Ice age

The discovery that glaciers had moved

Into the 18th century, huge boulders scattered across Alpine valleys were thought to have been carried by Noah's flood. In 1837 the Swiss-born naturalist Louis Agassiz proposed that these erratics and scratched bedrock were traces of far larger glaciers — an "Eiszeit", or ice age — and set out the case in his 1840 Études sur les glaciers. In the 1870s the Scottish scientist James Croll suggested that changes in Earth's orbit triggered ice ages, and the Serbian engineer Milutin Milanković, who began his hand calculations while interned during the First World War, completed his curves of sunlight by latitude in 1941. In 1896 Svante Arrhenius, trying to explain ice ages, made the first calculation that less carbon dioxide cools the planet and more warms it.

Orbital cycles and carbon dioxide

Milanković's theory holds that the shape of Earth's orbit (about 100,000 years), the tilt of its axis (about 41,000 years) and the wobble of that axis (about 23,000 years) combine to change how much summer sunshine reaches the northern hemisphere. Cool summers let winter snow survive and ice sheets grow. The theory was long doubted until Cesare Emiliani used oxygen isotopes in foraminifera shells to read past temperatures in 1955, and in 1976 James Hays, John Imbrie and Nicholas Shackleton found exactly these cycles in deep-sea sediments — the "pacemaker of the ice ages". The Vostok ice core from Antarctica, published in 1999, showed carbon dioxide and temperature rising and falling together over 420,000 years: orbital changes pull the trigger, and greenhouse gases and reflective ice amplify the change. The starting point is the pull of other planets in the solar system on Earth's orbit.

The last glacial period and Snowball Earth

At the Last Glacial Maximum about 20,000 years ago, ice kilometres thick covered North America and northern Europe, and sea level was about 120 m lower, joining Britain to Europe and Siberia to Alaska. Mammoths and woolly rhinos roamed, and people crossed the Bering land bridge into the Americas. About 11,700 years ago the warm Holocene interglacial began and farming spread. Because ice sheets still cover Antarctica and Greenland, geologists say we live in an interglacial within the Quaternary ice age that began about 2.6 million years ago. Much earlier, around 700 million years ago, Earth may have frozen to the equator — the "Snowball Earth" hypothesis advanced by Paul Hoffman and colleagues in 1998. Today's human greenhouse emissions are large enough to delay the next glacial period by tens of thousands of years, and are warming the planet far faster than any end-of-glacial warming. The disappearance of ice-age animals links to research on mass extinctions.

FAQ

When will the next ice age come?

Orbital calculations already put it tens of thousands of years away, and many studies find that human carbon dioxide will delay the next glacial period by at least 50,000 years, perhaps over 100,000.

Was the whole Earth covered in ice during the ice age?

No. Even in the last glacial period, ice sheets were mainly at high latitudes and in mountains; the tropics were only a few degrees cooler and drier. A fully frozen "Snowball Earth" happened hundreds of millions of years ago.

Sources reviewed: 2026-10-26

References

1. Agassiz, L. (1840). Études sur les glaciers. Neuchâtel: Jent et Gassmann.
2. Croll, J. (1875). Climate and Time in Their Geological Relations. London: Daldy, Isbister & Co.
3. Arrhenius, S. (1896). On the influence of carbonic acid in the air upon the temperature of the ground. Philosophical Magazine, 41(251), 237–276.
4. Milanković, M. (1941). Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitenproblem. Belgrade: Royal Serbian Academy.
5. Emiliani, C. (1955). Pleistocene temperatures. Journal of Geology, 63(6), 538–578.
6. Hays, J. D., Imbrie, J., & Shackleton, N. J. (1976). Variations in the Earth's orbit: Pacemaker of the ice ages. Science, 194(4270), 1121–1132.
7. Hoffman, P. F., Kaufman, A. J., Halverson, G. P., & Schrag, D. P. (1998). A Neoproterozoic snowball Earth. Science, 281(5381), 1342–1346.
8. Petit, J. R., et al. (1999). Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature, 399(6735), 429–436.
9. Lisiecki, L. E., & Raymo, M. E. (2005). A Pliocene-Pleistocene stack of 57 globally distributed benthic δ¹⁸O records. Paleoceanography, 20(1), PA1003.
10. Clark, P. U., et al. (2009). The Last Glacial Maximum. Science, 325(5941), 710–714.