The differences in the temperature responses can be explained by the smaller opacity of water vapor in the stratosphere compared to that of CO2; this means that, for an increase in SWV, the fractional difference in the instantaneous heating rate perturbation between the upper and lower stratosphere is significantly smaller than for an increase in CO2. Hence, for a SWV perturbation, the largest change in temperature required to balance the instantaneous heating rate perturbation is in the lower stratosphere, where gases are less able to effectively radiate to space. The background concentration of SWV is therefore essential for determining the structure of the temperature response to a SWV perturbation: indeed for background SWV concentrations ≥30ppmv, the instantaneous heating rate perturbation and temperature change resemble those from an increase in CO2.
In the extra-tropical lower stratosphere, the lower height of the tropopause is found to cause the enhancement in cooling due to a reduction in the net exchange of radiation between stratospheric layers. Meridional gradients in upwelling infra-red radiation from the troposphere and spatial differences in the background stratospheric temperature are found to have a minor effect on the cooling.