1) Human activity causes global cooling, and the natural heating cycle is overpowering it.
2) Human activity causes neither global warming nor global cooling.
3) Human activity causes global warming, and is either adding to the natural heating cycle OR overpowering a natural cooling cycle.
Now, since carbon dioxide is a KNOWN greenhouse gas (Venus, anyone), then I'd say there is a damn good chance that all the carbon that's been buried for 50 million years and is being released into the atmostphere is doing something to the planet.
The fact that the Earth is warmer is something we must deal with, regardless of the cause. The 90% certainty that WE are making is much worse is good enough odds to do something about it.
Remember, Cheney is "Mr. One-Percent Doctrine".
"If there's a 1% chance that Pakistani scientists are helping al-Qaeda build or develop a nuclear weapon, we have to treat it as a certainty in terms of our response. It's not about our analysis ... It's about our response."
http://en.wikipedia.org/wiki/1%25_doctrine So, if there's a 99% chance that Pakistani scientists are NOT helping Al-Qaeda, that is still reason to act.
Now, the recent global scientific report says that there is a 90% chance that global warming is caused by humans. That means there is only a 10% chance that global warming is NOT caused by humans.
According to the 1% Doctrine of Mr. Cheney, since 1% minimum is the threshold, and we have 90% certainty... then don't we have to "treat it as a certainty in terms of our response. It's not about our analysis ... It's about our response."
Game, set, match.