= Solution
At a fixed surface warming, a smaller <lapse rate (atmosphere)>[lapse rate] makes the upper troposphere warmer than in an unchanged-lapse profile. Its emission to space therefore increases, giving a <negative feedback>. <Water vapor> has the opposite effect under the given coefficients. The net restoring coefficient is $\lambda_P-2+0.5\simeq2.24$ W m$^{-2}$ K$^{-1}$, so $\boxed{\Delta T\simeq1.65\ \mathrm K}$. This calculation uses the same $S_0$ and linearized Planck model as the preceding part.
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