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Kenneth P. Green, a climate specialist at the conservative American Enterprise Institute for Public Policy Research, said keeping track of which projects would be eligible for inclusion is another flaw in the plan.

“Who is responsible if there’s a fire that burns down a [green] project? Will those just be wasted offsets?” he asked.

Mr. Green and others say the bill’s offset provisions are too vague and leave unanswered too many questions about which projects will qualify for the offsets and how many offsets would be offered for a given project.

“The key with offsets is ensuring that they generate ‘credible’ emission reductions,” said Evan Juska, North America senior policy manager for the Climate Group, which advises governments and business how to move to a low-carbon economy.

Mr. Juska said the bill, as written, “leaves much of it to be determined by the administrator after the program is enacted.”

While tree stands are a large absorber of carbon dioxide and other greenhouse gases, they may not be the only projects that qualify for offsets. Companies that erect wind farms, install solar panels, invest in devices that trap the methane gas in landfills, use less fertilizer, or upgrade equipment at their refineries and power plants might also be eligible for offsets.

The bill would only allow 2 billion tons, or about 30 percent, of carbon-dioxide emissions to be offset a year through the so-called “green” actions.

Half of the qualifying projects must be domestic and half must be overseas, but the bill includes the option to award more offsets to international projects if not enough domestic projects are available.

The CBO projects that the thousands of firms subject to the cap-and-trade program would utilize 230 million tons of domestic offsets and 190 million tons of international offsets in 2012, the year the legislation is proposed to take effect, instead of reducing their emissions levels.