The Younger Dryas plunged Greenland back into deep cold in three years or less
About 12,900 years ago, as the last ice age was ending, the North Atlantic world lurched back into the cold. Greenland's annual temperatures fell by up to 10 degrees Celsius, a drop that may have taken three years or less. About twelve centuries later the chill ended almost as abruptly as it had begun.
The episode is named for Dryas octopetala, a small white alpine wildflower that thrives only in glacial conditions. Early twentieth-century researchers studying Swedish and Danish bogs, notably the Allerød clay pit in Denmark, found its pollen suddenly taking over layers where forests had grown during the warm spell just before. In Britain the same interval is called the Loch Lomond Stadial.
Cooling was lopsided. Europe lost 2 to 6 degrees, British lowlands developed permafrost and glaciers returned to upland Britain, the Alps and the Rockies, while the southeastern United States grew warmer and wetter. Winters turned much colder, yet summers barely changed or even warmed slightly. Meanwhile the Southern Hemisphere warmed, with ice retreating in Antarctica, South America and New Zealand, so the net global change was a modest cooling of about 0.6 degrees, driven mainly by expanding sea ice reflecting sunlight.
That see-saw between hemispheres is the key clue. The Atlantic's overturning circulation carries heat from south to north; when it weakens, the north chills and the south warms. The consensus is that a severe weakening caused the Younger Dryas. Why it weakened is debated. The long-favoured idea, a flood of cold fresh water from North America's glacial Lake Agassiz, sits awkwardly with the lack of sea level rise at the time, and ice cores and cave deposits confirm unusually heavy volcanic activity just before the onset, pointing to an eruption as a possible trigger.
Shifting winds dried Scotland, Anatolia and the Sahara while wetting northern China. When the circulation recovered, previously cooled regions warmed back within 50 to 60 years, and carbon dioxide, which had risen from about 210 to 275 parts per million during the cold spell, helped carry the planet into the Holocene.
Source: Younger Dryas
