The stability of a thermodynamic climate model with three prognostic variables (two for temperature and one for humidity) is investigated. Two stable equilibrium points are found which, in agreement with earlier work, refer to the current (warm) climate and a cold climate. However, perturbations in global temperature must be extremely large (<-20 ¿C) to drive the warm climate into the cold climate. The domains of attraction in phase space are dependent on temperature, but also on humidity. Starting from an ice covered state, the climatic trajectory approaches the present state, if humidity is above some threshold value initially, but evolves into the cold state, if it lies below. Numerical experiments show that the model is not only remarkably stable to internal perturbation but also relatively insensitive to charges in external parameters. The solar constant must be reduced by approximately 20% to obtain total ice cover; the atmospheric CO2-content must be doubled to obtain 1.5 ¿C global warming. However, the surface temperature response of the model increases to 3.5 ¿C if cloud temperature rather than cloud height is held fixed. In addition to the cloud height feedback, other positive feedbacks are ice albedo, water vapor and the oceanic heat flux. The great stabilizers of the model are IR damping and the atmospheric heat transport which is assumed to depend quadratically on the meridional temperature gradient. In addition, precipitation and evaporation effectively damp the surface temperature response of the all ocean model to increased atmospheric trace gases. ¿ American Geophysical Union 1991