CO2 produced by humans is causing the recent increase in temperature is a hypothesis. But it has no evidence to support it. It's not even what one would label as a theory. It could be correct, it might not be correct. I have about the same opinion of it as I do for the hypothesis of "intelligent design". Maybe it's true, maybe it's not. But there isn't evidence. Given the movement's spotty record for yearly temperatures (it's actually gotten cooler since 1998 incidentally, not that you hear much about that little tidbit) it's hard to argue that it's firmly based on "science". Theories are science and they make testable predictions. (see theory vs. hypothesis).
Methane is also a green house gas. It traps heat at over 20X the level of CO2. Moreover, while CO2 in the atmosphere has gone up some since the start of the industrial revolution, the rise in methane has gone up vastly more in the past 50 years in particular.
But at the end of the day, the hypothesis remains: CO2 is a green house gas. Green house gases trap heat. More trapped heat will cause temperatures to rise. Temperatures have risen in the northern hemisphere since 1976 slightly. Therefore, it must be CO2.
Of course, temperatures in the southern hemisphere have not risen and the antarctic ice cap has grown.
But I will grant that the northern hemisphere has definitely increased in temperature. And I'm willing to also grant that humans are probably helping it a long.
Therefore, what should be done?
First, we have to decide whether global warming is a "bad thing". The case really hasn't been made that it really is a "bad thing" (Al Gore's ridiculous arguments that the sea level will suddenly rise by a dozen feet or whatever notwithstanding).
Then, we have to decide what we would do about it. Even if we removed every car off the road, it would make a trivial impact on worldwide CO2. Humans only produce 14% of the CO2 that goes into the air each year. And CO2 only accounts for a couple % of the green house effect. (this data is available on the global warming advocate websites btw, they don't dispute these facts).
The bottom line, there is really very little we can do to affect the situation other than largely eliminating all worldwide use of fossil fuels AND largely moving to a vegetarian diet (methane levels have gone up because the population of animals for meat is now several BILLION -- people don't like talking about this fact).
As a meat eater and someone who likes to use energy, I don't really want to give up my pork chops or my air conditioning in the Summer.
Talk about raising fuel standards or Kyoto accords or what have you are not serious discussions as they would have no impact.
Which is the problem with global warming as a religion. Because its adherents are generally ignorant of the details of it, they've simply been hammered with "the debate is over" mentra, they don't really understand the ramifications of their belief. It's sort of like the person who believes that the old testament of the bible must be followed literally only to get ticked off when we start talking about Leviticus and stoning his wife for being out in public during certain times of the month.
That is, if you believe that CO2 is the cause of global warming and that the CO2 in question is from humans then only deinustrializing most of the planet is going to cure that. Getting rid of SUVs or switching to hydrogen based cars or solar power isn't going to have much impact unless it's done completely, worldwide and even then, the effect is going to be miniscule (especially as long as we have billions of methane producing livestock hanging around).