The simplest way to explain the caveat is to compare the impact of CO2 on global warming to the impact of a beef vindaloo on the tongue.
The first bite of a vindaloo blows your head off. Similarly, if an atmosphere has no CO2 at all and you add 20 ppmv (parts per million volume), then the temperature will go up 1.5 degrees. However, like a vindaloo, each additional mouthful of steaming hot curry does not blow your head off to the same degree. After a few mouthfuls of vindaloo, you reach a tongue/curry saturation point, where the addition of more vindaloo fails to strip any more cells from the surface of your tongue. Similarly, an additional plateful will not double your misery the next morning when sitting on the toilet.
Going from 20ppmv to 40ppmv of CO2 adds about 0.3 degrees C to the temperature. Going from 40 to 60 adds 0.2 degrees and so on so that by the time you get to where we are today, adding another 20 ppmv adds about 0.02 degrees to the temperature. CO2, like vindaloo, has all its impact in the first few bites. After that, the marginal impact of more curry of CO2 is minimal.
That's a theoretical explanation anyone should be able to understand. So yes, it is a greenhouse gas, but not to the extent that people might have us believe.
And if you still think CO2 is the main driver, ponder this:
CO2 is more evenly distributed than water, so if CO2 caused warming it would have a proportionately greater effect in areas where there is little water vapor (such as deserts and in very cold regions), while in areas with a lot of water, the effect of CO2 may be insignificant (in terms of its effect on local temperature) compared to the effect of water vapor. This is one of many factors that mitigate against the idea of a "climate catastrophe."
CO2 levels have only increased by 23.7% since 1900.According to the US Department of Energy, only about 14.8% of this increase, or 11.88 ppm, is man-made. The remaining 68.5 ppm is caused by natural forces, such as volcanoes and forest fires. From this, researchers have estimated that, when water vapor is taken into account, anthropogenic CO2 contributions cause about 0.117% of the Earth's total greenhouse effect.