Earth remains warm enough to support life mainly because which mechanism operates in the lower atmosphere?

• A. Infrared radiation from Earth's surface is absorbed by gases in the lower atmosphere.
• B. The ozone layer blocks all UV radiation.
• C. Earth's core provides heat.
• D. The Sun is unusually close.

Answer: (A)

The main idea being tested is how the atmosphere helps keep Earth warm enough for life. The mechanism at work is the greenhouse effect: certain gases in the lower atmosphere (like water vapor, carbon dioxide, and methane) absorb infrared radiation that Earth’s surface emits. After absorbing this heat, these gases re-radiate infrared energy in all directions, including back toward the surface. That trapping of heat keeps the surface warmer than it would be if the atmosphere were transparent to infrared.

Ozone blocking UV sits in the stratosphere and protects against ultraviolet light, but that isn’t the process that keeps surface temperatures warm in the lower atmosphere. Earth’s core heat is tiny compared to the energy coming from the Sun, so it doesn’t drive the climate in this context. The Sun isn’t unusually close; the current warming operates primarily through atmospheric heat trapping, not changes in solar distance.