Absorption by greenhouse gases

What does the absorption of IR radiant energy do to a greenhouse gas molecule?

It increases the internal energy

What happens then when the greenhouse gas molecule returns?

When the molecule returns to a lower energy state it can re-emit the radiant energy it has absorbed in all directions

Is a greenhouse gas efficient in absorbing and re-emiting?

Yes efficient when absorbing and re-emitting infrared radiation

Why is it efficient?

primarily due to their molecular structure (specifically their vibrational movements) and the changes in their dipole moments during vibration

Emission from the atmosphere: What happens when greenhouse gasses are considered?

less IR energy is emitted directly from the planet’s surface to space (only 10% versus 90% that is emitted from the atmosphere)

Which radiative equilibrium temperature is higher (with or without atmosphere)?

Earth’s surface must be higher for an Earth with atmosphere than for an Earth without atmosphere, so as to make up for the ‘small’ emission from the atmosphere to space and match the incoming solar energy flux