The timing of the Younger Dryas was first established using radiocarbon dating and assigned to the approximate interval 11,000 to 10,000 years before the present (BP).
The radiocarbon timescale is now known to be in error for the late-glacial period, owing to rapid changes in global carbon reservoirs at that time. The true date of the Younger Dryas has now been established using cores of ice drilled from the Greenland ice sheet. Careful study of ice layers that built up near the centre of the ice sheet has established that the onset of the Younger Dryas was about 12,800 calendar years ago, and its termination about many years ago. The Greenland ice cores provide such a clear and strong record of the Younger Dryas that they can be used as a yardstick against which other types of evidence can be compared(Lehman 1992).
Reconstructed temperatures, based on the isotopic composition of oxygen locked up in the ice, are about 7 degrees C colder in central Greenland during the Younger Dryas than in the preceding and succeeding warm periods. The ice-core record shows that the cooling at the start of the Younger Dryas was gradual, but the warming at the end was exceedingly rapid, occurring within a few years or decades at most.
The rapidity of this climatic change, which is also recorded in marine and terrestrial records from around the North Atlantic, is one of the most intriguing aspects of the Younger Dryas; it is also characteristic of other relatively short-lived episodes during the last glacial period. The Greenland ice cores also record changes in atmospheric composition. Air bubbles trapped within the ice show that during the Younger Dryas concentrations of the ‘greenhouse’ gases, carbon dioxide and methane, were lower than modern natural levels, and were similar to those of full glacial times. Methane in particular shows a large oscillation, closely following the pattern of temperature change. Finally, precipitation changed during the Younger Dryas, with snow accumulation about half of that for the succeeding early Holocene(Broeker 1990).
This evidence for low precipitation is compatible with the temperature record, since cold air cannot carry as much moisture as warm air.
In Europe, the Younger Dryas cooling is most pronounced in Britain, southern Scandinavia, northern Germany, the Low Countries, and northern France, and declines in importance to the north, east, and south. On the other side of the Atlantic, the event is clearly recognizable in the maritime provinces of Canada and the north-eastern United States, but is less apparent further west. A short-lived cooling is also indicated by some pollen records from the Pacific coast of North America, as far north as Alaska. some scientists believe that the Younger Dryas can be recognized in pollen profiles from the northern Andes, Chile, Japan, and Australia, but others disagree.
There is intriguing evidence that the Younger Dryas brought changes in wind circulation and moisture balance to some low-latitude continental areas(Bond 1993).
Lake levels in East Africa fell, indicating dry conditions and a reduction in the on-land transfer of moist air from over the Indian Ocean. The Tibetan plateau is also thought to have been arid at that time. In contrast, the plateaux of Central China appear to have had a wetter climate than in the preceding and succeeding periods. This wet climate has been attributed to a heightened temperature gradient between a warm land surface and a relatively cool western Pacific, and a consequent strengthening of the summer monsoon. Additionally, the Altiplano in the high Andes also appears to have been wetter at that time.
The climate of the Younger Dryas created the conditions for glacier expansion, especially in north-west Europe. The Scandinavian ice sheet, which had been in retreat until about 12-000 (radiocarbon) years ago, underwent a significant readvance and formed prominent moraines at the limit of ice advance in Norway, Sweden, and Finland. Ice cover in Britain had probably been reduced to isolated remnants in Scotland between 13-000 and 12-000 radiocarbon years ago, but during the Younger Dryas a substantial ice field formed over the western Scottish Highlands (locally known as the Loch Lomond readvance) and smaller mountain glaciers became established elsewhere in Britain and Ireland.
Younger Dryas glacier advances of smaller magnitude have been identified in the European Alps, the North American Cordillera, the Andes, and New Zealand, although dating evidence is fragmentary and far from certain in many areas. In some areas, the glacial record hints at climatic changes within the Younger Dryas(Atkinson 1987).
In western Scotland, retreat of some glaciers was well under way before the temperature increase that marked the end of the stade, suggesting that deglaciation may have begun in response to a decrease in snowfall. Changes in the abundance of certain pollen types also indicate that the Younger Dryas in north-west Europe consisted of a moist, cold period followed by a more arid (but still cold) period.
References
Atkinson, T. C., Briffa, K. R., and Coope, G. R. (1987) Seasonal temperatures in Britain during the past 22 000 years, reconstructed using beetle remains. Nature, 325, 587–92.
Bond, G., Broeker, W., Johnsen, S., McManus, J., Labeyrie, L., Jouzel, J., and Bonani, G. (1993) Correlations between climate records from North Atlantic sediments and Greenland ice. Nature, 365, 143–7.
Broeker, W. S. and Denton, G. H. (1990) What drives glacial cycles? Scientific American, January 1990, 43–50.
Lehman, S. J, and Keigwin, L. D. (1992) Sudden changes in North Atlantic circulation during the last deglaciation. Nature, 356, 757–62.
Nicholas R. Golledge, 2009. Glaciation of Scotland during the Younger Dryas stadial: a review
