Greenhouse gases influence Earth's energy balance by which mechanism?

• A. Absorbing heat in the lower atmosphere
• B. Reflecting all sunlight back to space
• C. Cooling the surface by emitting cold air
• D. Preventing any heat from escaping

Answer: (A)

Greenhouse gases influence Earth's energy balance by absorbing infrared radiation emitted by the surface and re-radiating it in all directions, which traps heat in the lower atmosphere and at the surface. This means less heat escapes to space, so the planet warms until a new balance is reached. The sun’s energy that reaches Earth is mostly visible light and is mostly absorbed by the surface, but the heat that the surface emits as infrared is what greenhouse gases soak up. Because these molecules can store and re-emit that infrared, some of it is sent back downward, keeping the lower atmosphere and surface warmer than they would be otherwise. The other ideas miss this infrared absorption mechanism or overstate the effect—greenhouse gases don’t reflect all sunlight, they don’t cool by emitting cold air, and they don’t completely stop heat from escaping.