So what accounts for Cancún’s success?  I can see a number of factors that thankfully conspired to produce the most tangible progress in the U.N. climate talks in years.

The first, without doubt, is the savvy and skill of the Mexican diplomatic corps. The Mexicans have been widely praised for doing their utmost to keep the negotiations inclusive and above-board. Less noted, but equally important, was the firm hand they maintained in the crucial closing hours. Taking the very practical view that consensus does not mean strict unanimity, they refused to allow a vocal minority to impede the will of the vast majority. In short, they ensured that everyone had their say, even if all didn’t get their way.

A second big change from Copenhagen was the apparent non-aggression pact between the United States and China. Up until a few weeks ago, they were openly sniping, each working hard to make sure that in the event of a failure in Cancún the other would take the blame. Here, they seemed to appreciate that the best way to avoid blame was to avoid failure. They refrained from the open tit-for-tat we saw in Copenhagen.  And everyone else was a bit more comfortable seeing that the two big elephants in the room were playing nicely with each other.

The third big reason for Cancún’s success was the palpable sense among parties that the process itself was at stake.  It’s produced little to speak of in years and suffered a severe shock in Copenhagen. Another “failure” would have been crippling, if not fatal, to the whole enterprise. So countries that before insisted on binding-or-nothing were willing to declare a package of incremental steps a major success – if for no other reason than to keep intact the process they desperately hope will deliver much more down the road.

Finally, a greater sense of realism seems finally to have taken hold. For years, climate negotiators have seemed engaged in a collective illusion – that the binding deal was just around the corner. That Copenhagen would produce a binding treaty was a fantasy that some parties perpetrated and others fell sway to.  The political reality is that if it’s even possible, a binding deal is still years off. Few if any parties would openly acknowledge that.  But the Cancún agreement suggests they’re beginning to understand it, and are prepared to consider a more evolutionary approach.

Just as Copenhagen’s “failure” was way overblown (a victim of false expectations), it’s important not to overstate Cancún’s success.  Essentially, parties have imported the essential elements of the Copenhagen Accord into the U.N. process – as well as the Accord’s mitigation pledges – and taken initial steps to implement them.  To do that, they had to put aside differences over when and how to move toward a binding deal (keeping the Kyoto Protocol on life support in the meantime). We stand a better chance against climate change if, eventually, countries assume binding commitments. The Cancún agreement doesn’t create a pathway there – or even declare it a clear objective.

But if one accepts, as I do, that the only way there is step by step, then Cancún is an unmitigated success, one that’s long overdue.