B.2.8—Infrared absorption

Absorb at molecular frequencies

Greenhouse-gas molecules can absorb infrared radiation when its frequency matches an allowed molecular vibration or transition. The molecule moves to a higher energy state.

Re-emit in all directions

The excited molecule soon returns to a lower energy state and emits infrared radiation. The emission is in random directions, so some radiation continues upward and some is directed back toward the surface.

Why Earth’s radiation matters

The cooler Earth emits mainly longer-wavelength infrared radiation. Greenhouse gases absorb part of this outgoing radiation; they are less likely to absorb most of the Sun’s shorter-wavelength incoming radiation.

Model boundary from local Question Bank row 30018

Mars's atmosphere is mainly carbon dioxide, but its pressure is less than $1\%$ of Earth's. The low pressure means far fewer absorbing molecules per unit volume, so much less outgoing infrared radiation is absorbed and re-emitted toward the surface.

The presence of a greenhouse-gas species alone does not determine the size of the effect; the amount of gas also matters.

Common trap

The greenhouse effect is not mainly reflection of incoming sunlight. The key process is absorption and re-emission of outgoing infrared radiation.