Large, abrupt changes between warm and cold modes of North Atlantic climate exhibit spectral power at ~1500 years, yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic. We hypothesize that a weak periodic forcing has combined with noise from ice sheet-related events to cause the observed mode switches. This stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models. The ice isotopic data from central Greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested. We thus support arguments for the existence of a periodicity of ~1500 years in the North Atlantic climate system and for the importance of ice sheet events in forcing North Atlantic changes. ¿ 2001 American Geophysical Union