How do shortwave radiation and longwave radiation differ in Earth's energy balance?

• A. Shortwave radiation is infrared energy radiated by Earth; longwave radiation is solar energy
• B. Shortwave radiation is solar energy arriving at Earth (mostly visible light); longwave radiation is infrared energy radiated by Earth back into the atmosphere
• C. Shortwave is ultraviolet energy; longwave is microwave
• D. Shortwave is gamma radiation; longwave is radio waves

Answer: (B)

The main thing this question tests is how energy moves between the Sun, Earth, and space in different wavelength bands. Shortwave radiation refers to the Sun’s energy arriving at Earth, which is mostly visible light (with some near-ultraviolet and near-infrared). Longwave radiation is the infrared energy that Earth itself emits back into the atmosphere and space as heat, because the planet is much cooler than the Sun and radiates at longer wavelengths.

In Earth's energy balance, the Sun’s shortwave energy that reaches Earth drives warming. Some of it is reflected back by clouds and the surface (albedo), and the rest is absorbed and heats the surface and atmosphere. To balance this, Earth radiates energy outward as longwave infrared radiation. Greenhouse gases modify this balance by absorbing some of that longwave radiation, trapping heat and keeping the lower atmosphere warmer.

So the best description is that shortwave radiation is solar energy arriving at Earth (mostly visible light), while longwave radiation is infrared energy radiated by Earth back into space. The other options mix up the source or the wavelength (for example, infrared radiation radiated by Earth is longwave, not shortwave; solar energy is shortwave, not ultraviolet or other bands).