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How does climate sensitivity vary with the magnitude of climate forcing? This question was investigated with the use of a modified coupled atmosphere‐ocean model, whose stability was improved so that the model would accommodate large radiative forcings yet be fast enough to reach rapid equilibrium. Experiments were performed in which atmospheric CO 2 was multiplied by powers of 2, from 1/64 to 256 times the 1950 value. From 8 to 32 times, the 1950 CO 2 , climate sensitivity for doubling CO 2 reaches 8°C due to increases in water vapor absorption and cloud top height and to reductions in low level cloud cover. As CO 2 amount increases further, sensitivity drops as cloud cover and planetary albedo stabilize. No water vapor‐induced runaway greenhouse caused by increased CO 2 was found for the range of CO 2 examined. With CO 2 at or below 1/8 of the 1950 value, runaway sea ice does occur as the planet cascades to a snowball Earth climate with fully ice covered oceans and global mean surface temperatures near −30°C.
Russell et al. (Tue,) studied this question.