Why Does a CO2 Increase Cool the Stratosphere?

Abstract:

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 alone. We find the following: 1) The change in downward LW flux contributes at least as much to the CO₂ radiative cooling in the upper stratosphere as the change in upward LW flux; 2) the increasing with height temperature profile of the stratosphere results in a cooling due to the change in upward LW flux and a warming in the lower stratosphere due to the change in downward LW flux, which can offset or even overturn the cooling due to changes in upward LW flux.