The Copenhagen climate summit just keeps getting bigger and bigger. As a journalist who has covered the climate story for twenty years now, including the historic Earth Summit in Rio de Janeiro in 1992 that produced the climate treaty being updated in Copenhagen, I can’t recall a moment more filled with genuine possibility and hope. To be sure, there are a thousand ways things could still go wrong in Copenhagen. But make no mistake: momentum is building, governments are feeling the heat and Copenhagen could bring an historic breakthrough—if the public pressure that got us this far is sustained over the next fourteen days.

Last Friday’s stunning announcement by the White House that President Obama would attend the end of the climate summit, rather than the beginning, is just the latest in a series of developments that have upended the conventional wisdom about Copenhagen. Remember: it had long been assumed that Obama wouldn’t even attend the summit; administration officials were saying as much as recently as a month ago, in a clear attempt to lower expectations. But Obama declared otherwise in a Nov 9 interview with Reuters, saying he would come to Copenhagen if negotiations were making progress.

Obama followed that with a real game-changer, though it’s one that still hasn’t registered with most people: he reached a climate deal with China, in the culmination of backchannel talks that began in July 2008, before he was elected president.

When Obama and Chinese president Hu Jintao jointly announced on November 17 steps that each nation would take to tackle climate change, it marked the first time the two climate superpowers had publicly told the world they would limit their emissions. News accounts and activist comments generally saw this announcement as a glass half-empty, noting that neither side specified how much it would cut emissions by when. A few days later, both China and the US answered this criticism by outlining specific targets. Again, commentators complained they didn’t go far enough. That is true, but it misses the larger point.

To draw a parallel with the nuclear arms race of the Cold War, the Obama-Hu announcement was akin to the first meeting between US President Reagan and the Soviet leader, Mikhail Gorbachev, in 1985. In that meeting, the leaders of the two nuclear superpowers agreed for the first time to put down the pistols they had long had pointed at each other’s foreheads. It took the US and USSR more time and more meetings to agree on the necessary next steps: unloading the pistols and doing away with the bullets inside. The same will be true for today’s climate superpowers, China and the US, but the shift in mood and intent is unmistakable, and it may have encouraged some of the other remarkable announcements of recent weeks, including offers from Brazil, South Africa and India to limit their own emissions, even though as developing nations they are not legally obligated to do so.

But, again as with the nuclear arms talks, there remain serious obstacles to reaching a fair, ambitious and binding agreement on climate change, and it appears that the Obama administration is responsible for one of the biggest. Obama’s climate negotiators have been insisting that the existing international procedures for confronting climate change—codified in the Kyoto Protocol to the treaty signed in Rio in 1992—be scrapped and a new mechanism created. The arguments pro and con on this are long and complicated. The upshot, however, is not: the US proposal would create a system where future emissions reductions would in effect be voluntary—nations would pledge to reduce emissions by such-and-such amount by such-and-such date, but there would be no international enforcement of these pledges.

Such a voluntary scheme would plainly invite disaster, which helps explain why governments from rich and poor nations alike have rejected the US position. This is one of the stories I’m going to be following most closely in Copenhagen. It’s also one where pressure from civil society could do the most good. President Obama has given many signs that he is serious about tackling climate change, a very welcome shift from his predecessor’s malign neglect. But Obama cannot claim to be serious about combatting climate chaos if he meanwhile backs a voluntary approach to emissions reductions.

Nevertheless, the US position might well shift over the coming days, especially if more pressure is brought to bear. After all, the positions of many governments have changed substantially in recent weeks as the world’s attention focuses on Copenhagen and momentum builds. The UN has christened this summit “Hopenhagen,” a nice touch. Just remember: hope is a verb.