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Milankovitch cycles
worked out 1920s–1940s; confirmed 1976· Theory or idea· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
Slow, regular shifts in Earth's orbit and tilt that change how sunlight falls on the planet and pace the ice ages.
See it on the Life map →What happened
Earth's orbit stretches from nearly circular to slightly oval over about 100,000 and 400,000 years; its tilt rocks between about 22.1 and 24.5 degrees every 41,000 years; and a top-like axis wobble shifts which season falls nearest the Sun every 19,000 to 23,000 years. Serbian engineer and mathematician Milutin Milanković (1879–1958) calculated by hand how these cycles change summer sunshine in the far north. Cool northern summers let snow survive and ice sheets grow. James Croll had suggested an earlier version in the 1860s.
Why it matters
It showed that ice ages follow a predictable astronomical clock, and lets scientists date ancient sediments by counting the cycles recorded in them.
The evidence (fossils, genes, rocks)
James Hays, John Imbrie and Nicholas Shackleton matched the cycles to deep-sea sediment cores in 1976; Antarctic ice cores reaching back 800,000 years show the same rhythms in temperature and carbon dioxide.
Still debated
The '100,000-year problem': why ice ages switched from 41,000-year to roughly 100,000-year cycles about a million years ago, though the orbital push at that period is weak.
Connections (2)
▸Milankovitch cycles pace Pleistocene ice agesWell established
The connection
Ice sheets grow when northern summers are too cool to melt the winter's snow. Orbital cycles raise and lower summer sunshine at high northern latitudes, and feedbacks amplify the effect: ice reflects sunlight, and colder oceans absorb carbon dioxide. Over the last 2.6 million years ice ages came about every 41,000 years, later about every 100,000.
When and how
Milanković's calculations, from the 1920s to his 1941 'Canon of Insolation'; confirmed in 1976 by James Hays, John Imbrie and Nicholas Shackleton.
The evidence
Their paper, 'Pacemaker of the Ice Ages', found the predicted 100,000-, 41,000- and 23,000-year rhythms in deep-sea cores; ice cores show them too.
Debates and open questions
Why the rhythm switched from 41,000 to about 100,000 years around a million years ago, and exactly how the carbon dioxide feedback gets triggered.
▸Milankovitch cycles timed the warming that began End of the last ice age (Holocene begins)Strong evidence
The connection
After about 20,000 years ago, orbital shifts raised northern summer sunshine. Ice sheets began melting, carbon dioxide rose from about 190 to 280 parts per million, and warming spread worldwide, with setbacks such as the Younger Dryas cold snap. The Holocene, our current warm interval, began 11,700 years ago as that last cold spell ended.
When and how
Worldwide, about 20,000 to 11,700 years ago; the Holocene's start is officially defined in a Greenland ice core.
How sure are we?
Ice cores show carbon dioxide and temperature rising together; sea level rose about 120 meters as ice melted, recorded in drowned reefs and coastlines.
Debates and open questions
Why the Holocene has been so stable, and whether early farming's greenhouse gases helped keep the next ice age away, as William Ruddiman has argued.