Ice age

Lore & Background

His ideas were discussed by scientists in Sweden, Scotland, and Germany. The ice age theory gained full acceptance internationally in the second half of the 1870s, following James Croll's work, including 'Climate and Time' (1875).

Reader's Guide

The ice age concept fundamentally changed understanding of Earth's climatic history. The theory, developed through observations of erratic boulders, moraines, and striations, proposed that glaciers had once been far more extensive. Key figures included Pierre Martel, Jean-Pierre Perraudin, Jens Esmark, Karl Friedrich Schimper, and Louis Agassiz, who synthesized earlier work and coined the term 'ice age.' The theory faced initial skepticism because it contradicted the prevailing view of a gradually cooling Earth. Acceptance grew through geological fieldwork and the explanatory work of James Croll, who provided a credible orbital cause. The ice age framework now organizes Earth's recent climate history, with the current Late Cenozoic Ice Age containing the Quaternary glaciation, the Last Glacial Period, and the Holocene interglacial. Evidence includes geological features like moraines and erratics, chemical data from sediment and ice cores, and paleontological records. The discovery that glacials are long and interglacials short came from sediment and ice core analysis.

Did You Know?

• James Croll's 1875 book 'Climate and Time' provided a credible explanation for the causes of ice ages.

The Deep-Time Rhythm of Planetary Cooling

An ice age, in its broadest scientific sense, describes an extended stretch of planetary history during which surface and atmospheric temperatures fall low enough to sustain or expand continental glaciers and polar ice sheets. Earth's climate has oscillated between two broad states: a greenhouse mode with little or no permanent ice, and an icehouse mode where glaciers persist. For the majority of the planet's existence it has lived in the greenhouse configuration. The current icehouse phase, known as the Late Cenozoic Ice Age, began roughly 34 million years ago. Nested within this longer cooling are shorter cycles of glacials and interglacials. The Quaternary glaciation, a more recent sub-chapter, kicked in about 2.58 million years ago. Within it, the Last Interglacial closed its chapter 115,000 years ago, giving way to the Last Glacial Period. The coldest peak of that period, the Last Glacial Maximum, arrived between 26,000 and 20,000 years ago. The final cold snap, the Younger Dryas, ran from 12,800 to 11,700 years ago, after which the warm Holocene epoch began.

Mountain Folk and the First Hints of Ancient Ice

Long before formal glaciology existed, people living in and around the Alps and other mountain ranges had already pieced together a partial story. In 1742, the Geneva-based engineer and geographer Pierre Martel traveled to the Chamonix valley in Savoy and recorded, two years later, that local residents attributed scattered erratic boulders to glaciers that had once stretched much farther than they do today. Similar folk explanations surfaced across other Alpine regions and even in Goethe's scientific writings. In 1815, a chamois hunter named Jean-Pierre Perraudin in the Val de Bagnes pointed to deep rock striations and giant boulders as evidence of a former ice cover. Decades later, an unnamed woodcutter in Meiringen, Bernese Oberland, voiced a comparable idea to the geologist Jean de Charpentier. Even in the Chilean Andes, when the Bavarian naturalist Ernst von Bibra visited in 1849–1850, indigenous people explained fossil moraines as the legacy of ancient glaciers. These scattered, locally grounded observations formed a quiet undercurrent that would eventually reshape geology.

From Regional Speculation to a Global Theory

The scientific case for ancient glaciation evolved through a series of increasingly bold proposals. In 1742, the Swedish mining expert Daniel Tilas suggested drifting sea ice could account for erratic boulders in Scandinavia and the Baltic. In 1795, James Hutton invoked glacial action to explain Alpine erratics. A more sweeping idea arrived in 1818 when the Swedish botanist Göran Wahlenberg proposed that the Scandinavian peninsula had experienced a full glaciation, though he still treated it as a regional event. The Danish-Norwegian geologist Jens Esmark pushed the argument to a global scale in a 1824 paper, arguing for a sequence of worldwide ice ages driven by changes in Earth's orbit. He noted the striking resemblance between moraines near sea level in Rogaland and those at the branches of Jostedalsbreen. His ideas were subsequently discussed and partially adopted by Swedish, Scottish, and German scholars, including Albrecht Reinhard Bernhardi, who in 1832 speculated that polar ice caps had once reached temperate latitudes. Esmark's work, however, was later attributed or appropriated by Theodor Kjerulf and Louis Agassiz.

The Dam That Convinced a Skeptic

One of the most vivid episodes in the history of glacial science unfolded in the Val de Bagnes in 1818. The 1815 eruption of Mount Tambora had triggered the formation of a proglacial lake above the valley, held back by an ice dam that threatened a catastrophic flood. The engineer Ignatz Venetz joined Perraudin and the geologist Charpentier to inspect the site. Perraudin, already convinced that ancient ice had carved the valley, tried to persuade his companions, pointing to the deep striations and erratics that only glacial ice could produce. Charpentier, who had previously dismissed such explanations as absurd and favored a vast-flood hypothesis, remained unconvinced. But when the ice dam finally broke, the released water produced only minor erratics and no new striations — a result that could not be explained by flood action. Venetz concluded that Perraudin had been right all along. He read a prize-winning paper on the glacial theory to the Swiss Society in 1821, though it was not formally published until Charpentier, now himself converted, included it alongside his own more widely circulated paper in 1834.

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