For more, see Alex Lenferna's report, COP 17′s Youthful Hope in a Climate of Stale Politics.
The problem is, the US and other historically high emitters are responsible for a large fraction of the extra CO2 that's in the atmosphere now, and will be there for centuries. About 50% of emissions are absorbed quickly by the environment, mainly the oceans, but also an increase in absorption by the land, and enhanced plant growth. The rest is drawn down by much slower processes, and climate change caused by an increase in CO2 could be with us for as much as 1,000 years. That means the US is already responsible for a large fraction of the problem. What's more, the US can act unilaterally by both taking strong moves domestically to cut emissions, and by imposing a carbon tax on imports.
|Fraction of initial increase in greenhouse gases over time|
This picture (from the supplement to a paper published in 2009 on irreversible climate change) illustrates how a large fraction of CO2 added to the atmosphere will still be there in 800 years, even given the uncertainty in the rate at which the environment can absorb emissions (given by the grey area in the graph). This uncertainty reflects our inability to predict the climate that far out into the future with great precision. Nonetheless the fact that we are carrying out an experiment with consequences that will be felt for 1,000 years suggests we should be taking efforts to turn things around seriously.
Back to the effects that a unilateral US action could have; better still if Europe was dragged in.
The US and Europe have off-shored a lot of their pollution, especially to China. The US remains the world's biggest consumer nation, with nearly a third of worldwide consumption, and many of its imports are from countries that would have to adjust their policies if the US imposed a high carbon tax on imports. The US, therefore, is in a strong position to change the world emissions economy by acting unilaterally. As an example of how this kind of leverage works, I was told by someone in the US that Canada in the 1980s introduced a law requiring that new cars have an extended warranty against rust. Canada is a relatively small market and manufacturers could not justify building cars on a separate production line just for Canada, so all cars destined for North American buyers benefited. I can't find any definitive reference for this story, but that kind of leverage can work.
In short, the US was a major cause of the problem we now face and can't fall back on the logic of we can't move until anyone else moves. And as a major consumer nation, they can provide a strong incentive for others to clean up their act.