The Capitalist Imperial Pig discusses whether the Earth could go into meltdown, pointing to an article at the MIT Technology Review based on a submission by Colin Goldblatt and Andrew Watson available at arXiv. The manuscript itself is quite clear and differentiates different kinds of runaway warming, the second , and the one Eli will think a bit about comes from determining the vapor pressure of the stratosphere by temperature of the tropopause. In other words, the flux of water vapor into the stratosphere is determined by how much can get through the cold trap at the tropopause.
In radiative equilibrium, the upward longwave flux must equal the downward shortwave flux throughout. Two consequences of this are that the temperature increases with optical depth (this increase is rapid once optical depth > 0.1) and that a higher absorbed shortwave flux will mean that the stratosphere is warmer throughout.They show this in their Figure 2.
A limit on outgoing radiation from the stratosphere arises because the radiative equilibrium temperature profile needs to match the optical depth of the stratosphere, which is set by the saturation vapour pressure at the tropopause. Intuitively, one might think first of the tropopause temperature then of the associated outgoing flux, but the radiation limit is easier to demonstrate by starting with the outward flux. For some given flux, we follow the radiative equilibrium temperature–optical depth profile until it intersects the temperature–optical depth curve derived from the saturation vapour pressure (figure 2). The intersection gives the tropopause . If the outgoing longwave flux is large, no solution can be found—the radiation limit is then the highest flux for which a solution can be found, around 385Wm−2 here.Put this together with Susan Solomon's demonstration a couple of years ago that humans are wetting the stratosphere by increased methane releases, followed by oxidation to water vapor and the bunnies realize that there is another mechanism for what Goldblatt and Watson call "Radiation Limit 2: moist stratosphere upper limit (Komabayashi–Ingersoll limit)", blowing a bunch of methane into the stratosphere.
And where have we seen that discussed? and here too.
Eli, being a lazy bunny, thinks this would require an ungodly methane release, but a massive push of methane into the stratosphere with the associated increase in stratospheric water vapor would bring the Earth into an entirely unique and dangerous situation.