The long lag of CO2 is probably not the result of timing errors between ice age and gas age in the ice core, as methane more closely follows the temperature record (for the timing, see also the discussion at UKweatherworld). (realclimate.org)
We calculate the WD gas age - ice age difference (Delta age) using a combination of firn densification modeling, ice - flow modeling, and a data set of d15N - N2, a proxy for past firn column thickness. (usap-dc.org)
If you back - calculate the emissions vs. the increase in the atmosphere over the past 100 + years, the zero emission level is around 300 ppmv, not that far of what can be found in ice cores of 100 year old gas age. (jennifermarohasy.com)