Bad Astronomer Phil Plait writes at Slate:
First, it’s true that in the distant past (hundreds of thousands of years ago) a rise in carbon dioxide sometimes did follow a rise in temperature. As Skeptical Science points out, that’s to be expected: If the temperature goes up (which can have a number of initial causes), a lot of CO2 locked up in the oceans gets released. However, this does not mean carbon dioxide doesn’t cause warming; in fact we know an increase in CO2 causes an increase in temperature. That in turn increases the amount of CO2 released from the oceans, further increasing temperature. This is called a positive feedback loop. Happily, in general, positive feedback loops like this tend to flatten out, preventing the heat from cranking up past the point where temperatures become unstable.