"You have been negotiating all my life," 21-year-old Mima Haider of Lebanon told delegates at the United Nations climate negotiations in Cancún. "You cannot tell me you need more time." But that’s pretty much what they did tell her, and the rest of us. True, some important agreements were reached in Cancún. Rich countries reaffirmed their legal obligation to help poor countries fight climate change, and even promised sizable sums toward that end. The Cancún Agreements oblige rich countries to contribute $30 billion in new aid over the next three years—growing to $100 billion a year by 2020—to a Green Climate Fund. This fund will help developing countries both to reduce greenhouse gas emissions and install protections against the floods, droughts and other climate impacts that disproportionately punish the global poor.

But on the key questions determining whether children in rich and poor countries alike inherit a livable climate—how much will global emissions be reduced, and by when?—negotiators kicked the ball down the road. The Cancún text did "recognize that deep cuts in greenhouse gas emissions are required…to hold the increase in global average temperature below 2 degrees Celsius above the pre-industrial level." And it is significant that, for the first time, developed countries will not be the only ones who must make cuts. In a major concession to Washington, developing countries—including China, India and other rising powers—will henceforth also be required to reduce emissions, if only to below business-as-usual trajectories. But these are general statements of intent. Decisions on implementation—how steep the cuts will be, how this burden will be shared between developed and developing countries, and how all this will be enforced—were explicitly put off until the next round of negotiations, scheduled for December 2011 in Durban, South Africa.

No wonder many media outlets chose the word "modest" to describe the Cancún deal. Still, it could have been much worse. Going into the talks, expectations were very low; outright failure seemed a real possibility. The fact that, instead, compromises were reached on a range of second-tier issues revived many participants’ faith in the UN process. After the catastrophe in Copenhagen last December, when a majority of mainly developing countries angrily rejected a deal the United States, China and other big emitters had hammered out in secret, the UN’s role was in question. Some in rich countries complained that its multilateral, consensus-seeking approach was too unwieldy to make progress. Many in developing countries countered that only the UN process enabled democratic decision-making. In Cancún, thanks to the Mexican hosts’ diplomatic skills, negotiations went more smoothly and yielded real results, giving the UN process a new lease on life.

Those who see the Cancún glass as half full hope the trust and momentum achieved there will make it easier to tackle the knotty issues that await in Durban. As always, the dance of the two climate superpowers, the United States and China, will be crucial. After an acrimonious stand-off in Copenhagen, both sides’ negotiators showed surprising flexibility in Cancún. China accepted that it too had to limit emissions and even accepted a degree of outside monitoring. In a compromise proposed by India, projects to reduce emissions in developing countries that are financed by international sources will be internationally verified, while domestic projects will only be domestically verified. For its part, the United States not only accepted the latter stipulation, it agreed that rich countries must cut emissions more and sooner than developing nations, even as they supply $100 billion a year to the Green Climate Fund.

How rich countries envision finding that $100 billion is suggested by a little-noticed provision concerning deforestation. Many participants have cited the Cancún Agreements concerning REDD (Reductions in Emissions from Deforestation and Forest Degradation) as another cause for optimism. After all, deforestation causes roughly as many emissions globally as transportation does, and the Agreements pledge to give developing countries financial incentives to leave forests standing. Where will the money for the developing countries come from? The Cancún text specifically authorizes the use of "market-based mechanisms"—which is to say, cap-and-trade. Under cap-and-trade, polluters in rich countries would be credited for reducing emissions not because they burned less fossil fuel at home but because they paid to keep forests intact in developing countries. Given how unpopular cap-and-trade is on both the right and the left (and not only in the United States), more fights on this front seem certain before a final deal is reached in Durban.

Arguably the most important question left dangling after Cancún is the future of the Kyoto Protocol. The advantage of Kyoto from a scientific perspective is that it imposes mandatory rather than voluntary emissions reductions, at least on rich nations; developing nations are exempted on the grounds that overcoming poverty must be their first priority. Of course, the mandatory nature of Kyoto is precisely why the United States—alone among rich industrial countries—has refused to ratify it. In Cancún, other rich nations signaled that they’ve had enough. First Japan and then Russia and Canada announced they would abandon the protocol if other big emitters—read, the United States and China—remained outside its purview. The Cancún Agreements, however, may have opened a door to resolving this dispute, for they oblige all nations to reduce future emissions. The challenge between now and next December in Durban is to translate that general principle into specific, proportional, binding targets for rich and poor countries alike and, much harder, generate the political pressure to compel national leaders to accept those targets.

A stiff challenge? No doubt, not least because the cuts that countries have pledged so far fall well short of limiting future temperature rise to 2 C above pre-industrial levels. Thus, future cuts will either have to be significantly larger or humanity will have to endure even higher temperatures and the stronger impacts they unleash—not a smart move. For years, a 2 C rise has been seen as a relatively safe increase, but that is no longer so. A landmark study the British Royal Society released on the eve of the Cancún conference found that the latest scientific assessments project "a significant increase in the severity of some impacts for a 2 C temperature rise." As a result, 2 C "now represents a threshold, not between acceptable and dangerous climate change, but between dangerous and ‘extremely dangerous’ climate change." In short, our civilization is already locked in to a very perilous future, and governments will have to go well beyond what is "politically realistic" if we are to avoid utter catastrophe. Like the 21-year-old from Lebanon told the Cancún delegates, we shouldn’t have waited so long to get started.