A box model of the ocean-atmosphere-sea ice-land ice climate system is used to study a novel mechanism for self-sustained oscillations of the climate system on a time scale of 100,000 years, without external forcing. The oscillation in land ice volume has the familiar sawtooth shape of climate proxy records. The most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice. The sea ice acts as a switch of the climate system, switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillation's sawtooth structure. A simple explanation of the 100-kyr timescale is formulated on the basis of the mechanism seen in the model. Finally, rapid sea ice changes such as those seen in our model, and their drastic effects on the climate system, may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord. ¿ 2001 American Geophysical Union