Why does a CO2 increase cool the stratosphere?

The cooling of the stratosphere is a robust feature of climate simulations forced by an increase in CO2. This feature is primarily a consequence of the CO2 forcing alone (no feedbacks), which causes longwave (LW) radiative cooling in the stratosphere. In this study, we first separately diagnose upward and downward LW radiative flux perturbations caused by an increase of CO2. We find the following: 1) the change in downward LW flux contributes at least as much to the CO2 radiative cooling in the stratosphere as the change in upward LW flux; 2) the change in downward LW flux is responsible for most of the CO2 radiative heating seen at the surface and in the lower troposphere; 3) the vertical band structure of the radiative heating profile is a result of offsetting heating and cooling contributions by upward and downward fluxes, respectively, in the upper troposphere. Using a grey model of the atmosphere, we can qualitatively explain the vertical structure of the radiative heating profile due to a CO2 increase alone.