The Last Glacial Maximum: when ice sheets were largest

When was the Last Glacial Maximum?

A widely used synthesis places the LGM at approximately 26,500–19,000 years before present. Individual ice sheets and their margins did not all reach their maximum position simultaneously. The atlas opens at 21,000 BP as a convenient view within this interval, not the exact year of maximum ice everywhere.

References: Clark et al. (2009) — The Last Glacial Maximum

What changed on the map?

Large ice sheets covered much of Canada and northern Europe. Lower global sea level exposed continental shelves that are now submerged. The IPCC assesses global mean sea level during the LGM at about 125–134 metres below present; this global estimate is not a local shoreline elevation for every coast.

Exposed shelf and glacier ice are different things. An exposed region could be dry land without being covered by an ice sheet. Beringia is an important example of why those layers should be read separately.

References: Gowan et al. (2021) — A new global ice sheet reconstruction for the past 80,000 yearsIPCC (2021) — AR6 Working Group I, Chapter 9, Table 9.6USGS — Geology of Bering Land Bridge National Preserve

The end of the glacial period

The transition to the Holocene came later than the LGM. Its formal boundary is about 11,700 years before AD 2000, equivalent to about 11,650 years before the BP reference year 1950. The map uses BP dates and an approximate era label; the slider does not locate a sharply dated worldwide ice retreat.

References: International Commission on Stratigraphy — Holocene GSSP

Explore the interval

Compare 25,000, 20,000 and 15,000 BP. Keep the view fixed so you can separate changing ice margins from changes in the exposed shelf. When inspecting a point, read the model snapshot date as well as the selected date: the thickness data change in 2,500-year steps.