From experiments on marathon speed, CO2 production:
1 from each tile of rail laid.
1 for the creation of a fuel using unit, doesn't seem to be a difference between coal and oil.
1 each turn for the maintenance of a fuel using unit.
So no matter what your power production is, even having a small military is going to jack you into the game's AGW spiral whether you like it or not.
Anyone have any idea if having a lumber mill on a forest tile hurts it as a CO2 sink? I don't mind covering the world in forests, but it'd be nice to get something out of them. Theoretically, enough forests and it should start reducing CO2.