  This article is concerned with the dynamics of glacial cycles observed in the
geological record of the Pleistocene Epoch. It focuses on a conceptual model
proposed by Maasch and Saltzman [J. Geophys. Res.,95, D2 (1990), pp.
1955-1963], which is based on physical arguments and emphasizes the role of
atmospheric CO2 in the generation and persistence of periodic orbits (limit
cycles). The model consists of three ordinary differential equations with four
parameters for the anomalies of the total global ice mass, the atmospheric CO2
concentration, and the volume of the North Atlantic Deep Water (NADW). In this
article, it is shown that a simplified two-dimensional symmetric version
displays many of the essential features of the full model, including
equilibrium states, limit cycles, their basic bifurcations, and a
Bogdanov-Takens point that serves as an organizing center for the local and
global dynamics. Also, symmetry breaking splits the Bogdanov-Takens point into
two, with different local dynamics in their neighborhoods.
