Questions: Climate Sensitivity and Radiative Feedbacks

Question 1

Why is equilibrium climate sensitivity (ECS) larger than the transient climate response (TCR) for the same CO₂ doubling scenario?

• A. ECS includes fast feedbacks; TCR includes only slow feedbacks
• B. ECS is reached after the ocean has fully equilibrated; TCR is measured while the ocean is still absorbing heat
• C. ECS assumes no cloud feedback; TCR includes all feedbacks
• D. ECS is measured over land only; TCR is the global average

Answer: (B)

The ocean has enormous heat capacity and takes centuries to equilibrate with a new forcing. TCR is measured at the moment of CO₂ doubling (typically ~70 years into a 1% per year ramp-up), when the ocean is still absorbing heat and suppressing surface warming. ECS is the warming after full ocean equilibration, so it is always larger than TCR. The ratio ECS/TCR reflects how much additional warming is 'in the pipeline' even if emissions stopped.

Question 2

A positive feedback in the climate system will cause temperatures to rise without bound (runaway warming) if the feedback is strong enough.

• T. True
• F. False

Answer: False

Positive feedbacks amplify warming but do not automatically cause runaway. The climate system stabilizes when the increased outgoing longwave radiation from a warmer surface restores radiative balance. Runaway warming (like Venus) requires the feedback to overwhelm this restoring force entirely — a specific condition that Earth's current state does not satisfy for finite CO₂ increases. Positive feedbacks increase climate sensitivity; they do not eliminate the equilibrium.

Question 3

Explain the water vapor feedback mechanism and why it is considered a positive feedback.

Model answer: As surface temperature rises, the atmosphere holds more water vapor (following the Clausius-Clapeyron relation). Water vapor is a potent greenhouse gas, so higher concentrations absorb more outgoing infrared radiation, causing additional warming — which in turn drives more evaporation. The feedback amplifies the initial temperature perturbation.

Water vapor feedback roughly doubles the warming that would occur from CO₂ alone (in the absence of other feedbacks). It is the single largest positive feedback in the climate system. Crucially, water vapor is not a forcing — humans do not directly add atmospheric water vapor at climatically relevant scales — it responds to temperature, making it a feedback rather than a driver.