It would be a slightly darker earth. The net warming from CO2 addition is a few watts/m^2. Insolation is something shy of two orders of magnitude higher. Dimming sunlight by a few percent is not a large change.
There are far larger changes in insolation over the day and over seasons. Life is obviously less sensitive to these flucuations, than life would be to changes in temperature.
One goes from 1000 W/m^2 in noon sun to, what, milliwatts/m^2 from starlight? Many orders of magnitude. Temperature goes from maybe 250K to 320K over the year, maybe, a change of maybe 30%?
What a double standard you have.