The greenhouse effect involves which energy flow?

• A. Longwave infrared radiation from Earth's surface being absorbed by greenhouse gases
• B. Shortwave solar radiation reflected by the atmosphere
• C. Cosmic radiation heating the surface
• D. Solar wind heating the upper atmosphere

Answer: (A)

The greenhouse effect is about infrared energy being trapped by gases in the atmosphere. When sunlight reaches Earth, it provides energy in the form of shortwave radiation that heats the surface. The surface then emits longwave infrared radiation. Greenhouse gases such as water vapor, carbon dioxide, and methane absorb much of that infrared energy 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 if the atmosphere were transparent to infrared.

That’s why the option describing longwave infrared radiation from Earth’s surface being absorbed by greenhouse gases is the best fit. The other options describe other, less relevant processes: reflecting shortwave solar radiation relates to albedo and not the IR trapping; cosmic radiation and solar wind heating are not the mechanisms behind the greenhouse effect.