Which statement correctly describes the energy balance mechanism that keeps Earth's surface warmer than it would be otherwise?

• A. The greenhouse effect, by trapping heat through greenhouse gases
• B. The albedo effect, by reflecting all sunlight away
• C. The ozone layer, by absorbing all UV
• D. Ocean currents, by cooling the surface

Answer: (A)

The energy balance keeping Earth's surface warmer is the greenhouse effect. Sunlight reaches Earth and heats the surface, which then radiates heat as infrared energy. Greenhouse gases in the atmosphere—like water vapor, carbon dioxide, and methane—are good absorbers of that infrared radiation. They trap a portion of the heat, re-radiating it back toward the surface as well as within the lower atmosphere. This trapping of infrared energy means less heat escapes to space, so the surface settles at a warmer temperature than it would if the atmosphere were transparent to infrared.

Understanding the other options helps clarify why they’re not the primary mechanism here. The albedo effect would cool the planet by reflecting more sunlight away before it even warms the surface, not keep it warmer. The ozone layer mainly protects against ultraviolet radiation and isn’t the main driver of surface warming. Ocean currents redistribute heat around the globe, influencing regional climates, but they don’t create the baseline energy-trapping mechanism that makes the average surface temperature higher.