Lubos Motl discusses the lag between temperature rises and CO2 levels in the atmosphere
As we have explained in 2006, Vostok ice core records show that the carbon dioxide concentration averaged over a few centuries has been correlated with temperature at least for half a million of years. However, we know for sure that the temperature was the cause and the CO2 concentration was its consequence, not the other way around. It follows that the greenhouse effect hasn't been important in the last half a million of years.
There are many ways to see it. The 800-year lag is the most popular one, it has been featured in the Global Warming Swindle, and we will discuss it below. However, there are other ways to see that the influence of temperature on the concentration of gases has been more important than any influence in the opposite direction. For example, the ice core records show that the concentration of methane was correlated with temperature, too. If the CO2 concentration were the primary cause, we would have no explanation why the CH4 concentration was also correlated. In fact, CO2 and CH4 play the very same role in the ice core records. If some combination of them determined the temperature, we would still have no explanation why these two concentrations were correlated with one another.
Moreover, easy reasoning can be used to show that the ability of oceans to store gases decreases with increasing temperature and this effect is clearly much stronger than the greenhouse effect.
The 800-year lag
However, the most popular - and the most straightforward - explanation of the direction of the causal relationship is the fact that in all cases, the CO2 concentration only changed its trend roughly 800 years after temperature had done the same thing. There have been many papers that showed this fact and incidentally, no one seems to disagree with it.
Every sane person knows that this detailed insight implies that the greenhouse effect couldn't have been among the most important effects.