Ice ages and glaciation

An ice age is any period in Earth's history during which large ice sheets persist over the poles and, during their colder phases, extend far into the mid-latitudes. By this definition Earth is currently in an ice age, because the Antarctic and Greenland ice sheets have been continuous features of the planet for millions of years. Within each ice age, climate oscillates between colder glacials, during which ice sheets advance, and warmer interglacials, during which they retreat. The present geological moment is an interglacial known as the Holocene. Understanding glaciation requires drawing together evidence from geology, geochemistry, and astronomy, and the story that emerges spans more than two billion years of planetary history.^{10, 24}

Louis Agassiz and the recognition of past glaciation

The scientific recognition that ice sheets once covered much of the Northern Hemisphere came gradually during the early nineteenth century, and it was the Swiss-American naturalist Louis Agassiz who synthesized the evidence into a coherent theory. In his landmark 1840 work Études sur les glaciers, Agassiz argued that the boulders scattered across the Swiss lowlands — erratics entirely foreign to the local bedrock — could only have been transported by glaciers on a continental scale.¹ He described the scratched and polished surfaces of bedrock outcrops beneath former glaciers, the ridges of debris (moraines) deposited at glacier margins, and the flat-bottomed, steep-walled valleys carved by glacial erosion. Although the idea initially met resistance from geologists who favored explanation by flooding, the physical evidence proved overwhelming within a generation.^{1, 22}

The sedimentary record of glaciation takes several forms that geologists use as diagnostic evidence. Glacial striations are parallel grooves scratched into bedrock by rocks frozen into the base of a moving glacier; their orientations reveal the former flow directions of ice. Erratics are boulders transported far from their source region and deposited when the ice melted; matching an erratic to its bedrock of origin allows geologists to reconstruct ice-flow paths over distances of hundreds of kilometers. Till is the unsorted mixture of clay, sand, pebbles, and boulders deposited directly by melting ice, in contrast to the well-sorted sediments laid down by meltwater streams. Ancient, lithified tills are called tillites, and their identification in Precambrian and Paleozoic rock sequences is the primary evidence for glaciations in the deep past.²²

Major glacial periods in Earth history

The geological record contains evidence for at least five major glacial eras, each separated by long intervals of warmer global climate. The oldest well-documented glaciation is the Huronian glaciation, which struck between approximately 2.4 and 2.1 billion years ago during the Paleoproterozoic era. Tillites of Huronian age are exposed across the Canadian Shield in Ontario, and correlative formations exist in South Africa and Australia. The Huronian event is likely connected to the Great Oxidation Event, when cyanobacterial photosynthesis flooded the atmosphere with oxygen and consumed the methane that had been warming the early Earth, precipitating a collapse in greenhouse forcing.⁷

The most dramatic glaciation in Earth's history occurred during the Cryogenian period, approximately 720 to 635 million years ago. Two separate intervals — the Sturtian (~720–660 Ma) and the Marinoan (~650–635 Ma) — left tillite deposits on virtually every craton, including those that were then positioned near the equator. The geochemist Paul Hoffman and colleagues proposed in 1998 that these glaciations represent a state in which the entire ocean surface froze over, a scenario they called Snowball Earth.⁶ The hypothesis explains both the global distribution of tillites and the peculiar cap carbonates — thick limestone layers deposited abruptly above the glacial sediments — which are interpreted as the product of massive CO2 buildup by volcanic outgassing during the frozen interval, followed by rapid greenhouse warming once ice retreated. The Snowball Earth hypothesis remains under active research and debate, with some workers proposing a softer "Slushball" state in which the tropical oceans remained partially open.⁶

A geologically briefer but biologically significant glaciation occurred during the Late Ordovician, approximately 445 million years ago, centered on Gondwana when the supercontinent lay over the south pole. Finn and colleagues demonstrated using oxygen isotope ratios in brachiopod fossils that seawater temperatures fell by at least 5°C as ice sheets grew across what is now northern Africa.⁸ The glaciation was associated with a severe drop in sea level and is linked to the Late Ordovician mass extinction, which eliminated approximately 85 percent of marine species.⁸

The Carboniferous-Permian glaciation (roughly 360–260 Ma) was the longest of the Phanerozoic ice ages and left an extensive tillite record across the Gondwanan continents of what are now South America, southern Africa, Antarctica, India, and Australia. This glaciation, sometimes called the Karoo Ice Age from the basin in South Africa where tillites are well exposed, coincided with a dramatic drawdown of atmospheric CO2 as vast coal swamp forests buried organic carbon across tropical Euramerica.^{9, 11} Atmospheric CO2 fell from values exceeding 1,000 parts per million early in the Carboniferous to near modern values during the glacial maximum, illustrating with unusual clarity the relationship between the carbon cycle and global temperature.⁹

The planet remained largely ice-free through most of the Mesozoic and early Cenozoic, but the progressive closure of the Tethys Ocean, the growth of the Himalayas, and other tectonic events gradually drew down CO2 through silicate weathering. By around 34 million years ago, at the Eocene-Oligocene boundary, the Antarctic ice sheet nucleated for the first time in the Cenozoic, an event recorded by an abrupt shift in deep-sea oxygen isotope ratios.^{12, 24} The opening of the Drake Passage between South America and Antarctica isolated the Southern Ocean thermally, reinforcing Antarctic glaciation. By 2.6 million years ago, ice sheets had grown across the Northern Hemisphere, marking the formal beginning of the Quaternary ice age.^{10, 24}

Milankovitch cycles and orbital forcing

The timing of glacial and interglacial intervals within the Quaternary ice age is not random; it follows periodicities that the Serbian mathematician Milutin Milankovitch calculated in the early twentieth century. Milankovitch recognized that three distinct variations in Earth's orbit and axial geometry combine to alter the distribution of solar radiation across latitude and season, without changing the total amount of energy the Earth receives from the Sun.²

The first cycle involves the eccentricity of Earth's orbit — the degree to which the orbit departs from a perfect circle. Eccentricity varies on two main periodicities, approximately 100,000 and 413,000 years, driven by gravitational interactions with Jupiter and Saturn. When eccentricity is high, Earth is closer to the Sun at one point in its orbit and farther at the opposite point, amplifying the seasonal contrast in that hemisphere. When eccentricity is low, the orbit is nearly circular and seasonal contrasts are muted.^{2, 3}

The second cycle involves the obliquity — the tilt of Earth's rotational axis relative to the plane of its orbit. Obliquity varies between roughly 22.1° and 24.5° over a period of approximately 41,000 years. Higher obliquity produces stronger seasonal contrasts at high latitudes: warmer summers that melt ice and colder winters that bring snow. Lower obliquity produces milder summers at high latitudes, reducing summer melting and allowing snow to persist year-round, which is the key condition for ice-sheet growth.^{2, 3}

The third cycle involves precession — the slow wobble of Earth's rotational axis, like a spinning top, with a period of approximately 26,000 years (combined with the simultaneous rotation of the elliptical orbit itself, the dominant climate-relevant period is approximately 23,000 years). Precession determines which hemisphere experiences summer when Earth is closest to the Sun. When the Northern Hemisphere has summer near perihelion (closest approach), northern summers are intensified; when it has summer near aphelion, northern summers are cooler. Because ice sheets grow primarily on the large northern land masses, the precession cycle has a strong influence on glacial inception.^{2, 3}

The most direct test of the Milankovitch theory came in 1976 when James Hays, John Imbrie, and Nicholas Shackleton analyzed oxygen isotope and microfossil records from deep-sea sediment cores spanning the last 450,000 years. They demonstrated that the dominant periodicities in the climate record — 100,000, 41,000, and approximately 23,000 years — match the eccentricity, obliquity, and precession cycles, providing what they called the "pacemaker" evidence for orbital forcing of ice ages.³

The three Milankovitch cycles and their properties^{2, 3}

A persistent puzzle, sometimes called the "100-kyr problem," is that the 100,000-year eccentricity cycle dominates the climate record of the last million years despite producing only a small change in total solar energy received. Before roughly one million years ago, the dominant glacial periodicity was 41,000 years (obliquity-dominated). The transition to 100,000-year cycles, known as the Mid-Pleistocene Transition, is thought to involve feedbacks between ice volume, bedrock erosion, and CO2, but the precise mechanism remains a subject of active research.²³

Ice core climate records

Among the most powerful archives of past climate are the ice cores extracted from the Antarctic and Greenland ice sheets. Snow falling each year traps tiny air bubbles as it compresses into ice, preserving samples of the ancient atmosphere. The chemical composition of the ice itself encodes past temperature: the ratio of heavy to light oxygen isotopes (oxygen-18 to oxygen-16, expressed as δ¹⁸O) in the ice is temperature-dependent, becoming more negative as temperature falls. By measuring both the gas in the bubbles and the isotope ratios in the surrounding ice, scientists can reconstruct past temperature and atmospheric composition at the same time.^{4, 5}

The Vostok ice core, drilled by a joint Russian-French-American team at Vostok Station in East Antarctica, was the first to extend the climate record back 400,000 years, spanning four glacial-interglacial cycles. The record showed that local Antarctic temperature and atmospheric CO2 concentration had varied in close parallel, with CO2 ranging from roughly 180 parts per million during glacials to 280–300 ppm during interglacials.⁴

The EPICA (European Project for Ice Coring in Antarctica) Dome C ice core, reported in 2004, extended the continuous record to 800,000 years before present, spanning eight complete glacial-interglacial cycles.⁵ This record confirmed and extended the Vostok findings, demonstrating that the CO2-temperature coupling is a robust feature of Quaternary climate across the full range of glacial intensities. Atmospheric CO2 concentrations remained within a narrow band of 180–300 ppm throughout the 800,000-year record — until the twentieth century, when anthropogenic emissions pushed the concentration above 400 ppm.⁵

Atmospheric CO2 across eight glacial cycles from the EPICA Dome C ice core⁵

An important question raised by the ice cores is the precise relationship between CO2 and temperature: does warming cause CO2 to rise, or does CO2 drive the warming? Detailed analysis by Shakun and colleagues in 2012 showed that during the last glacial termination, CO2 rose slightly after initial high-latitude warming driven by orbital forcing but before global mean temperature peaked, consistent with CO2 acting as an amplifying feedback that helped spread deglacial warming globally.²¹ This finding does not contradict the Milankovitch hypothesis; rather, it clarifies that orbital forcing initiates the change while the CO2 feedback amplifies and globalizes it.^{21, 13}

Rapid climate events within glacials

Superimposed on the slow orbital pacing of glacial cycles are shorter, abrupt climate oscillations documented primarily in Greenland ice cores and North Atlantic sediment records. Dansgaard-Oeschger events are rapid warmings in Greenland temperature — sometimes exceeding 10°C within decades — followed by gradual cooling over centuries to millennia, and then an abrupt return to cold conditions. The original Greenland GRIP ice core identified more than twenty such cycles during the last glacial period, with a dominant spacing of approximately 1,470 years.¹⁶ The mechanism is thought to involve reorganizations of Atlantic Ocean circulation, specifically changes in the formation of North Atlantic Deep Water, which transports heat northward.¹⁶

Heinrich events are episodic discharges of icebergs from the Laurentide Ice Sheet into the North Atlantic, first described by Hartmut Heinrich in 1988 from layers of ice-rafted debris in North Atlantic sediment cores.¹⁵ These events, which occurred approximately six times during the last glacial cycle at intervals of 7,000–10,000 years, temporarily flooded the North Atlantic with fresh water, disrupting the thermohaline circulation and triggering rapid cooling across the North Atlantic region. Heinrich events are recognized as important drivers of millennial-scale climate variability and correlate with major reorganizations of atmospheric circulation as far away as East Asia and the tropical Americas.^{15, 16}

Glacial geomorphology and landscape change

The physical imprint of glaciation on Earth's surface is profound and pervasive across formerly glaciated regions. Moving ice acts as an extraordinarily powerful agent of erosion, transport, and deposition. In upland areas, glaciers carve bowl-shaped hollows called cirques at their heads and produce the characteristic U-shaped valleys that distinguish glacially eroded landscapes from the V-shaped valleys cut by rivers. Where several cirques erode toward each other from different directions, the intervening ridgeline is sharpened into an arête, and if erosion proceeds from three or more directions, an isolated pyramidal peak called a horn is produced. The Matterhorn in the Swiss Alps is a classic example.²²

At the margins of former ice sheets, moraines mark the positions where the ice front paused during retreat. Terminal moraines mark the maximum advance of a glacier; recessional moraines mark temporary pauses during retreat; and lateral moraines parallel the flanks of former valley glaciers. Across the lowlands of North America and Europe, the continental ice sheets deposited vast sheets of till, reshaped the courses of rivers, and created the present drainage patterns of the Great Lakes and the river systems of the northern plains.²² Wind-blown silt, called loess, was deflated from glacial outwash plains and redeposited in thick sheets across wide areas, creating some of the most fertile soils in the world, including the Eurasian steppes and the American Midwest.²⁰

Sea level, post-glacial rebound, and biogeography

At the peak of the Last Glacial Maximum, approximately 21,000 years ago, global sea level stood roughly 120–130 meters below its present value.^{18, 19} This dramatic lowering of the oceans exposed vast continental shelves, creating land bridges that had major consequences for biogeography and human prehistory. The Bering Land Bridge (Beringia) connected Asia to North America; Britain and Ireland were joined to continental Europe; the Indonesian archipelago was partially fused into a larger land mass (Sundaland); and Australia, Tasmania, and New Guinea formed a single continent known as Sahul. These connections allowed the dispersal of animals and humans into regions that had previously been inaccessible.¹⁹

As ice sheets melted during the Holocene, returning vast quantities of water to the oceans, sea level rose at rates of up to 1–2 centimeters per year during the most rapid phases of deglaciation, drowning the exposed shelves and isolating previously connected populations.^{14, 18} The removal of the weight of ice sheets also caused the underlying crust to slowly rebound upward — a process called post-glacial rebound (or isostatic rebound). Scandinavia and Hudson Bay, which were depressed under kilometers-thick ice, are still rising at rates of up to 1 centimeter per year today, thousands of years after the ice disappeared.¹⁷ Ancient shorelines are found at elevations hundreds of meters above present sea level in formerly glaciated regions, recording the combined effects of ice unloading and changing ocean volumes.¹⁷

Approximate sea-level change since the Last Glacial Maximum (~21,000 years ago)^{18, 19}

The CO2–temperature feedback

The ice core and sediment records make clear that glacial-interglacial climate swings involve a tight coupling between temperature and atmospheric CO2, but understanding the direction and magnitude of causality has required careful analysis. Orbital forcing alone — the changes in insolation driven by Milankovitch cycles — is insufficient to explain the full amplitude of glacial-interglacial temperature change, which reaches 4–6°C in global mean temperature and 8–12°C in Antarctica.¹³ The additional amplification comes from a cascade of positive feedbacks: as the planet warms, ice-albedo feedback reduces the fraction of sunlight reflected back to space; warming oceans outgas CO2 and methane; changes in vegetation alter surface properties; and the additional greenhouse warming drives further melting.²¹

During glacial inceptions, the sequence runs in reverse: orbital cooling reduces summer insolation at high northern latitudes, allowing perennial snow cover to develop, which raises albedo and promotes further cooling, which drives CO2 into the ocean through increased solubility and biological productivity changes, which reduces the greenhouse effect and amplifies the cooling.^{13, 21} This system is inherently self-reinforcing within each direction of change, which is why ice ages, once initiated, tend to amplify well beyond what the orbital forcing alone would predict. The same physics that explains past glacial cycles is central to understanding the sensitivity of the climate system to present-day changes in greenhouse gas concentrations.²¹

During the Carboniferous-Permian glaciation, the relationship between CO2 and climate is particularly direct: the burial of organic carbon in vast coal swamps removed CO2 from the atmosphere on a geological timescale, providing a natural experiment in which declining greenhouse gas concentrations tracked the onset and intensification of glaciation over tens of millions of years.⁹ Conversely, the long ice-free intervals of the Mesozoic corresponded to CO2 concentrations estimated at 1,000–2,000 ppm, maintained by elevated volcanic activity, and sustained global temperatures 8–14°C warmer than today.^{9, 11}

The Quaternary glaciations in detail

The Quaternary ice age, formally defined as beginning 2.588 million years ago at the base of the Gelasian Stage, encompasses the Pleistocene epoch (2.588 Ma to 11,700 years ago) and the Holocene (11,700 years ago to present).¹⁰ During the Pleistocene, ice sheets advanced and retreated repeatedly across North America and Eurasia. In North America the Laurentide Ice Sheet, at its maximum, extended as far south as the present position of Long Island, southern Ohio, and the Missouri River, reaching thicknesses of 3–4 kilometers over Hudson Bay. The Cordilleran Ice Sheet covered western Canada and extended into the northwestern United States. In Eurasia, the Fennoscandian Ice Sheet covered Scandinavia and the British Isles, while the Siberian and Barents Sea sheets occupied much of northern Russia and the Arctic shelf.^{22, 14}

Before the Mid-Pleistocene Transition approximately one million years ago, glacial cycles followed the 41,000-year obliquity rhythm, with ice sheets growing and shrinking relatively modestly. After the transition, cycles became longer, more asymmetric, and more intense: ice sheets now grew slowly over ~90,000 years before collapsing in relatively rapid terminations spanning 10,000–15,000 years, with the dominant period shifting to ~100,000 years.²³ The last glacial maximum, approximately 21,000 years ago, was the most recent peak of this cycle. Deglaciation began around 19,000–18,000 years ago and was completed, at least in the Northern Hemisphere, by roughly 7,000 years ago.¹⁴

The Holocene, the current interglacial, has been the warmest sustained interval since the last interglacial roughly 125,000 years ago. The Antarctic and Greenland ice sheets have persisted throughout, which is why geologically the Earth remains in an ice age even today. Natural orbital projections, extrapolated from the pattern of previous cycles, suggest that the current interglacial might extend another 50,000 years before the next glacial inception — but anthropogenic CO2 emissions have added a forcing to the climate system with no analog in the 800,000-year ice core record, making confident extrapolation of the natural glacial cycle increasingly uncertain.^{5, 21}