Water vapor is 84x better at absorbing radiation than CO2.
Yes it is; it accounts for more than 90% of the atmosphere's ability to intercept heat. But that isn't the entire story on the comparison between the two. In his book,
Unsettled: What Climate Science Tells Us, What It Doesn't, and Why It Matters (pages 50-53), Steve Koonin debunks the notion that a few ppm of CO2 can be ignored because the influence of water vapor is so great.
Paraphrasing him, the absorption spectra for H2O and CO2 are different. H2O is really efficient in absorbing the parts of the EM spectrum it absorbs; so much so that these portions of the spectrum are almost fully absorbed at low concentrations of H2O; and doubling the amount of H2O has a negligible effect on additional absorption. The metaphor he uses is that painting a window pane with black paint stops nearly all the light from passing through, but doubling the amount of black paint on the glass by adding a second coat of paint really doesn't stop any more light from passing through (because it was effectively all stopped by the first coat of paint). CO2 is equally effective at absorbing the portions of the spectrum which it absorbs; but because it's at such a low concentration in the atmosphere it's nowhere near the concentration at which it can absorb all the radiation from those portions of the spectrum. Metaphorically, it's like drawing a very sparse crosshatch on the window, which absorbs very little light and allows much more light to pass through. But if you make the crosshatch denser, by adding more lines to it, the result is an outsized impact on the amount of light absorbed.
Basically, adding fewer than three molecules of CO2 to 10,000 atmospheric particles (a mere 0.03 percent change), can increase the atmosphere’s heat intercepting ability by about thirty times that amount (1 percent).
--- Edited (05-07-2024) to add
Forgot that I had already posted the extract from Koonin's book.
Find it here.
For more on Koonin and his book, see jmdrake's post
The shameless attack on a climate change denier (actually, he's a skeptic) from the OBAMA administration
--- Edited (05-21-2024) to add
The key absorption bands for water vapor are centered around 0.72 μm and 0.81 μm.
The key absorption bands for CO2 are centered around 15 μm and 4.3 μm
