Observations of secular trends in the E and F1 regions of the ionosphere indicate that electron densities have increased, and that the height of the E-region peak has decreased, during the past several decades. Detection of trends in the upper ionosphere through analysis of F2-layer parameters has been more complex and controversial. In order to facilitate observational detection of long-term trends in the ionosphere, simulations were performed using a single-column upper atmosphere model. CO2 concentrations for the year 2000 and projected for the year 2100 were used to investigate changes of electron densities and the altitudes of ionospheric layers. Results show that increased CO2 concentration increases electron density in the lower regions of the ionosphere, but decreases electron density in the upper ionosphere. The transition altitude occurs slightly below the F2 peak altitude (hmF2). The proximity of hmF2 to the transition altitude may explain why different analyses of long-term trends in F2 peak density have shown both positive and negative trends. The altitudes of the E, F1 and F2 regions all decrease with increased CO2 concentration.