The concentration in the last 400 million years has probably changed between 15 and 30 percent, changing total atmospheric pressure I assume. (realclimate.org)
But Caldeira says he doubts that anyone knows what will happen to total atmospheric pressure in the distant future. (sciencemag.org)
Low atmospheric CO2 levels during the Permo - Carboniferous...... at a time when total atmospheric pressure was similar or slight higher than now. (realclimate.org)