On 18 December, the last day of the Copenhagen climate meeting, what will President Barack Obama tell the world that the United States is prepared to do?
With a month to go, that's the challenge. We already know Copenhagen won't  have legally binding agreements, but how the nations of the world use the meeting to tee up negotiations in 2010 will determine whether the event will be deemed a barely marginal success or a total repudiation of the U.N. approach.
Today Obama and Chinese President Hu Jintao raised the stakes in a Joint Agreement  they signed. And they put a big onus on the U.S. Senate to come up with numbers for its emissions cuts goals. Hu and Obama said in their statement that "an agreed outcome in Copenhagen" should:
include emission reduction targets of developed countries and nationally appropriate mitigation actions of developing countries.
The word "targets" on 18 December would mean actual commitments to emissions cuts. Getting them would be a pretty good accomplishment for the meeting, especially with organizers lowering expectations by the day. The hope was that the U.S. Senate would have passed its version of the climate bill by then, so Obama's negotiators could have real legislated targets to bring to the table in December.
That's not going to happen. The next best thing is Obama's challenge: coming up with a number from Congress with some credibility, with a month to go. Here's how it might go down.
The National Resources Defense Council's International Climate Policy Director Jake Schmidt lays it out. The key trifecta  of Senators John Kerry (D–MA), Lindsey Graham (R–SC), and Joe Lieberman (I–CT) try to get as many senators as possible to back a piece of paper—not necessarily a proposed bill—in which they commit to a range of emissions cuts, something like "10% to 20% cut by 2030." That effort attracts a range of moderate Republican senators and doesn't alienate key liberal Democrats. Depending on what they get, Obama can point to the House of Representatives bill—which is a cut of 17% below 2005 levels by 2020—with the Senate paper as a picture of the U.S. commitment.
Not pretty—and far from ideal—but it might convince the European Union, Japan, Australia, and Canada that the United States is serious, and they'll come on board with their numbers. And China, India, Indonesia, and Brazil, the main developing nations, could come forward with aggressive promises on cutting emissions.
But it's a very tough slog. The liberals in the Senate won't back a paper whose numbers are too low. But the moderates won't sign a paper whose numbers are too high. They won't like nuclear power, which is sure to be the price  that players like Senators Lamar Alexander (R–TN) and Jim Webb (D–VA) exact for their support.
And if Obama can't round up the people? Negotiators in Copenhagen go home with little more than a pledge to keep negotiating—maybe a deadline they agree to in 2010. That, in turn, could make the effort more difficult that Obama will have to mount  in the early spring to get U.S. lawmakers on board for an agreement.
Could China's actions help rouse support here? Schmidt argued today  that negotiators in Beijing made solid gains today toward setting up the next best thing: a solid platform on which to build the real deal later next year. "Subtle but Important Shifts in Global Warming Positions" is how he described the steps in statements the two countries signed and released this afternoon. Among those shifts was a pledge to "stand behind" its commitments—Kyoto-speak for allowing open analysis of its emissions performance. It's a new step for China.
Scientific American, which sees the agreement as potentially "more important than Copenhagen," nicely lays out the energy-related agreements the two superpowers struck :
• the opening of a joint clean energy research center  (pdf) with $75 million in funding from China and the U.S. over the next 5 years. Goals? Energy efficiency, "clean coal" and clean vehicles, among others;
• Electric vehicle demonstration projects  (pdf) and the development of joint standards for the new technology;
• Joint building efficiency standards , including inspector and auditor training;
• Renewable energy development  roadmaps for both countries, including grid modernization;
• U.S. assessment of Chinese shale gas  (pdf) potential as well as help with development of this lower carbon fuel;
• 22 U.S. companies to help develop clean energy projects in China , including alternative energy, a "smart" grid, and greater energy efficiency, among others;
• corporate and government cooperation on "21st Century Coal," such as developing carbon capture and storage at the so-called GreenGen plant , and gasification of coal  to help remove pollution before combustion, among other efforts.
Not the thinnest of gruel, but far from comprehensive promises on its emissions that would have real force on Capitol Hill. Add yet another weight onto Barack Obama's shoulders.