A one-dimensional nonlinear ice age model developed by J. Imbrie and J. Z. Imbrie was used to investigate the 100,000 year cycle in climate records. It was already known that the model could mimic the 100,000 year cycle in the climate response, but it was not clear how this strong response was created. It is shown in this paper that the interference between the spectral components in the solar heating of 19,000 years and 23,000 years gives rise to an amplitude modulation, and how this modulation is converted by the nonlinearity to a 100,000 year cycle in the climate response.