FIG. 1: Paleoclimatic data showing ice ages and interglacials $^{2–5}$. IV. LIMIT STATES This sensitivity of opacity to water vapor pressure, combined with the sensitivity of equilibrium vapor pressures to temperature, qualitatively suggests a bistable system: There may be a cold state in which the water precipitates and the atmosphere is transparent, with little water vapor absorption, and a warm state in which the atmosphere is warm, humid and opaque and in which absorbed solar energy is carried through the troposphere by convection (resembling present tropical or temperate summer conditions). These limit states are stable: in the cold state perturbations to the temperature have little effect on the infrared opacity because it remains very low, while in the warm state the opacity has little effect on the heat transfer. In either case there is little feedback to the heat balance from changes in the tropospheric water vapor content. At intermediate temperatures increases in temperature produce a positive feedback by increasing the infrared opacity (and vice versa for decreases), so that perturbations grow exponentially until one of the stable limit states is approached. 3