The repeated advance and retreat of ice sheets in North America and globally occurred as part of glacial–interglacial cycles, during which climate alternated between colder and warmer states. These were primarily driven by Milankovitch cycles1,2, which describe periodic changes in Earth’s orbit and orientation relative to the Sun, particularly affecting summer solar radiation in the Northern Hemisphere.
Although changes in solar radiation associated with orbital forcing are relatively small, their climatic effects were amplified by feedbacks within the climate system3, including changes in ice cover, greenhouse gas concentrations, and ocean circulation. Together, these processes created the conditions for repeated ice-sheet growth and decay.
100,000 year cycles
Over the past million years, glacial–interglacial cycles have followed a dominant pattern of roughly 100,000 years5. However, the climate is never completely stable, and even within glacial periods there were fluctuations between colder stadial phases and relatively warmer interstadial phases.
These long- and short-term changes are captured in the Marine Isotope Stage (MIS) framework, a global climate record based on variations in oxygen isotopes preserved in deep-sea sediments, in which even-numbered stages correspond to colder stadial or glacial conditions and odd-numbered stages correspond to warmer interstadial or interglacial conditions5.
We are currently in an interglacial period known as the Holocene, which began around 11,700 years ago and corresponds to MIS 1.
More Information
NASA – Milankovitch Cycles
NASA – Why Milankovitch Cycles can’t explain current warming
Nature – Milankovitch cycles
About the Author
Jakob Hamann is an IAPETUS PhD student at Newcastle University, studying the former Cordilleran Ice Sheet.
