I just came across the most hopeful article I've read on climate change for ages.
The one paragraph summary is that there is already a limited cap and trade program involving ten states on the east coast of the U.S. Another seven states in the western U.S. are negotiating to set up their own separate cap and trade program by 2015 (along with four Canadian provinces). At that stage, American industry groups might start to lobby for uniform national regulations, rather than having to deal with a variety of carbon emissions regulations across the country. That has occurred in the past with other pollution measures.
That suggests to me a plausible route to a global agreement on climate change:
1. The more environmentally conscious American states restrict emissions of greenhouse gases.
2. The U.S. congress imposes national regulations on greenhouse gas emissions, at the behest of industry lobby groups that want uniform environmental laws across the country.
3. The U.S. then insists that other countries control greenhouse gases, and threatens trade sanctions if they don't comply. Major greenhouse gas emitters bring in these controls, because they don't want to risk losing the American market.
Why I could see this happening is that it doesn't rely on heroic, selfless gestures. Step 1 is already in progress, step 2 assumes an American government dominated by selfish lobbyists and step 3 assumes that the U.S. is a domineering superpower. Surely even the most jaded left-wing pessimist would concede those two points.
Of course this process might lead to quite weak global regulations, that don't emerge until some time in the 2020s. Hopefully it will be enough to spur big improvements in renewable energy and efficiency that cut greenhouse gas emissions in time. It's a bit of a long shot, but I'm revising my outlook from "Climate change will almost certainly end our civilisation in the latter 21st century" to "Climate change will probably end our civilisation in the latter 21st century."