*Almost* no extra effect. Thing is, in the grand scheme of things nobody's claiming it *is* a big effect in a physics sense. The average temperature of the earth is currently roughly 300 Kelvin; we're talking about an increase of 2-8K, depending on scenario - 0.6-2.6%. The thing is, even though that's a small change on the scale of absolute physics, in terms of its effect on the climate it's pretty huge.
