Explain the greenhouse effect in terms of the absorption of infrared radiation by greenhouse gas molecules.

What this dot point is asking

You must explain the greenhouse effect at the molecular level: why certain gases absorb IR and others do not, and how increasing their concentration enhances warming.

The natural greenhouse effect

The Sun emits mostly visible and UV radiation, which passes through the atmosphere and warms Earth's surface. The warm surface re-emits energy as longer-wavelength infrared radiation. Greenhouse gases absorb some of this outgoing IR and re-radiate it in all directions, including back towards the surface. This keeps Earth roughly $33\,^\circ\text{C}$ warmer than it would otherwise be.

Why only some gases absorb IR

A molecule absorbs IR only if the vibration changes its dipole moment - the separation of charge across the molecule.

• $\text{N}_2$ and $\text{O}_2$ are symmetrical diatomic molecules. Their bond vibration does not create a changing dipole, so they do not absorb IR and are not greenhouse gases (even though they make up most of the air).
• $\text{CO}_2$, although linear and overall non-polar, has bending and asymmetric stretching vibrations that produce a changing dipole, so it absorbs IR.
• $\text{H}_2\text{O}$ and $\text{CH}_4$ also have vibrational modes that change their dipole and absorb IR strongly.

The enhanced greenhouse effect

Human activities - burning fossil fuels, agriculture, deforestation - raise the concentration of greenhouse gases. More greenhouse-gas molecules absorb more outgoing IR, increasing the energy retained in the lower atmosphere. This is the enhanced (or anthropogenic) greenhouse effect, the driver of global warming.

Complete combustion of any hydrocarbon fuel adds $\text{CO}_2$ to the atmosphere.

Global warming potential

Gases differ in how strongly and at which wavelengths they absorb, and in how long they persist. Per molecule, methane absorbs IR far more effectively than $\text{CO}_2$, but $\text{CO}_2$ is emitted in much larger quantities and persists for longer, making it the dominant contributor overall.