A Colorful look at Climate Sensitivity

Abstract. The radiative response to warming, and to changing concentrations of CO₂, is studied in spectral space. If relative humidity does not change with temperature, clear-sky emissions over spectral intervals in which water vapor is optically thick become independent of surface temperature, giving rise to the idea of spectral masking. It is demonstrated that this idea allows one to derive simple, physically informative, and surprisingly accurate, expressions for the clear sky radiative forcing, radiative response to warming and hence climate sensitivity. Extending these concepts to include the effects of clouds, leads to the expectation that (i) clouds damp the clear-sky response to forcing, (ii) that diminutive clouds near the surface, which are often thought to be unimportant, may be particularly effective at enhancing the clear-sky sensitivity over deep moist tropical boundary layers; and (iii) even small changes in high-clouds over deep moist regions in the tropics makes these regions radiatively more responsive to warming that previously believed. The analysis demonstrates that the net effect of clouds on warming is ambiguous, justifying the assertion that the clear-sky (fixed RH) climate sensitivity – which after accounting for clear-sky surface albedo feedbacks, is about 3 K – provides a reasonable prior for Bayesian updates accounting for how clouds are distributed, how they they might change, and for deviations associated with changes in relative humidity with temperature. These effects are best assessed by quantifying the distribution of clouds and water vapor, and how they change, in temperature, rather than geographic space.