co2, which would otherwise reduce the atmosphere’s reflectivity

Just to be pedantic CO2 absorbs bands in the infrared and reemits it, energy that otherwise could be lost to space. This is part of the reason you can’t do infrared telescopes from earth.

…columbia.edu/…/carbon-dioxide-cause-global-warmi…

Water is an even more powerful greenhouse gas but fortunately the earth is cool enough for it to condense back out of the atmosphere. If temps got high enough that more evaporated than condensed then you’d get a runaway greenhouse effect and we’d be truly fucked.