Nate has all the gory methodological details in his post. I just want to make two points:
1) This bill is, by itself, inadequate, has gotten watered down considerably already, and will undoubtedly be further watered down in the Senate; and, indeed, I'd be shocked if it passed the Senate at all. But the way to think about it, I think, is as a contribution to a conditional chain: if the US government fails to do anything in the reasonably near future to fight global warming, then horrible catastrophe is inevitable; but if the US does pass even a weak bill, then an international agreement becomes more likely; and if that happens, then altering the energy-intensive development of China becomes a possibility; and if that happens, then we might be able to moderate the slew of catastrophic consequences that are gathering for the end of this century.
2) The United States is not really a democracy, not by modern standards. I'm not talking about all the corruption, the lobbying, and the tilting of the playing field toward special interests, though you could surely make a decent case for the non-democraticness of the US on those grounds alone.
What I'm talking about, though, is the US Senate. Wyoming, which has about half a million people, has two senators. And New York, which has about 19,000,000 people, also has two senators. Florida, which might well be drowned in a century or two by rising seas, has 18,000,000 people and two senators. West Virginia, which produces a lot of coal, has less than 2 million people - and two senators. You see where I'm going with this? The United States government, which was revolutionary and awesome back in the 18th Century, should no longer be considered to have a legislature that meets modern standards for representative democracy. I'm not the first to point this out, of course, but it really doesn't get the attention it deserves. I mean, the form of government of the US is obsolete: why isn't this a matter for public discussion? And of course, the skewing of representative democracy tends to pull in favor of rural areas, which tend to both use and produce more in the way of CO2-heavy fossil fuels, and against urban areas, which are more energy-efficient and more supportive of efforts to fight global warming. So, to the litany of insidious aspects of the global warming challenge, add this: the outmoded institutional structure of the United States government.