The southern amplifier

Earth's natural climate variability of the past ∼2.6 million years has been dominated by glacial-interglacial cycles. These cycles are paced by variations in incoming solar radiation due to changes in Earth's orbit around the Sun. Feedbacks in the Earth system, including variations in greenhouse gas concentrations and the growth and retreat of Northern Hemispheric ice sheets, amplify the effects of the insolation changes. The characteristics and amplitude of the cycles changed fundamentally at the mid-Pleistocene transition, between 1.25 million and 700,000 years ago. On page 1080 of this issue, Hasenfratz et al. (1) report that this transition was associated with increased Southern Ocean stratification and reduced upwelling during glacial times.