A glacial period is a geologic interval of sustained cooling within an ice age during which glaciers and ice sheets expand over land, alternating with warmer interglacials. In Quaternary Earth history, these cycles dominated the Pleistocene Epoch and shaped modern landscapes across mid- and high latitudes. According to Encyclopaedia Britannica, a glacial period (or glacial stage) is a cold episode of an ice age, contrasted with interglacial stages of relative warmth. Britannica.

Temporal framework

The most recent glacial period in the Northern Hemisphere began about 115,000 years ago and ended at 11,700 years ago with the onset of the Holocene interglacial, a span often regionally termed Weichselian (northern Europe) or Würm (Alps). Britannica; Britannica.

Orbital pacing and climate drivers

Glacial–interglacial cycles are paced primarily by variations in Earth’s orbit—eccentricity (~100 kyr), axial tilt (obliquity; ~41 kyr), and precession (~23 kyr)—which modulate the seasonal and latitudinal distribution of incoming solar radiation. This Milankovitch framework is supported by classic spectral analyses of deep-sea cores and ice-core records. NASA Science; Science (Hays, Imbrie & Shackleton, 1976); Paleoceanography (Lisiecki & Raymo, 2005).

The Last Glacial Maximum (LGM)

The Last Glacial Maximum marks the time of greatest global ice volume and ice-sheet extent during the most recent glacial period. Britannica places the LGM at roughly 29,000–19,000 years ago, with permanent summer ice covering about 8% of Earth’s surface and sea level approximately 125 m below present. Britannica. Assessments by the IPCC conclude that global mean surface temperature during the LGM (around 23–19 ka) was about 5–7°C colder than 1850–1900. IPCC AR6 WG1, Chapter 2. The U.S. Geological Survey similarly reports a sea-level low stand on the order of ~125 m at the LGM. USGS.

Ice sheets and geographic extent

During the LGM, large ice sheets included the Laurentide Ice Sheet over North America and the Fennoscandian ice sheet over northern Europe, among others. The Laurentide reached thicknesses of several kilometers and extended into the mid-latitudes of the United States. Britannica. Lower sea level exposed continental shelves and land bridges, such as the Bering Land Bridge connecting Asia and North America. USGS – Bering Land Bridge Geology; NPS.

Atmospheric composition and feedbacks

Ice-core records from Antarctica show atmospheric CO₂ ranged roughly 172–300 ppm over the last 800,000 years, with minima during glacials and maxima during interglacials. These greenhouse gas variations acted as powerful feedbacks amplifying orbitally paced temperature change. Nature (Lüthi et al., 2008). High-resolution records demonstrate that CO₂ rose by ~80 ppm from the LGM into the early Holocene, consistent with deglacial warming. Geophysical Research Letters.

Abrupt variability within glacials

Glacial climates were punctuated by abrupt events. Millennial-scale Dansgaard–Oeschger (D–O) oscillations involved rapid warmings over Greenland and reorganizations of atmospheric circulation recorded globally in caves and other proxies. PNAS; Britannica. Heinrich events reflect massive iceberg discharges into the North Atlantic, altering ocean salinity and circulation. Quaternary Science summaries; Overview. Near the end of the last glacial period, the Younger Dryas stadial (≈12,900–11,600 years ago) produced rapid Northern Hemisphere cooling before abrupt termination at the onset of the Holocene. Britannica.

Sea level and exposed landscapes

Global sea level fell by roughly 120–125 m at the LGM, exposing continental margins and altering coastlines worldwide; subsequent deglaciation drove a multi-millennial sea-level rise to near-modern values. USGS. These changes connected and then re-isolated biogeographic regions, including the exposure and later inundation of Beringia. USGS – Bering Land Bridge Geology; NPS.

Landforms and sediments

Glacial erosion and deposition during glacial periods created characteristic landforms: moraines (ridges of till at former ice margins), drumlins (streamlined hills of till), eskers (sinuous ridges of meltwater-laid sand and gravel), and widespread ground moraine. Britannica – Moraine; Britannica – Drumlin; Britannica – Glacial landform. Windblown silt (loess) sourced from glacial outwash plains accumulated downwind of ice margins, forming thick deposits such as the Loess Hills of the central United States. USGS.

Evidence and methods

Multiple archives document glacial periods and their variability: marine isotope records of benthic foraminifera that track ice volume and deep-ocean temperature (e.g., the LR04 stack), ice cores with greenhouse gases and stable isotopes, terrestrial geomorphology, and sediments. The LR04 stack aligns 57 globally distributed δ18O records over 5.3 Myr and demonstrates strong coherence with orbital cycles. Paleoceanography (Lisiecki & Raymo, 2005). The “Pacemaker of the Ice Ages” study linked paleoclimate spectra to orbital periods, establishing the foundation for Milankovitch pacing. Science (Hays, Imbrie & Shackleton, 1976). Modern syntheses quantify the magnitude of LGM cooling using global proxy compilations and data assimilation. IPCC AR6 WG1, Chapter 2.

Postglacial responses and crustal adjustment

After deglaciation, formerly ice-loaded regions continue to experience Isostatic rebound (glacial isostatic adjustment), the viscoelastic uplift of the crust and complementary subsidence of peripheral forebulge regions (e.g., the U.S. mid-Atlantic). NSIDC; NOAA National Geodetic Survey. This ongoing solid Earth response influences relative sea level, coastal evolution, and geodetic reference frames. NOAA National Geodetic Survey.

Terminology and regional naming

The most recent glacial period carries regional names reflecting local stratigraphic traditions: Weichselian (northern Europe), Würm (Alpine region), and Wisconsin (North America). These refer to the same global interval of maximum ice volume and attendant climatic conditions. Britannica; Britannica.

Context in Earth history

Glacial periods recur during long ice ages, with the latest spanning the Quaternary since ~2.6 million years ago. The alternation of glacials and interglacials is a fundamental rhythm of Quaternary climate, driven by orbital forcing and amplified by feedbacks involving greenhouse gases, ice–albedo, and ocean circulation. NASA Science; IPCC AR6 WG1, Chapter 2.