Ice cores show that CO₂ increases follow rather than precede the warming at the end of ice ages. What should we make of this?

The contribution of CO2 to the glacial-interglacial coolings and warmings amounts to about one-third of the full amplitude, about one-half if you include methane and nitrous oxide.

So one should not claim that greenhouse gases are the major cause of the ice ages. No credible scientist has argued that position (even though Al Gore implied as much in his movie). The fundamental driver has long been thought, and continues to be thought, to be the distribution of sunshine over the Earth's surface as it is modified by orbital variations. This hypothesis was proposed by James Croll in the 19th century, mathematically refined by Milankovitch in the 1940s, and continues to pass numerous critical tests even today.

The greenhouse gases are best regarded as a biogeochemical feedback, initiated by the orbital variations, but then feeding back to amplify the warming once it is already underway. By the way, the lag of CO2 of about 1000 years corresponds rather closely to the expected time it takes to flush excess respiration-derived CO2 out of the deep ocean via natural ocean currents. So the lag is quite close to what would be expected, if CO2 were acting as a feedback.