Ice age

An ice age is a prolonged interval in Earth’s climatic history marked by significant reductions in global temperatures and the growth or persistence of polar and continental ice sheets. During these intervals, alpine glaciers expand, sea levels fall and large portions of the Earth’s surface are reshaped by ice. Earth alternates between icehouse conditions, in which substantial glaciation is present, and greenhouse phases, when permanent ice is absent. The planet is currently within the Quaternary glaciation, an ongoing ice age that began over two million years ago. The present Holocene epoch represents merely an interglacial warm interval within this broader glacial framework.

Terminology and Climatic Context

Within an ice age, colder phases are known as glacial periods or glacials, during which ice sheets advance. Warmer episodes, known as interglacials or interstadials, separate individual glacial stages. Glaciology defines an ice age strictly by the existence of substantial ice sheets in both the Northern and Southern Hemispheres. Based on palaeoclimatic trends and current anthropogenic greenhouse gas accumulation, the onset of the next glacial stage is expected to be delayed significantly.

Early Observations and the Development of Ice Age Theory

The modern understanding of ice ages emerged gradually from observations made in the eighteenth and nineteenth centuries. In 1742 Pierre Martel documented claims by Alpine villagers that glaciers had once extended far beyond their contemporary limits. By the early nineteenth century similar explanations for erratic boulders and striated rocks were recorded throughout the Alps. Jean-Pierre Perraudin, a chamois hunter, drew attention to glacial features in Val de Bagnes; initially rejected by the geologist Jean de Charpentier, his explanations were later vindicated through field investigations.
Other observers contributed key insights. Daniel Tilas proposed drifting sea ice as a mechanism for the transport of erratics in Scandinavia, and James Hutton suggested glacial action in the Alps. By 1818 Göran Wahlenberg argued that Scandinavia had once been glaciated. Soon afterwards Jens Esmark proposed that global climate changes, driven by variations in Earth’s orbit, could account for multiple glaciations. His observations of moraines in Norway supported this early hypothesis.
During the 1830s scientists such as Karl Friedrich Schimper, Ignatz Venetz and Charpentier advanced ideas pointing towards extensive ice coverage in the past. Schimper coined the term Eiszeit (ice age) in 1837. Louis Agassiz, after collaborating with these researchers, presented a synthesis of glacial theory in 1837. His later work Études sur les glaciers (1840) popularised the idea of past glaciations, although disputes over priority and attribution accompanied its publication.
It was not until the 1870s, following the work of James Croll and his theory linking ice ages to orbital cycles, that glacial theory gained wide scientific acceptance.

Lines of Evidence for Past Glaciations

Multiple types of evidence support the reconstruction of ancient ice ages.
Geological EvidenceRock scouring, striations, U-shaped valleys, moraines, drumlins and glacial till provide tangible records of former glacial activity. Glacial erratics—large transported boulders resting far from their source—represent some of the earliest recognised indicators. However, successive glaciations often erode or obscure earlier deposits, complicating interpretation. Advances in stratigraphy and radiometric methods gradually allowed scientists to establish the true durations of glacials and interglacials.
Chemical EvidenceIsotopic analyses of marine and lacustrine sediments reveal variations in global temperatures and ice volume. Ratios of oxygen isotopes in microscopic shells record changes in evaporation and precipitation patterns. Ice cores drilled from Greenland and Antarctica contain layered chemical archives of temperature, greenhouse gas concentrations and atmospheric particulates. Bubbles trapped in the ice preserve samples of ancient air, offering detailed insights into past climate dynamics.
Palaeontological EvidenceFossil assemblages shift geographically in response to changing climate. During cold stages, species adapted to cooler conditions migrate towards lower latitudes or lower elevations, while warm-adapted species retreat. These biogeographical patterns provide indirect but powerful indicators of glacial extent and temperature fluctuations.

The Last Glacial Maximum

The Last Glacial Maximum (LGM), occurring approximately 26,000 to 19,000 years ago, was the period during which global ice sheets reached their maximum extent within the current ice age. Vast ice sheets covered much of northern North America, Scandinavia and northern Asia, while mountain glaciers expanded worldwide. Sea levels fell by more than one hundred metres, exposing continental shelves and reshaping coastlines.
Climatic conditions during the LGM were markedly colder and drier than today. Dust deposition increased, vegetation zones shifted southward and human populations adapted by migrating, developing new technologies and exploiting new ecological niches. In many regions the LGM left a profound geological legacy, carving valleys, depositing moraines and influencing drainage systems.
As the climate warmed after the LGM, ice sheets gradually retreated, initiating the transition towards the present interglacial. Meltwater pulses raised sea levels and altered ocean circulation patterns, contributing to the complex climatic oscillations of the late Pleistocene.