Skeptic vs Warmist, page-5

That being said, I've often found myself idly wondering if that much-trumpeted "lag" between temperature and CO2 in ice cores is simply an artefact of the snow-ice consolidation process. CO2 is collected from trapped gas bubbles, while temperature comes from the ice itself (specifically the relative amounts of "normal" 16-O and "heavy" 18-O water). As the interminable discussion of the "firning" process highlighted, consolidation to final "blue" ice takes up to a few thousand years, and as we all know air is much, much lighter than water. Doesn't seem much of a leap to imagine the entrained air, when it's eventually "permanently" trapped in fully compacted ice, being a few hundred years out of step with the water it originally came down with.

No idea if this has been checked or not. Not really that important in the grand scheme of things - in the end, it's the physics of CO2 that make warming inescapable.