Ice Sheets in Transition

It is now well known that periodic variations in ice-age climate correspond to variations in Earth's orbit around the Sun (1, 2). However, some notable aspects of this climate variability appear to have little or no relation to orbital forcing. One such example is the middle Pleistocene transition (MPT), which began at some time after 1.25 million years ago. Before the MPT, climate was dominated by low-amplitude 41,000-year cycles; over the course of the MPT, these were replaced by dominant high-amplitude 100,000-year cycles in the absence of any change in orbital forcing (see the first figure) (3). On page 704 of this issue, Elderfield et al. (4) provide important new insights into ice-age variability and the origin and timing of the MPT.