Yeah, understanding the real reason for the greenhouse effect was tricky for me. CO2 makes the atmosphere opaque to infrared even on the scale of meters, so it's not like a regular greenhouse. If the CO2 already absorbs all the infrared emitted from the ground, why does increasing CO2 decrease the amount of energy reaching space? Because what space sees is the temperature of the last atom to emit infrared, and as you add more CO2, the last atom gets higher and higher on average, and thus colder and colder.

This is more like a "warm, clear blanket" effect than a greenhouse effect. (That is, more like diffusion than reflection).

Though note that neither greenhouses nor warm blankets violate the second law - they just can't get any warmer than the sun, which is pouring in energy at wavelengths for which the atmosphere is mostly transparent. Good ol' sun.