Taking the height of the Ice Wall to be finite and no other containment mechanisms in place, the atmosphere should leak out at some rate due to diffusion. Using geologic evidence, the composition and density of the atmosphere can be traced back several hundred million years. I am not familiar with the available evidence, but I'm fairly certain it has not changed much. Using gas diffusion laws, it should be possible to estimate how high the Ice Wall would be to be consistent with this evidence. Perhaps some of our members can collaborate and we can evaluate this bound.