How does the greenhouse effect maintain Earth's surface temperature?

• A. By absorbing and re-radiating heat back toward the surface
• B. By reflecting sunlight away from Earth
• C. By cooling the atmosphere
• D. By emitting only visible light

Answer: (A)

The main idea is that the atmosphere traps heat by letting sunlight reach the surface but then absorbing the infrared radiation that the surface emits as it heats up. Greenhouse gases like water vapor, carbon dioxide, and methane are especially good at absorbing infrared energy. After they absorb this heat, they re-emit infrared radiation in all directions, including back toward the surface. This downward component of the emitted infrared adds warmth near the ground, so the surface and lower atmosphere stay warmer than they would if those gases didn’t exist. If the atmosphere were simply reflecting sunlight away, or if it cooled the air, or if it only emitted visible light, those processes would not explain the sustained warming mechanism. The key is the absorption of infrared radiation and its re-radiation back toward the surface, which helps maintain Earth's relatively warm climate.