A slowing of ocean circulation in the waters surrounding Antarctica drastically altered the strength and more than doubled the length of global ice ages following the mid-Pleistocene transition, a new study finds. For the last several million years, the natural cycle of Earth's climate has been dominated by the regular ebb and flow of glacial and interglacial periods. These cycles are generally thought to have largely kept pace with our wobbling planet's elliptical path around the sun. Variation in incoming solar radiation due to each orbital period greatly influences Earth's climate, including the strength and duration of glacial cycles. However, during the mid-Pleistocene transition - a period occurring between roughly 1.25 million and 700,000 years ago - the tempo of glacial cycles drastically changed. Glacial cycles became colder and longer - extending from 41,000 years in length to nearly 100,000. The mid-Pleistocene transition (MPT), which has no obvious orbital cause, remains largely unexplained. What's more, understanding the characteristics of the MPT in detail is problematic as few available paleorecords span the transition in detail. Adam Hasenfratz and colleagues present a new 1.5 million-year record of temperature and salinity in the Southern Ocean. Using trace elements and oxygen isotopes locked inside the microscopic shells of foraminifera, a species of single-celled planktonic animals, Hasenfratz et al. evaluated changes in Southern Ocean circulation across the MPT. Evidenced by a reduction in deep water supply and a freshening of surface water, the results suggest that the emergence of the 100,000 year-cycle coincided with increased ocean stratification and reduced deep-ocean ventilation - where CO2 laden deep water is transported to the surface and released into the atmosphere. As a result, atmospheric CO2 was reduced, allowing glacial periods to persist despite the orbitally paced drivers. In a related Perspective, Laurie Menviel discusses the study's findings in more detail.