**Latitude of ice margin as a function of a non-dimensional total solar irradiance in the diffusive energy balance climate model described by North 1975, for different values of the non-dimensional diffusion . Stable states are indicated by a thicker line.**

When we were first starting out as graduate students, Max Suarez and I became interested in ice age theories and found it very helpful as a starting point to think about energy balance models for the latitudinal structure of the surface temperature. At about the same time, Jerry North had simplified this kind of model to its bare essence: linear diffusion on the sphere with constant diffusivity, outgoing infrared flux that is a linear function of surface temperature, and absorbed solar flux equal to a specified function of latitude multiplied by a co-albedo that is itself a function of temperature to capture the different planetary albedos for ice-free and ice-covered areas. Playing with this kind of “toy” model is valuable pedagogically – I certainly learned a lot by building and elaborating this kind of model — and can even lead to some nuggets of insight about the climate system.