In earth's past, CO2 rise lagged temperature, so CO2 can't possibly cause global warming? This is like saying "'I saw a chicken lay an egg. That proves chickens don't come from eggs.'"
— Dr. Richard Alley
Notes: Variation: Chickens do not lay eggs, because they have been observed to hatch from them."
Skeptics often point to the historic lag between temperature and CO2 as "proof" that CO2 doesn't cause warming. This relies on the logical fallacy that causality can only occur in one direction, not two.
Ice core data show CO2 and temperature varied closely together for at least 800,000 years. The earth begins to come out of an ice age when small changes in its orbit trigger a bit of warming, not enough for a major shift. However, warming oceans start releasing more CO2 into the air, which in turn greatly amplifies the initial rise.
This "vicious cycle" (positive feedback) can start with either a rise in CO2 or with a temperature rise, with similar results, because CO2 causes warming and warming causes CO2 to rise.
For more on the CO2-temperature lag, see Skeptical Science, "CO2 lags temperature - what does it mean?"
Bite Source: Richard Alley. Variation, Tim Lambert, at Skeptical Science
Image Source: here