The Younger Dryas: Extreme climate change just 12.000 years ago
The Younger Dryas is one of the best known examples of abrupt climate change. About 14.500 years ago, the Earth's climate began to shift from a cold glacial world to a warmer interglacial state. Partway through this transition, temperatures in the Northern Hemisphere suddenly returned to near-glacial conditions. This near glacial period is called the Younger Dryas, named after a flower (Dryas octopetala) that grows in cold, artic conditions and became common in Europe during this time. The end of the Younger Dryas, about 11.500 years ago, was particularly abrupt. In Greenland, temperatures rose 10°C in about a decade.
Temperature fluctuations over the past 17.000 years showing the abrupt cooling during the Younger Dryas. The late Pleistocene cold glacial climate that built immense ice sheets terminated suddenly about 14.500 years ago, causing glaciers to melt dramatically. About 12.800 years ago, after about 2000 years of fluctuating climate, temperatures plunged suddenly and remained cool for 1300 years. About 11.500 years ago, the climate again warmed suddenly and the Younger Dryas ended.
The Younger Dryas interruption of the global warming that resulted in the abrupt, wholesale melting of the huge late Pleistocene ice sheets was first discovered in European pollen studies about 75 years ago. Terrestrial plants and pollen indicate that arboreal forests were replaced by tundra vegetation during a cool climate. The Younger Dryas return to a cold, glacial climate was first considered to be a regional event restricted to Europe, but later studies have shown that it was a world-wide event.
The problem became even more complicated when oxygen isotope data from ice cores in Antarctica and Greenland showed not only the Younger Dryas cooling, but several other cooling/warming events, of which the Younger Dryas is the longest and coldest. Among these abrupt changes in climate, now known as Dansgaard-Oerscher events, are:
- Sudden global warming 14.500 years ago that sent the immense Pleistocene ice sheets into rapid retreat
- Several episodes of climatic warming and cooling between ~14.400 and 12.800 years ago
- Sudden cooling 12.800 years ago at the beginning of the Younger Dryas
- Abrupt climatic warming of up to 10º C in just a few decades ~11.500 years ago
The Younger Dryas was not just a single climatic event. Late Pleistocene climatic warming and cooling not only occurred before and after the YD, but also within it. All three major Pleistocene ice sheets, the Scandinavian, Laurentide, and Cordilleran, experienced double moraine-building episodes, as did a large number of alpine glaciers. Multiple YD moraines of the Scandinavian Ice Sheet have long been documented and a vast literature exists. The Scandinavian Ice Sheet readvanced during the YD and built two extensive end moraines across southern Finland, the central Swedish moraines, and the Ra moraines of southwestern Norway. 14C dates indicate they were separated by about 500 years.
Among the first multiple Younger Dryas moraines to be recognized were the Loch Lomond moraines of the Scotish Highlands. Alpine glaciers and icefields in Britain readvanced or re-formed during the YD and built extensive moraines at the glacier margins. The largest Younger Dryas icefield at this time was the Scotish Highland glacier complex, but smaller alpine glaciers occurred in the Hebrides and Cairngorms of Scotland, in the English Lake District, and in Ireland. The Loch Lomond moraines consist of multiple moraines. Radiocarbon dates constrain the age of the Loch Lomond moraines between 12.900 and 11.500 calendar years ago. Multiple Younger Dryas moraines of alpine glaciers also occur throughout the world, e.g., the European Alps, the Rocky Mts., Alaska, the Cascade Range, the Andes, the New Zealand Alps, and elsewhere.
Radiocarbon and cosmogenic dating of glacial moraines in regions all over the world and abrupt changes in oxygen isotope ratios in ice cores indicate that the Younger Dryas cooling was globally synchronous. Evidence of Younger Dryas advance of continental ice sheets is reported from the Scandinavian ice sheet, the Laurentide ice sheet in eastern North America, the Cordilleran ice sheet in western North America, and the Siberian ice sheet in Russia. Alpine and ice cap glaciers also responded to the abrupt Younger Dryas cooling in both the Northern and Southern hemispheres, e.g., many places in the Rocky Mts. of the U.S. and Canada, the Cascade Mts. of Washington, the European Alps, the Southern Alps of New Zealand, and the Andes Mts. in Patagonia of South America.