CO₂ concentrations double, causing initial warming. As the atmosphere warms, it holds more water vapor, which traps additional heat and causes further warming. Is the increase in water vapor a forcing or a feedback?
AA forcing, because water vapor is a greenhouse gas that directly changes the energy balance
BA feedback, because the water vapor increase is a response to the temperature change caused by the CO₂ forcing
CBoth a forcing and a feedback, because it independently perturbs the energy balance
DNeither — water vapor changes are absorbed into the Planck response and not counted separately
A forcing is an externally imposed perturbation to the energy balance (here, the CO₂ doubling). A feedback is an internal climate system response to the temperature change that the forcing produces. Water vapor increases because the temperature rose — it is responding to the climate system's own warming. This makes it a feedback (a positive one), not a forcing. If humans were directly injecting water vapor into the atmosphere, that would be a forcing; but the natural atmospheric moistening in response to warming is a feedback.
Question 2 Multiple Choice
If all climate feedbacks were somehow eliminated and only the Planck (blackbody) response remained, how much would Earth warm in response to a doubling of CO₂?
AAbout 3°C — the standard 'climate sensitivity' estimate already assumes no feedbacks
BAbout 1.1°C — the no-feedback response from the Planck blackbody adjustment alone
CZero degrees — without feedbacks, the system cannot reach a new equilibrium
DAbout 5°C — removing feedbacks amplifies the direct CO₂ effect
The Planck response is the basic blackbody adjustment: a warmer planet emits more longwave radiation until energy balance is restored. For CO₂ doubling (forcing ~3.7 W/m²), this no-feedback response yields about 1.1°C. Real climate sensitivity is 2–5°C because positive feedbacks (water vapor, ice-albedo) amplify this initial response significantly. The difference between 1.1°C and the full sensitivity estimate is entirely attributable to the net effect of feedbacks — making this the key number for understanding why feedbacks matter so much.
Question 3 True / False
The water vapor feedback is a positive feedback in the climate system because warmer temperatures increase atmospheric water vapor content, and water vapor is itself a greenhouse gas that traps additional heat.
TTrue
FFalse
Answer: True
Correct. The Clausius-Clapeyron relation tells us that warmer air can hold exponentially more water vapor. Water vapor is a potent greenhouse gas, so this additional moisture traps more outgoing longwave radiation, causing further warming — a self-amplifying (positive) feedback loop. The water vapor feedback is the single strongest positive feedback in the climate system, roughly doubling the warming that CO₂ alone would produce.
Question 4 True / False
A climate system with primarily negative feedbacks would warm less than a system with net positive feedbacks in response to the same forcing, but both systems would eventually reach a new stable equilibrium at a higher temperature.
TTrue
FFalse
Answer: False
This statement is misleading about the behavior when net feedbacks are strongly positive. The equilibrium temperature change is ΔT = F / (λ₀ − Σλᵢ), where λ₀ is the Planck response parameter and Σλᵢ is the sum of all feedback parameters. When positive feedbacks are net negative (all feedbacks negative), the denominator is large and ΔT is small. But if positive feedbacks approach λ₀ in magnitude, the denominator approaches zero and ΔT becomes very large — potentially a runaway. A system with only negative feedbacks would warm less than a no-feedback system, not more, and would certainly not behave the same as a net-positive-feedback system.
Question 5 Short Answer
Why does climate sensitivity become very large — or potentially runaway — when the sum of positive feedbacks approaches the Planck feedback parameter, and what does this tell us about the mathematical structure of the forcing-feedback framework?
Think about your answer, then reveal below.
Model answer: The equilibrium warming is ΔT = F / (λ₀ − Σλᵢ), where λ₀ is the Planck response (a stabilizing negative feedback from increased thermal emission) and Σλᵢ is the net sum of all feedbacks. As positive feedbacks grow, their sum approaches λ₀, and the denominator shrinks toward zero — causing ΔT to grow without bound. Physically, the system's self-stabilizing mechanism (radiating away more energy when warmer) is being canceled by the self-amplifying feedbacks, so it takes an ever-larger temperature increase to restore energy balance. If positive feedbacks exceed λ₀, there is no stable equilibrium: this is the runaway greenhouse state, where warming cannot be arrested by any finite temperature increase.
This reveals that the forcing-feedback framework is not just about quantifying warming — it identifies the conditions under which a climate system can become fundamentally unstable. The ratio structure of the sensitivity equation is the mathematical encoding of this instability threshold, making it one of the most important insights in climate science.