Democrats Waxman and Markey propose a multi-part effort to reduce greenhouse gases and promote renewable energy.

House Democratic leaders on Tuesday unveiled a sweeping plan to fight climate change and boost renewable energy, including mandates for renewable electricity nationwide and a market-based system for reducing greenhouse gas emissions.

It’s a starting point in what promises to be a long and winding congressional debate over energy and climate policy this year. And it leaves some crucial questions unanswered -- most notably, how to allocate emissions permits and what to do with any revenue from selling those permits.

The plan, posted on the House Energy and Commerce Committee website, is a “discussion draft” sponsored by Rep. Henry A. Waxman (D-Beverly Hills), the committee chairman, and Rep. Edward J. Markey (D-Mass.), who chairs the House Select Committee on Energy Independence and Global Warming. Among the bill’s provisions:

* A nationwide mandate for renewable energy -- such as wind, solar and biomass -- in electric power generation, starting at 6% in 2012 and rising to 25% by 2025.

* A “cap-and-trade” program to restrict greenhouse gas emissions by requiring utilities and other emitters to hold “allowances” for the carbon dioxide they send into the atmosphere. The level of allowances would shrink annually to reduce carbon emissions to 3% below 2005 levels by 2012, to 20% below 2005 levels by 2020 and to 83% below 2005 levels by 2050.

* A mechanism for emitters to buy “offsets” -- a sort of emissions credit that comes from spending money to reduce emissions outside the scope of the cap-and-trade system.

* A national standard, akin to California’s, limiting carbon dioxide emissions from vehicles and a new low-carbon fuel standard to further support biofuels and low-emission alternatives to gasoline.

* Support for carbon capture-and-storage technology, which, if successful, would remove the carbon emissions from coal-fired power plants and store them underground, essentially allowing coal burning, a major carbon-emitter, to maintain its place in American power generation.

* New mandates for energy efficiency in appliances, lighting, vehicles and buildings.

* A variety of measures supporting “green jobs,” including worker training.

* Rebates for manufacturers hit hard by additional energy costs imposed by the bill.

Many environmentalists immediately applauded the plan.

Frances Beinecke, president of the Natural Resources Defense Council, said in a statement that “Chairmen Waxman and Markey are setting the stage for the strong action on energy and climate we need to move America in a new direction.”

David Moulton, climate policy director for the Wilderness Society, said the congressmen “deserve high praise for introducing legislation that could break the congressional stalemate that has prevented this nation from tackling the potentially catastrophic challenge posed by global warming.”

Other environmental groups’ reviews were mixed.

Emily Figdor, the federal global warming program director for Environment America, called the draft a “pragmatic bill” with strong clean-energy standards and emission-reduction goals.

But, she said, “we’re disappointed that the bill includes sky-high levels of carbon offsets, which provide less-certain reductions in emissions, and lavish subsidies, including from ratepayers, for still-unproven carbon capture-and-storage technology.”

Next up is a lengthy congressional debate: Waxman has promised to pass a bill from the full committee by Memorial Day.

The full House is likely to approve whatever emerges. And then, perhaps, the real work begins in the Senate, where Sen. Barbara Boxer (D-Calif.) is writing her own climate bill, mindful of divisions in both parties over many key details.

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jtankersley@tribune.com