In the basic energy balance of the climate system, which statement best describes the balance in a steady climate?

• A. Incoming solar radiation equals Earth’s emitted longwave radiation.
• B. Net energy at the top of the atmosphere is constant regardless of drivers.
• C. Absorbed solar radiation equals outgoing longwave radiation in a steady climate.
• D. Greenhouse gases absorb all infrared radiation.

Answer: (C)

In a steady climate, the Earth is in radiative equilibrium: the energy it absorbs from the Sun must be balanced by the energy it emits back to space as infrared radiation. Because some of the incoming solar energy is reflected (albedo) and some is absorbed, the meaningful balance is between absorbed solar radiation and outgoing longwave radiation. When those two are equal on average, the climate remains stable. That makes the statement about absorbed solar radiation matching outgoing longwave radiation the best description. The idea that incoming solar equals emitted longwave ignores reflection and what is actually absorbed, while claiming the top-of-atmosphere net energy is constant regardless of drivers is too absolute, and greenhouse gases do not absorb all infrared—absorption occurs only in select wavelengths and amounts, with some IR still escaping to space.