how exactly does carbon dioxide absorb black body radiation?

Answer 1

Black body radiation consists of all frequencies, from infrared to visible and even ultraviolet light. When a photon of the correct frequency hits a molecule, it is absorbed and the energy is transferred to the molecule. If the photon is the wrong frequency, it just keeps on going and the molecule does not gain any energy. Now CO2 absorbs infrared frequencies, but not visible frequencies. So visible light from the Sun passes through the clear air and hits the ground, which DOES absorb visible light frequencies. This heats up the ground enough that it re-radiates other photons of light, but infrared light, not visible. That infrared light tries to go back up through the air to outer space, effectively cooling the Earth, but if it hits a carbon dioxide molecule before reaching space, it gets absorbed and the heat is retained in the air. By the way, water molecules absorb infrared radiation too, which is why a dry clear night is much cooler than a cloudy, or even clear but humid, night.