What is the effect of greenhouse gases on Earth's average temperature?

• A. They trap heat in the surface layer only, with no atmospheric warming.
• B. They primarily cause rapid cooling in deserts.
• C. They increase cloud reflectivity, causing cooling.
• D. They trap heat in the lower atmosphere, warming the surface.

Answer: (D)

Greenhouse gases warm Earth by trapping heat that would otherwise escape to space. They absorb infrared radiation emitted by the surface and then re-emit it in all directions. A portion of that energy returns toward the surface, so the lower atmosphere and the surface both end up warmer than they would be otherwise. This downward trapping is the key reason the average temperature rises.

So the idea that they trap heat in the lower atmosphere, warming the surface, captures what actually happens: infrared heat is held in by these gases, leading to a warmer surface. The other options mischaracterize the effect—greenhouse gases don’t primarily cause rapid desert cooling, nor do they mainly increase cloud reflectivity to produce cooling, and heat trapping isn’t confined to just the surface layer without atmospheric warming.