The effect of CO2 has been likened to layers of tint put over a window. The first layer blocks (or absorbs in the case of CO2) a lot of light. The next layer less, the next layer still less and so on. The overall effect of increasing amounts of CO2 isn't a runaway effect, it is a declining effect. Once it gets to a certain level the effect levels off.

Despite all the fairly wide variations in temperatures over the millenia, there's still no runaway trend. Earth won't ever turn into another Venus.

As noted, there are a lot of factors still not accounted for in the computer models that the "experts" are basing their theories on.