Describe the greenhouse effect and why increasing greenhouse gases leads to warming.

• A. GHGs trap infrared radiation warming the lower atmosphere; human emissions intensify this effect.
• B. GHGs block solar radiation, cooling the surface.
• C. GHGs reflect visible light; no warming occurs.
• D. GHGs only affect the stratosphere; no surface warming.

Answer: (A)

The essential idea is that greenhouse gases trap heat. Solar radiation from the sun arrives mainly as visible light and heats the Earth's surface. That surface then emits infrared radiation as heat. Greenhouse gases—like carbon dioxide, methane, and nitrous oxide—are efficient at absorbing this infrared radiation. When they absorb it, the molecules vibrate and re-emit infrared energy in all directions, including back toward the surface. This trapping of heat slows the loss of energy to space, warming the air near the surface and the lower atmosphere. When human activities raise the concentrations of these gases, more infrared energy is absorbed and re-emitted downward, boosting the surface temperature.

These gases don’t block sunlight or reflect visible light, so they don’t cool the surface in the way some people imagine. Their warming effect comes from absorbing infrared radiation, not from blocking visible light, and their impact is strongest in the lower atmosphere and at the surface rather than being confined to the stratosphere.