(PS regarding Venus — as I have understood it, a
runaway water vapor feedback would have occured when solar heating increasing to become greater than a limiting OLR value (Simpson - Kombayashi - Ingersoll limit — see http://chriscolose.wordpress.com/2010/08/23/climate-feedbacks-part-1/ — although I should add that at more «moderate» temperatures (
warmer than today), stratospheric H2O increases to a point where H escape to space becomes a significant H2O sink — if that stage worked fast enough relative to solar brightening, a
runaway H2O case could be
prevented, and it would be a dry (er) heat.