What is the greenhouse effect?

• A. The greenhouse effect is caused by the reflection of sunlight by clouds.
• B. Greenhouse gases emit visible light to heat the surface.
• C. Heat from the sun warms Earth; greenhouse gases absorb infrared radiation, trapping heat.
• D. Heat from Earth escapes freely to space, cooling the planet.

Answer: (C)

The greenhouse effect happens because sunlight warms the Earth, and the Earth, in turn, emits heat as infrared radiation. Greenhouse gases in the atmosphere—like water vapor, carbon dioxide, and methane—absorb much of that infrared heat and then re-radiate it in all directions, including back toward the surface. This trapping of heat keeps the lower atmosphere and surface warmer than they would be otherwise; without it, Earth would be much cooler (about 33°C cooler on average).

So, the best description is that heat from the sun warms Earth, greenhouse gases absorb infrared radiation, and trap heat. It’s not about reflecting sunlight or emitting visible light to heat the surface, and it does not mean heat escapes freely to space.