Milankovitch Cycles and Glacial Cycles

How slow, regular changes in Earth's orbit redistribute sunlight and pace the advance and retreat of the great ice sheets over tens of thousands of years.

Definition

Milankovitch cycles are the periodic variations in Earth's orbital eccentricity, axial tilt, and precession that alter the distribution of incoming sunlight and pace the glacial cycles, the recurring growth and decay of continental ice sheets during the Quaternary.

Scope

This topic covers the astronomical theory of the ice ages and the glacial-interglacial cycles it explains. It treats the three orbital parameters, eccentricity, axial tilt, and precession, that vary on characteristic timescales and change the seasonal and latitudinal distribution of solar radiation, the spectral evidence from marine sediment cores that confirms their pacing of the ice ages, and the feedbacks involving ice, albedo, and greenhouse gases that amplify the orbital signal into full glacial cycles.

Core questions

• Which orbital parameters vary and on what timescales?
• How do orbital changes alter the distribution of sunlight?
• What evidence confirms that orbits pace the ice ages?
• What feedbacks amplify the orbital signal into full glacial cycles?

Key theories

Astronomical theory of the ice ages

Variations in eccentricity, tilt, and precession change high-latitude summer insolation, and when northern summers are cool enough that winter snow survives, ice sheets grow, pacing the glacial cycles.

Feedback amplification of orbital forcing

The relatively small orbital change in sunlight is amplified by ice-albedo and greenhouse gas feedbacks, so falling carbon dioxide and growing ice reinforce the cooling into a full glacial state.

Mechanisms

Gravitational interactions with other planets cause Earth's orbital eccentricity, axial tilt, and the precession of the equinoxes to vary over roughly 100,000, 41,000, and 23,000 years, changing how much sunlight reaches each latitude in each season. Cool northern summers allow snow to persist and ice sheets to grow; the brighter ice reflects more sunlight and the ocean and biosphere draw down carbon dioxide, amplifying the cooling, with the reverse driving deglaciation.

Clinical relevance

The orbital theory explains the natural timing of ice ages and shows that the present interglacial would, without human influence, persist for tens of thousands of years, providing a baseline against which human-caused change is measured.

History

Building on nineteenth-century ideas of Adhemar and Croll, Milankovitch computed the orbital insolation curves in the early twentieth century; the theory was largely confirmed in 1976 when Hays, Imbrie, and Shackleton found the predicted orbital frequencies in the spectra of deep-sea sediment records.

Debates

The 100,000-year problem

Why the late Quaternary ice ages are dominated by a roughly 100,000-year cycle, even though the eccentricity forcing at that period is weak, remains debated and points to the importance of nonlinear feedbacks.

Key figures

• Milutin Milankovitch
• James Hays
• John Imbrie
• Nicholas Shackleton

Seminal works

• hays1976
• imbrie1979

Frequently asked questions

What are Milankovitch cycles?

They are slow, regular changes in Earth's orbital shape, axial tilt, and precession that alter how sunlight is distributed across seasons and latitudes, pacing the ice ages.

If orbits drive ice ages, what about current warming?

Orbital changes act over tens of thousands of years and currently favor only gradual change, so they cannot explain the rapid recent warming, which is driven by greenhouse gases.