So what are U.N. climate negotiators actually trying to do? A few charts should help lay out the basic shape of the talks.
1) What past U.N. climate treaties have--and haven't--achieved. Under the Kyoto Protocol, which was finalized in 1997, a bunch of countries committed to targets on their greenhouse-gas emissions. Some wealthy nations, like those in Europe, agreed to cut emissions 5 percent below 1990 levels by 2012. Other developing countries, like China and India, were exempt from cuts. Still others, like the United States, never ratified the treaty in the first place.
In the years that followed, here's how emissions shrank and grew, via the Guardian's Duncan Clark:
There were some gains: Russia's emissions plummeted after the Soviet Union dissolved and its heavy industry collapsed. The same goes for much of Eastern Europe. Western Europe, meanwhile, did manage to make some actual emissions cuts. Japan mostly kept its emissions flat. And the United States, which never fell under Kyoto, has recently started reducing its carbon-dioxide emissions after years of growth.
But those cuts were all swamped by the rise of China, India, and other developing nations. Global emissions have exploded since 1990. And, to be fair, this isn't all China's fault. Part of China's emissions growth is due to the fact that it makes a bunch of products for Europe and the United States. One recent study found that E.U. emissions have actually risen 7 percent between 1990 and 2008 if you include imports from abroad.
2) What past treaty failures mean for global warming. Over the years, most nations have agreed that they'd like to limit global warming to below 2°C, although others have said even that's too dangerous. (Global temperatures have already risen about 1°C since pre-industrial times.)
But right now, 2°C is still just an ambitious goal. According to the World Bank, if you add up all the emissions pledges that countries have currently made, ”the world [is] on a trajectory for a global mean warming of well over 3°C.” Here's the relevant chart. The purple line represents our current path:
We don't want to be the purple line. The key question at the talks is how the world could become the orange line or blue line or even the green line instead.
3) What a new climate agreement would have to do. So how do we avoid 3°C or 4°C of global warming? That's the tricky part: Last year, the International Energy Agency issued a report arguing that global emissions would need to start peaking at or around 2017 if the world wants a decent shot at staying below that 2°C limit:
Why 2017? Because, the IEA said, that's when the world reaches the point of "infrastructure lock-in." We'll have built enough coal plants and energy-hogging buildings with the capacity to push the world past its emissions targets. (That doesn't mean it's game over — it's not impossible to shutter a coal plant or retrofit an inefficient building once it gets built, but it's much more difficult.)
So the goal of the U.N. climate talks is to make this last graph work. A successor treaty to the Kyoto Protocol would have to figure out how to get emissions to peak within the next five years or so. It would have to commit China and India to reductions of some sort. And the new treaty would have to divide up responsibility for cuts fairly, so that poorer nations still have room to develop.
That's an enormously complicated task. Which is why the climate talks are proceeding so slowly. Last year, at Durban, U.N. delegates agreed to negotiate a deal by 2015 that would take "legal force" by 2020. But the details aren't even remotely clear at this point.