Suppose that CO2 in the atmosphere was held at a constant, natural level for a few thousand years. Then, CO2 was added to double the atmospheric level rapidly, and this new, doubled level was maintained for a few thousand years. What would be the most likely change in the typical average temperature of the planet?

Question

BlackTom AI · Accepted Answer

Doubling CO2 increases radiative forcing by about 3.7 W/m^2, which leads to a multi‑degree Celsius equilibrium warming once the climate system adjusts.

1) Relevant knowledge points:
- Greenhouse gases trap outgoing longwave radiation; increasing CO2 reduces outgoing flux and produces a radiative forcing.
- Radiative forcing for CO2 doubling: ΔF = 5.35 · ln(C/C0); for C/C0 = 2, ΔF ≈ 3.71 W/m^2.
- Equilibrium climate sensitivity (ECS) is the global mean temperature change after the system equilibrates to a forcing. Best estimates: ~1.5–4.5 °C per CO2 doubling, central value ≈3 °C.
- A “few thousand years” of maintained forcing is long enough to approach equilibrium temperature (allowing deep-ocean adjustment).

2) Step-by-step reasoning:
- Compute forcing: ΔF = 5.35 · ln2 = 5.35 · 0.693 = 3.71 W/m^2.
- The no‑feedback (Planck) response gives ~0.3 K per W/m^2, so baseline warming ≈0.3 · 3.71 ≈ 1.1 °C.
- Feedbacks (water vapor, lapse rate, ice–albedo, clouds) amplify that baseline. Multiplying by observed feedbacks yields the ECS range; the common central estimate is ≈3 °C for one doubling.

3) Intermediate calculations shown above (5.35·ln2 = 3.71; 0.3·3.71 ≈1.1).

4) Why this answer is correct:
- Climate physics and empirical/Model evidence give a warming of order 1–4 °C for CO2 doubling, typically described as “a few degrees” rather than tens of degrees.

5) Why other choices are incorrect:
- “30 or 40 degrees” would require an enormously larger forcing (order 100 W/m^2), far beyond a single CO2 doubling.
- “A few degrees lower” or “the same” contradicts the positive radiative forcing from added greenhouse gas; CO2 increase cannot produce net global cooling.
- “30 or 40 degrees lower” is physically impossible from doubling CO2.

6) Conclusion:
The most likely change is a few degrees higher average global temperature, which matches the provided correct answer.

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