What is the primary source of the longwave radiation that is trapped by greenhouse gases?

• A. Earth's surface
• B. The Sun
• C. The Moon
• D. The upper atmosphere

Answer: (A)

The key idea is that greenhouse gases interact with infrared radiation, not with visible sunlight. Objects at Earth-like temperatures emit longwave infrared radiation, and the Earth’s surface does this as it absorbs energy from the Sun. Greenhouse gases have absorption bands in the infrared, so they soak up much of that longwave radiation and then re-emit it in all directions, including back toward the surface. That trapping and re-radiation is what keeps surface temperatures higher.

The Sun provides most of the energy, but its radiation is primarily shortwave (visible/near-IR) and passes through the atmosphere with relatively little absorption by these gases. The Moon and the upper atmosphere aren’t the main sources of the longwave radiation that greenhouse gases trap. So the longwave energy that greenhouse gases trap mainly comes from the Earth’s own surface.