In what direction do greenhouse gases re-emit the energy they have absorbed?

• A. In all (random) directions
• B. Only upward
• C. Only downward
• D. Only horizontally

Answer: (A)

Greenhouse gases re-emit energy in all directions. When these gases absorb infrared energy from Earth's surface, their molecules rise to excited states and then release photons of infrared radiation. This emission happens in a random, isotropic way, so photons are sent out in every direction rather than a single preferred path. As a result, some of the re-emitted energy travels upward toward space, while some is directed downward back toward the surface, and some goes sideways through the atmosphere. This omnidirectional emission is what produces the greenhouse effect: energy is trapped near the surface because the gases keep bouncing infrared radiation around, warming the lower atmosphere and surface.