Time evolution of the fraction of water vapor in the atmosphere over the total water amount and of the mean surface temperature for Gl 581 c with a 200 mbar background atmosphere. The black solid curve represents a model initialized with a water vapor column of 150 kg/m² and the gray dashed one with 250 kg/m². After a short transitional period, either the water collapses on the ground in a few hundred days (solid curve) or the atmosphere reaches a runaway greenhouse state (dashed curve).