When various factors are taken into account, there appears to be a fairly consistent 800 year lag between the end of an ice age and a rise in atmospheric CO2 levels on earth as indicated by gases trapped inside ice cores.

Climate change deniers claim this clearly means CO2 cannot be linked to global warming, because CO2 levels did not rise in lock-step with the warming of earth after the ice age. This view is mistaken.

Did a CO2 increase cause the end of the ice ages via the greenhouse effect? No, probably not. Some other event caused the end of each ice age, and the resulting temperature increase appears to lead to an eventual increase of atmospheric CO2, possibly released from the ocean, which may then contribute to further warming.

The data is somewhat unclear, and there is controversy about the timing and the exact interplay of ice ages ending and CO2 levels rising. However CO2 levels not being the proximal cause behind ice ages ending, does not mean CO2 cannot cause the greenhouse effect. It simply means it is probably not the initial factor in the case of ice ages.

New Scientist has a particularly lucid explanation of this issue (much of the information out there is murky), and Wikipedia has detailed information about Ice Ages.

A quick note, something I will expand upon later. The 800-year (+/- 200-years) lag and differs from the recent anthropogenic CO2 increase. The 800-year lag was noted in Antarctic deglaciation, however, CO2 preceded temperature rise in Northern Hemisphere deglaciaion. The 800-year lag would seem to be specific to Antarctic deglaciation and may not be generalizable. In addition, there was no human activity contributing to deglaciation until the 18th century.1

1 Timing of Atmospheric CO2 and Antarctic Temperature Changes Across Termination III

Nicolas Caillon, Jeffrey P. Severinghaus, Jean Jouzel, Jean-Marc Barnola, Jiancheng Kang, Volodya Y. Lipenkov

Science 14 March 2003:Vol. 299. no. 5613, pp. 1728 - 1731 DOI: 10.1126/science.1078758