What best describes the greenhouse effect?

• A. Trapping of infrared radiation by greenhouse gases
• B. Reflecting infrared radiation away from the planet
• C. Blocking sunlight entirely
• D. Producing ultraviolet radiation

Answer: (A)

The greenhouse effect means that greenhouse gases in the atmosphere trap heat by absorbing infrared radiation emitted by the Earth's surface. Sunlight reaches Earth as visible light and warms the surface; the surface then radiates heat as infrared. Greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone are effective at absorbing this infrared energy. After absorbing it, they re-emit infrared radiation in all directions, including back toward the surface, which slows heat loss to space and raises the temperature of the lower atmosphere and surface. This warming keeps Earth habitable, and increasing levels of these gases heighten the effect. It is not about reflecting infrared away, blocking sunlight entirely, or producing ultraviolet radiation.