A simple rule to determine which insolation cycles lead to interglacials

The pacing of glacial–interglacial cycles during the Quaternary period (the past 2.6 million years) is attributed to astronomically driven changes in high-latitude insolation. However, it has not been clear how astronomical forcing translates into the observed sequence of interglacials. Here we show that before one million years ago interglacials occurred when the energy related to summer insolation exceeded a simple threshold, about every 41,000 years. Over the past one million years, fewer of these insolation peaks resulted in deglaciation (that is, more insolation peaks were ‘skipped’), implying that the energy threshold for deglaciation had risen, which led to longer glacials. However, as a glacial lengthens, the energy needed for deglaciation decreases. A statistical model that combines these observations correctly predicts every complete deglaciation of the past million years and shows that the sequence of interglacials that has occurred is one of a small set of possibilities. The model accounts for the dominance of obliquity-paced glacial–interglacial cycles early in the Quaternary and for the change in their frequency about one million years ago. We propose that the appearance of larger ice sheets over the past million years was a consequence of an increase in the deglaciation threshold and in the number of skipped insolation peaks. A simple model, based on only summer insolation energy and time since the previous deglaciation, correctly predicts the deglaciation history of the past 2.6 million years, including the change in frequency of glacial–interglacial cycles about one million years ago. The timing of Pleistocene interglacials The existence of warm periods, or interglacials, in the generally cold climate of the Pleistocene epoch is well known from multiple lines of evidence. The timing of changes to the amount of solar radiation is regulated by small variations in Earth–Sun geometry. But a firm explanation for the timing of interglacials, and for the apparent changes in orbital configurations required to trigger them, has remained elusive. Chronis Tzedakis et al . now present a simple statistical model to predict interglacials on the basis of a threshold of summer insolation. The model successfully predicts the shift from glacial cycles about every 41,000 years to every 100,000 years one million years ago, and the increased likelihood of deglaciations as a function of time since the previous interglacial.