Around 20 thousand years ago, the Earth began to emerge from the grip of the Last Glacial Maximum (LGM). Huge ice sheets had locked up so much water that sea level was then about 125 m lower than it is today. At 12,870 years ago the warming and sea-level rise were reversed for 1,170 years in the Northern Hemisphere: an episode of near-full glacial conditions known as the Younger Dryas (YD). The adjectives ‘sudden’ or ‘abrupt’ grossly understate the pace of initial cooling – 3°, 6° and 15° C in North America, Europe and Greenland, respectively. Isotopic evidence from Greenland ice cores suggest that the cooling took place over three years or less. Such a degree of precision stems from the continuous annual layering in the Greenland ice cap. As far as humans were concerned, this would have been catastrophic for hunter gatherers following game northwards in Eurasia and North America as conditions ameliorated during the seven thousand years since the LGM. The archaeological record, or rather the lack of one, for what are now temperate zones suggests humans either retreated south or were blotted out.

There is no counterpart for the YD in the end stages of early glacial episodes. Some authors have suggested that it was the outcome of an appropriately catastrophic geological event, such as a large meteorite strike, as proposed in 2007 (See: Whizz-bang view of Younger Dryas; July 2007). This hypothesis gained traction in 2013, at least for its authors, with the discovery of anomalously high concentrations of the noble metal platinum (Pt) and other platinum Group metals, such as iridium (Ir) at or around the start of the YD in the GISP2 ice core. New research on this anomaly (Green, C.E. et al 2025. A possible volcanic origin for the Greenland ice core Pt anomaly near the Bølling-Allerød/Younger Dryas boundary. PLOS One, v. 20, article  e0331811; DOI: 10.1371/journal.pone.0331811) offers a different scenario. Charlotte Green of Royal Holloway, University of London and colleagues from universities in the UK, Germany and Austria examine the timing of this Pt spike and its detailed geochemistry.

The ‘killer’ observation is that the anomaly occurs in ice that formed 45 years after the onset of the Younger Dryas and has a spread of about 14 years. Whatever kind of event released the platinum, it definitely did not somehow trigger the onset of the YD. Moreover, the anomaly was significantly deficient in iridium compared with a wide range of meteorites and terrestrial igneous rocks. It also differed markedly in other elements, such as lutetium and hafnium, and in all three elements in melt rocks and ejecta sediments associated with five proven impact structures. The closest match is to volcanic gas condensates from a recent eruption of a submarine volcano near Tonga

Both the GISP2 and NGRIP cores through the Greenland ice also record a large, 12-year long spike in sulfate of volcanic origin spread across the very start of the YD. That roughly matches the age of an explosive eruption, which formed the circular Laacher See in the Eifel volcanic field in Germany. That eruption is thought to have blasted 6.3 km³ of highly alkaline magma into the atmosphere: about the magnitude of the 1991 Pinatubo eruption, but insufficient to yield the size and duration of the sulfate spike that coincides with the start of the YD. The sulfate anomaly suggests a far larger, currently unknown eruption at 12,870 years ago. The Pt and Ir data from the Laacher See event rule it out as a source for the younger Pt anomaly in the GISP2 ice core. One possibility is a nearby Icelandic subglacial fissure eruption at that time.

So, as regards what started the Younger Dryas, there is support for a very large, but so-far unknown volcanic event, and an as yet unresolved, perturbation in the Atlantic Meridional Overturning Circulation (AMOC) resulting from drainage of a huge glacial lake in northern North America (see: The Younger Dryas and the Flood; June 2006), but no support whatever for an impact event. Climatology of the distant past is always likely to be difficult to pin down. That is because, as now, it involves linkages between a large number of variables: not only physical ones, but issues of biogeochemistry, the inner Earth, the rest of the solar system and even cosmology. That is, it is as complex as human affairs and their history. Common sense, linear thinking and the like, simply will not do.

See also: Scientists solve 12,800-year-old climate mystery hidden in Greenland ice. Science Daily, 20 March 2026