Negotiators aim to produce a blueprint for a legally binding international treaty that would replace the Kyoto Protocol when it expires in 2012 and govern individual countries' greenhouse gas emissions.

Although the proposals from the world's two biggest greenhouse-gas emitters have boosted the prospects for a deal, they demonstrate something else as well: No one wants to shoulder the blame for failure at Copenhagen, even if it means the final outcome falls short of what many had envisioned a year or two ago. The U.S. pledge to cut its emissions by 2020 and China's offer to lower its carbon dioxide output relative to the size of its economy by the same date are more modest than what their negotiating partners had demanded.

The fact that countries are defining their climate goals in varied ways -- including different baseline years and efficiency targets rather than absolute cuts -- makes it hard to assess their commitments. The United States has pledged cuts that are modest in the first decade but ambitious 15 and 20 years from now, while China has set a target that could amount to a meaningful reduction if the country's growth rate slows somewhat.

Keya Chatterjee, the U.S. director for the World Wildlife Fund climate change program, likened the developments to "a phoenix . . . rising from the ashes." She added that, under a best-case scenario, "It's not a deal that's going to solve the problem of climate change a hundred percent. . . . But it is a deal that's going to create a foundation and an international architecture for resolving this issue over time."

A senior Obama administration official offered a more cautious assessment: "There's a very real chance of getting this done, but hurdles remain."

The biggest remaining obstacle is money, including how much the developed world will give developing nations to cope with the impact of global warming and to acquire technology to curb their emissions. The United States has not said how much it would pay into any global fund, which the Europeans have estimated would require at least $10 billion annually beginning next year.

And on Thursday, Brazilian President Luiz Inácio Lula da Silva said at a meeting of Amazon nations that wealthier countries must "pay the price" for protecting rain forests that are vulnerable to clear-cutting and burning by farmers and ranchers, activities that help fuel global warming.

Connie Hedegaard, the Danish minister for the climate conference, said "the decision on finance" was the most pressing issue developed countries face.

The Obama administration has allocated about $1.2 billion toward international climate programs as part of its proposed fiscal 2010 budget. Sen. John F. Kerry (D-Mass.), chairman of the Foreign Relations Committee, said in an interview that it would take at least twice as much to help seal a deal in Copenhagen.

China's announcement Thursday that it would send Premier Wen Jiabao to the talks and improve its economy's energy efficiency -- by as much as 45 percent by 2020, compared with 2005 levels -- makes it easier for other countries to commit to a treaty, but it remains unclear how the outside world would verify these cuts.

"It's great the Chinese have come forward with a plan, but are they willing to have that part of a binding agreement?" said Stephen Eule, vice president for climate and technology at the U.S. Chamber of Commerce's Energy Institute.

South Korea's climate change ambassador, Chung Rae-kwon, whose country just pledged to cut its emissions 4 percent below 2005 levels by 2020, wrote in an e-mail to The Washington Post that China's proposal was "a great step forward" but added, "The issue now is how this Chinese target can be captured in the agreement to be achieved in Copenhagen."