CO2 concentrations from past years can be accurately determined from the bubbles trapped in the ice. The time resolution is significantly better at the top of an ice core than near the bottom, where some ice flow and compression of the layers occurs. Measuring the pCO2 of a trapped air sample in a bubble is essentially the same procedure as measuring the CO2 content the atmosphere, so to merge the ice core and present-day atmospheric CO2 measurements is a very straightforward procedure. I.e., you just plot the time-series on the same graph.