  A "toy" model, simple and elementary enough for an undergraduate class, of
the temperature dependence of the greenhouse (mid-IR) absorption by atmospheric
water vapor implies a bistable climate system. The stable states are glaciation
and warm interglacials, while intermediate states are unstable. This is in
qualitative accord with the paleoclimatic data. The present climate may be
unstable, with or without anthropogenic interventions such as CO$_2$ emission,
unless there is additional stabilizing feedback such as "geoengineering".
