth Beaujolais Nouveau goes ‘green’ ‘Read green’ and tame the paper tiger Green wheels are the new hot wheels   Add Green headlines to your news reader: