Questions: Climate Feedback Mechanisms

Question 1

Which of the following correctly describes the role of water vapor in the climate system?

• A. Water vapor is a primary forcing that directly drives warming when humans emit it
• B. Water vapor is a feedback that amplifies warming because a warmer atmosphere holds more moisture
• C. Water vapor is a negative feedback because clouds formed from it reflect sunlight
• D. Water vapor has negligible effect on climate because it is too short-lived in the atmosphere

Answer: (B)

Water vapor is a feedback, not a forcing. Its atmospheric concentration is controlled by temperature: as the surface warms (from CO₂ forcing), evaporation increases, raising atmospheric humidity, which then traps more longwave radiation and amplifies the initial warming. Humans do not meaningfully emit water vapor as a climate forcing — it rains out quickly.

Question 2

A positive climate feedback means the climate system will inevitably spiral into a runaway warming state with no stable endpoint.

• T. True
• F. False

Answer: False

Positive feedback means amplification of an initial perturbation, not unbounded runaway. The system has both positive and negative feedbacks operating simultaneously. The Planck response (increased outgoing longwave radiation from a warmer planet) is a strong negative feedback that ultimately stabilizes the system at a new, warmer equilibrium. Runaway warming is a specific threshold phenomenon (relevant to Venus-like scenarios), not a consequence of ordinary positive feedbacks.

Question 3

Why are cloud feedbacks the largest source of uncertainty in estimates of equilibrium climate sensitivity?

Model answer: Low clouds cool the planet by reflecting incoming sunlight, while high clouds warm it by trapping outgoing longwave radiation. Whether warming increases or decreases each cloud type — and by how much — is difficult to determine from observations and varies across climate models, creating a wide range of possible net cloud feedback values.

Cloud behavior depends on many interacting processes (convection, humidity, aerosols) that operate at scales too small for global climate models to fully resolve. Since low and high clouds have opposite effects and both respond to warming in uncertain ways, cloud feedback can be anywhere from mildly negative to strongly positive, which directly widens the range of projected equilibrium climate sensitivity.