Terraforming and Atmosphere

[waldronate]

The effects on pressure would depend largely on the change in composition. In Earth's atmosphere, for example, changing all of the CO2 to O2 would have a negligible effect on air pressure. It would have an effect on temperature through reduction in greenhouse warming. How much of an effect is, of course, a subject of much debate between various entrenched groups.
The magnitude of a greenhouse effect depends on the spectrum of the host planet's star as well as atmospheric composition. On Earth, for example, we have a star with a spectral peak right around 500 nm ("green"). Where we are relative to the sun ends up at an equilibrium temperature right around 11 nm (far infrared) so items are absorbing the sun's energy in the visible (and UV and IR) and reradiating it as far IR.
The primary greenhouse gases in the atmosphere are water vapor and methane, both of whic have an absotrption spectrum that's relatively transparent across optical wavelengths but absorbs and re-radiates stringly in the far IR, which is the classical greenhouse effect. CO2 and other gases also perform this behavior, but they are at much lower concentration than the main ones.

I recommend the book World-Building by Stephen Gillett. It discusses many of the basics of atmospheres and world-building in general. It's for science fiction writers but is a good book nonetheless.

[Redrobes]

Would you not get other effects happening too which would be more important. I was under the idea that if the O2 rises then there would be many more fires on the planet which would put up a lot of smoke into the atmosphere and cause it to heat up that way.