  We propose a general principle that under the radiative-convective
equilibrium, the spatial and temporal variations in a planet's surface and
atmosphere tend to increase its cooling. This principle is based on Jensen's
inequality and the curvature of the response functions of surface temperature
and outgoing cooling flux to changes in incoming stellar flux and atmospheric
opacity. We use an analytical model to demonstrate that this principle holds
for various planet types: (1) on an airless planet, the mean surface
temperature is lower than its equilibrium temperature; (2) on terrestrial
planets with atmospheres, the inhomogeneity of incoming stellar flux and
atmospheric opacity reduces the mean surface temperature; (3) on giant planets,
inhomogeneously distributed stellar flux and atmospheric opacity increase the
outgoing infrared flux, cooling the interior. Although the inhomogeneity of
visible opacity might sometimes heat the atmosphere, the effect is generally
much smaller than the inhomogeneous cooling effect of infrared opacity.
Compared with the homogeneous case, the mean surface temperature on
inhomogeneous terrestrial planets can decrease by more than 20\%, and the
internal heat flux on giant planets can increase by over an order of magnitude.
Despite simplifications in our analytical framework, the effect of stellar flux
inhomogeneity appears to be robust, while further research is needed to fully
understand the effects of opacity inhomogeneity in more realistic situations.
This principle impacts our understanding of planetary habitability and the
evolution of giant planets using low-resolution and one-dimensional frameworks
that may have previously overlooked the role of inhomogeneity.
