Which statement best describes the Earth's energy budget?

• A. The energy budget refers to solar energy input only.
• B. It is the balance between incoming solar radiation and outgoing infrared radiation.
• C. It only concerns ultraviolet radiation.
• D. It refers to the energy stored in Earth's core.

Answer: (B)

The energy budget is about how much energy the Earth receives from the Sun compared with how much energy the Earth radiates back into space. Solar energy arrives as shortwave radiation, with some of it reflected by clouds, atmosphere, and the surface (albedo) and some absorbed, warming air, water, and land. The planet then must emit heat as infrared radiation to space. Greenhouse gases trap some of that infrared, which keeps the average surface temperature higher than it would be otherwise. When the incoming energy roughly equals the outgoing energy, the climate is in balance. If more energy comes in than goes out, the surface warms; if more energy goes out than comes in, it cools. This balance between input from the Sun and output as infrared radiation is the essence of the Earth's energy budget. The other statements aren’t correct because they ignore either the outward infrared radiation, overemphasize a part of the spectrum (like ultraviolet), or refer to heat from Earth’s interior, which is not what drives the planetary energy budget.