The cooling of the upper atmosphere in response to an increase in CO2 was predicted over 50 years ago:
The larger the mixing ratio of carbon dioxide…
1) …the warmer is the equilibrium temperature of the earth's surface and troposphere.
2) …the colder is the equilibrium temperature of the stratosphere.
(Manabe and Wetherald, 1966; Journal of the Atmospheric Sciences, p251)
One reason for this is that greenhouse gases absorb infrared radiation that is emitted by the Earth’s surface; without them, infrared radiation emitted by Earth would proceed uninterrupted to space. As the concentration of greenhouse gases increase, any particular photon has a greater chance of being absorbed by a greenhouse gas molecule. Since a greenhouse gas molecule which absorbed the photon can re-radiate it in a random direction, this has the immediate effect of preventing its energy from reaching the upper atmosphere. Until equilibrium is re-established by surface warming (which generates more IR photons to compensate), the stratosphere receives less energy with an increase in the strength of the greenhouse effect in the troposphere, and therefore cools.
It is not necessary for a lack of equilibrium to exist, however. Consider that CO2 is less abundant in the stratosphere than it is in the troposphere. Put another way, more of the stratosphere is composed of gases that are not greenhouse gases, like O2 and N2, than it is by greenhouse gasses. Although N2 and O2 do not absorb longwave IR, they are able to absorb shorter wavelengths (from the sun). This increases their energy and they move faster, which is synonymous with being warmer. Because these molecules cannot emit longwave IR, energy transfer between molecules can only happen s via collisions (conduction). When an O2 or N2 molecule collides with another O2 or N2 molecule, kinetic energy can be transferred from one to the other.
CO2 is also capable of moving, but it is also able to vibrate in an excited state when it absorbs energy. When an N2 or O2 collides with a CO2 molecule, then, the N2/O2 will cool after transferring its energy to the CO2. But if the collision results not in a 1:1 transfer of kinetic energy, and instead the CO2 molecule moves to an excited vibrational state
, kinetic energy (temperature) is no longer conserved and thus the average temperature of the collided molecules is less. The excited state is then relaxed by the emission of an IR photon. And because of the lower concentration of greenhouse gases in the upper atmosphere, any emitted photon has a greater chance of reaching space than would a photon emitted by a CO2 molecule in the troposphere.
Increasing CO2 concentrations in the troposphere will also increase them in the stratosphere, wherein they enable a more effective radiation of the photons, and thus lead to a cooler temperature.
By contrast, a warming caused by an increase in solar forcing, for example – either directly from the sun being more active, or from an increase in clouds (which would reflect shortwave, rather than longwave, back to the upper atmosphere) – would increase the shortwave absorption by N2 and O2 and therefore warm the stratosphere. Only a greenhouse-gas induced warming produces the signature of a warming lower atmosphere and cooling upper atmosphere. (Collapse)