How do greenhouse gases trap heat, and why is water vapor considered both a key absorber and a feedback?

• A. Greenhouse gases absorb infrared radiation emitted by Earth and re-radiate it back, trapping heat.
• B. Water vapor is a poor absorber and not a feedback.
• C. Greenhouse gases trap infrared radiation and water vapor acts as a powerful absorber that responds to temperature, amplifying warming.
• D. Water vapor acts as a primary forcing that initiates warming.

Answer: (C)

Heat is trapped when greenhouse gases absorb infrared radiation emitted by Earth's surface and then re-radiate it in all directions, including back toward the surface. Water vapor is the most abundant of these greenhouse gases and absorbs infrared strongly. Its amount in the atmosphere increases as temperature rises, because warmer air can hold more moisture. This makes water vapor act as a powerful absorber and, crucially, a feedback: once warming starts (from something like CO2 or other forcings), more water vapor enters the atmosphere, trapping more heat and causing even more warming. Water vapor is not the initial forcing that starts warming; it amplifies the effect once warming has begun.