We introduce a new one-level nonlinear seasonal energy balance climate model with a two-dimensional land-sea geography to study the astronomical theory of ice ages. The new model is more physically consistent and mathematically reliable than those previously reported. The model reproduces the current climate state and explains how the Earth's orbit causes ice ages. We have found a set of bifurcation points of warm and cold orbit for both backward and forward processes. Moreover, we found that because the Earth's orbit varies slowly, gradual increases (or decreases) in the concentration of CO2 will also cause sudden climate transitions. Thus, variations in the Earth's orbit and the concentration of CO2 appear to be the two major mechanisms that cause ice ages or climate changes; furthermore, these changes are usually sudden. Finally, we predict the near-future climate resulting from a continual increase in CO2 concentration. The model does not yet include slower components such as the deep-ocean circulation. Therefore, variations occur more rapidly than in the real world. ¿ American Geophysical Union 1992