I believe that I just indicated that H₂O is not well-mixed, and that CO₂ is. Why the echo?

That last bit really reveals your lack of even the most basic understanding of atmospheric physics. It is not heat capacity (which both gases have) that is of importance. CO₂ and water vapor are both examples of a handful of trace "greenhouse" gases. These gases consist of polyatomic molecules that are infrared active. They selectively absorb certain wavelengths of IR emitted by the solar-warmed Earth, and then reemit that radiation in all directions. The radiation directed back toward the surface adds to the energy that must be moved to space, resulting in warming. Yes, Virginia, CO₂ is a powerful (20% of the GHE) GHG. Why are you so confident in your assertions while lacking even this most basic bit of knowledge?

What has caused modern warming?