It is not difficult to show that there has been a regular temperature cycle over the last half million years. It is also obvious from the data that CO2 levels track this rise and fall quite closely.

From there, everything else is speculation. I have found during this controversy that for almost every argument in the realm of speculation, a credible scientific paper can be found for either side. This is true even on the RealClimate web site.

I don't know if CO2 lags temperature or not. IMO, over the time frame we're talking, and based on the methods used, the 800 years could be data noise.

Where common sense comes in is that I can think of a lot of theoretical reasons, mostly astronomical but possibly geological, why temperature would have a regular cycle in the 100,000 year range. I can think of no reason at all why CO2 would do this, never mind CH4.

If, as some say, a small temperature spike drives CO2 levels up, and increasing temperatures creates a CO2 feedback that increases temperatures even further (quite plausible), then why the hell would either ever drop back down to the previous level? The link you posted doesn't really address that. At most, when whatever drove the first temperature rise petered out, the temperature should only drop by about 1/6th, if 5/6ths of the rise was driven by the CO2 still sitting in the atmosphere.

It makes more sense to believe that CO2 is driven directly by temperature, which would explain why it drops off again when temperature drops.

I believe the CO2 levels are driven by temperature, and we are riding an upward cycle right now. If I'm right, then the reason for any perceived lag in CO2 levels would be obvious, and not require lengthy explanation.

Where the CO2 comes from when it gets warm, and where it goes when it gets cold, is a whole different subject. Warm weather could release ocean floor methanes, but why would colder weather put them back?

The reason this is important in the current controversy is that a lot of people who have made this into a "cause" (mostly non-scientists) are not factoring out natural variations. The people who want to make this into a political weapon have a lot to gain by emphasizing the "human-caused" (often meaning "Western Capitalist caused") problem.