Centrist Republican John W. Warner joined independent Joseph I. Lieberman on Thursday in unveiling legislation designed to curb greenhouse gas emissions, setting the stage for a Senate debate on climate change this fall.

The senators, bidding to write a consensus bill that can bridge partisan divides, drew from four already introduced bills that would cap greenhouse gas emissions. The bill would limit emissions of carbon dioxide and allow polluters to trade emissions allowances.

The proposal, which the two plan to introduce in September, is expected to emerge as the main alternative to a bill (S 1766) that Energy and Natural Resources Chairman Jeff Bingaman, D-N.M., introduced last month.

Bingaman’s bill also would cap emissions and create a mechanism for trading pollution credits, but environmentalists have criticized a provision that would create a “safety valve” if the price of emissions allowances rose too high. Bingaman’s approach was designed to draw support from industry and organized labor.

“It’s between Bingaman and Lie­berman-Warner,” said Brooks Yeager, vice president of the nonpartisan Climate Policy Center.

Warner, of Virginia, and Lieberman, of Connecticut, have been charged by Barbara Boxer, D-Calif., chairwoman of the Environment and Public Works Committee, with writing the committee’s bill. Boxer called their plan a “huge breakthrough in the fight against global warming.”

Majority Leader Harry Reid, D-Nev., wants to bring a climate-change bill to the Senate floor “as quickly as possible when we come back this fall,” said spokesman Jim Manley.

In many ways, the Lieberman-Warner plan is similar to Bingaman’s. In addition to capping smokestack emissions of carbon dioxide, which scientists cite as a main contributor to global warming, and creating an emissions-trading market, both bills would fund technology partnerships designed to help developing countries reduce emissions. Under both bills, Congress and the president could assess progress in the United States and adjust emissions targets in light of efforts by major trading partners. Critics of U.S. efforts to reduce greenhouse gas emissions say the effort is wasted if developing economies such as China and India pollute at higher rates.

The bills’ biggest differences center on the targets for reducing emissions and the working of the cap-and-trade mechanism.

The Bingaman bill would require ­greenhouse gas emissions to be cut to 2006 levels by 2020 and to 1990 levels by 2030, with a 60 percent reduction in current levels by 2050, contingent on science, technology development and international effort.

The Lieberman-Warner proposal would reduce emissions to 10 percent below current levels by 2020 and to 70 percent below current levels by 2050.

Bingaman said it is difficult to project emissions reductions that will be achievable more than four decades from now. “What people want to accomplish by 2050 is not really the right measurement,” he said.

Along with a less ambitious reduction target, Bingaman’s bill would set a $12-per-ton limit on how high the price of emissions credits can rise. The goal is to prevent market volatility and limit compliance costs for utilities and other industrial polluters.

The proposal also would offer protection for business from escalating prices of emissions credits. But instead of a price trigger, their bill would create a Federal Reserve-style Carbon Market Efficiency Board with the authority to implement cost-relief measures if prices exceeded expectations.

The board could adjust the short-term cost of the program by distributing extra emissions permits to businesses while ensuring that the cap on pollution remained the same over the long term. It also could expand the ability of individual companies to borrow emissions allowances and repay them in the future.

Environmental groups that criticized the Bingaman bill as too weak said they were happier with the Lieberman-Warner plan.

“This is a pretty good starting point,” said Daniel Lashof, of the advocacy group National Resources Defense Council. “It has stronger targets than Bingaman-Specter, and it moves the debate in the right direction –– though we’d still like to see some tweaks and adjustments.”

Industry and labor groups that supported Bingaman’s plan also expressed interest in the Lieberman-Warner approach.

“There is a cost-containment mechanism — in Bingaman it was a safety valve, here it’s something else,” said American Electric Power spokeswoman Melissa McHenry. “There are some positive things we see. It seems well thought-out, but we haven’t taken a formal position yet.”