Questions: Climate Feedbacks: Ice-Albedo and Water Vapor Feedback

Question 1

If doubling atmospheric CO₂ would warm the Earth by approximately 1.1°C through its direct radiative effect alone, why do climate models project equilibrium warming of roughly 2.5–4°C per CO₂ doubling?

• A. Climate models overestimate CO₂ forcing because they do not account for negative feedbacks
• B. Positive feedbacks — primarily water vapor and ice-albedo — amplify the initial warming by roughly 2–4 times
• C. CO₂ directly heats the atmosphere more than the 1.1°C estimate suggests, making feedbacks minor
• D. Each additional degree triggers more CO₂ release from oceans, further amplifying the forcing

Answer: (B)

The 1.1°C figure is the direct radiative effect of doubled CO₂ alone, before any feedbacks respond. But warming triggers ice melt (exposing darker surfaces, reducing albedo) and increases atmospheric moisture (since warmer air holds more water vapor, itself a greenhouse gas). These positive feedbacks amplify the initial warming considerably. Together, ice-albedo and water vapor feedbacks roughly double the equilibrium response. Option D describes a real but secondary effect (outgassing from warmer oceans); the dominant amplification comes from water vapor and ice-albedo.

Question 2

A climate scientist says 'water vapor is the most powerful greenhouse gas in the atmosphere, but it is not the cause of current climate change.' Which statement best explains this distinction?

• A. Water vapor is only a greenhouse gas at high altitudes; near the surface it has no warming effect
• B. Water vapor is a feedback that responds to temperature, not an independent forcing — it amplifies warming driven by CO₂ but does not initiate it
• C. Water vapor concentrations have been declining due to human activity, offsetting CO₂ warming
• D. Water vapor is a forcing like CO₂ but its emissions are natural, so it is not considered a cause of human-induced climate change

Answer: (B)

This is the key forcing vs. feedback distinction. Water vapor is indeed the most abundant greenhouse gas by warming effect, but its concentration in the atmosphere is controlled by temperature — warmer air holds more moisture (Clausius-Clapeyron relation). It does not accumulate independently the way CO₂ does; if CO₂ forcing were removed and temperatures fell, water vapor would decrease correspondingly. CO₂ is a forcing because humans are adding it regardless of temperature. Water vapor is a feedback because it responds to temperature changes caused by that forcing.

Question 3

The ice-albedo feedback is a positive feedback: melting ice exposes darker surfaces that absorb more solar radiation, causing further warming and further ice loss.

• T. True
• F. False

Answer: True

This is a textbook positive feedback loop in the climate system. Ice reflects 60–90% of incident solar radiation; open ocean or exposed land absorbs most of it. When initial warming melts some ice, the newly exposed surface absorbs more energy, warming the surface further, melting more ice. This self-reinforcing cycle is observed today in the Arctic, where sea ice decline contributes to the Arctic warming roughly 2–3 times faster than the global average — a phenomenon called Arctic amplification.

Question 4

Because the water vapor feedback is a positive feedback, removing most anthropogenic CO₂ from the atmosphere would cause water vapor to continue warming the climate indefinitely.

• T. True
• F. False

Answer: False

Water vapor is a feedback, not an independent forcing. If CO₂ were removed and temperatures dropped, the cooler atmosphere would hold less water vapor, reducing this greenhouse effect further. The system would find a new equilibrium rather than continuing to warm. Positive feedbacks amplify perturbations from an initial forcing — they do not cause runaway warming on their own without a sustained forcing to maintain the temperature departure. True runaway warming (like Venus) requires the forcing to exceed specific thresholds that are far beyond current projections for Earth.

Question 5

Why do positive climate feedbacks like water vapor and ice-albedo not necessarily lead to runaway warming, and what determines where the system stabilizes?

Model answer: Positive feedbacks amplify an initial forcing, but the system also contains negative feedbacks and increased radiative emission that eventually restore equilibrium. As Earth warms, it emits more longwave radiation to space (Stefan-Boltzmann law), which is a powerful stabilizing effect. Negative feedbacks like the lapse-rate feedback (upper troposphere warming faster, increasing outgoing radiation) and some cloud responses partially offset the positive feedbacks. The system stabilizes at a new equilibrium where the increased outgoing radiation balances the increased absorbed energy. Equilibrium climate sensitivity — roughly 2.5–4°C per CO₂ doubling — reflects this balance of amplifying and stabilizing processes.

Runaway warming would require positive feedbacks to completely overwhelm all restoring forces, which doesn't happen under plausible CO₂ scenarios for Earth. The concern in climate science is not runaway warming in the Venus sense, but the substantial amplification of forcing that positive feedbacks produce — turning a 1.1°C direct effect into a 3°C equilibrium response — with cascading consequences for ecosystems, sea level, and weather patterns.