Why is water vapor considered a feedback rather than a forcing in climate change?

• A. Water vapor changes in response to temperature changes and acts as a feedback that amplifies warming.
• B. Water vapor is the primary forcing behind CO2 warming.
• C. Water vapor has no radiative effect.
• D. Water vapor decreases as temperatures rise.

Answer: (B)

Water vapor acts as a feedback because its amount in the atmosphere responds to warming and then amplifies that warming. When something external, like more CO2, increases the planet’s temperature, the air can hold more moisture. Since water vapor is a powerful greenhouse gas, this extra moisture traps more infrared radiation, pushing temperatures up even further. In this way, the initial warming is caused by the forcing (the CO2 rise), and water vapor then enhances that warming as a consequence, not as the original driver. Water vapor does have a radiative effect, so saying it has no radiative effect isn’t correct, and it doesn’t decrease with higher temperatures—it increases because warmer air can hold more moisture.