Runaway Greenhouse Effect - Physics of The Runaway Greenhouse

Physics of The Runaway Greenhouse

Normally, when a planet's radiation balance is perturbed (e.g., by increasing the amount of sunlight it gets or changing the greenhouse concentration, see Radiative Forcing) it will transition to a new temperature until a stabilizing feedback known as the Stefan-Boltzmann response restores an equilibrium between the amount of energy the body absorbs and that which it emits. For example, if the Earth received more sunlight it would result in a temporary disequilibrium (more energy in than out) and result in warming. However, because the Stefan-Boltzmann response mandates that this hotter planet emit more energy, eventually a new radiation balance can be reached and the temperature will be maintained at its new, higher value.

However, when the planet has an operating water vapor feedback, the efficiency of the greenhouse effect increases as the temperature does, and so the outgoing radiation to space increases less rapidly than for a pure Stefan-Boltzmann radiator behaving like a blackbody. Eventually, the infrared absorption increases so much that the amount of energy escaping to space no longer depends on the temperature at the surface, and asymptotes to a fixed value. If the amount of energy that the planet receives from the star (or from internal heat sources) exceeds this value, radiative equilibrium can never be achieved. The result is a runaway that continues until the water vapor feedback ceases, which may be when the entire ocean is evaporated and dispersed to space.