Terminations are the periods when large Northern Hemisphere (NH) ice sheets collapse, propelling the Earth’s climate from a glacial to an interglacial state. The idea that these transitions are driven by variations in Earth’s orbital geometry arose almost 200 years ago, and was integrated into the astronomical theory of ice ages by Milankovitch in the early 20th century.

Studies of ocean sediments, which best preserve terminations, largely support the theory, but nagging questions remain, including: which orbital parameter is most important? The biggest obstacle is chronology: ocean sediments cannot be adequately dated beyond the last 40,000 years. Improvements in geochronology and our understanding of spatial responses to terminations have enabled speleothems to be used to indirectly date ocean sediments. A landmark study in 2016 used Chinese speleothems to date terminations since 640,000 years ago. It suggested they are paced by Earth’s climatic precession, which controls when NH summer insolation is sufficiently intense to melt the ice sheets.

We recently dated two older terminations from Italian speleothems, which suggest pacing by changes in Earth’s tilt angle (obliquity). After re-assessing all radiometrically dated terminations of the last million years, we find that changes in Earth’s obliquity exerts equal, if not greater, influence on both termination timing and duration. The results suggest that a NH summer energy metric that combines ~equal contributions from obliquity and precession best explains termination timing.