Environmentally Speaking....

      THE YOUNGER DRYAS

The Younger Dryas stadial, was a geologically brief cold climate period following the Bolling/Allerod interstadial at the end of the Pleistocene between approximately 12,800 to 11,500 years ago, and preceding the Preboreal of the early Holocene.  An Older Dryas stadial had preceded the Allerod, approximately 1,000 years before the Younger Dryas; it lasted 300 years.

 

The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12,900-11,500 years before present in sharp contrast to the warming of the preceding interstadial deglaciation.  It has been believed that the transitions each occurred over a period of a decade or so, but the onset may have been faster.  Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was ~15ºC colder during the Younger Dryas than today.  In the UK, coleopteran (fossil beetle) evidence suggests mean annual temperature dropped to approximately 5ºC, and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas.  Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.  

 

 

Was the Younger Dryas global? 

 

In western North America it is likely that the effects of the Younger Dryas were less intense than in Europe; however, evidence of glacial re-advance indicated younger Dryas cooling occurred in the Pacific Northwest.

 

Other features are:

 

• Replacement of forest in Scandinavia with glacial tundra (which is the habitat of the plant Dryas octopetala).
• Glaciation or increased snow in mountain ranges around the world.
• Formation of solifluction layers and loess deposits in Northern Europe
• More dust in the atmosphere, origination from deserts in Asia
• Drought in the Levant, perhaps motivating the Natufian culture to invent agriculture.
• The Huelmo/Mascardi Cold Reversal in the Southern Hemisphere began slightly before the younger Dryas and ended at the same time.
• Decline of the Clovis Culture and extinction of animal species in North America.

Causes of the Younger Dryas

 

Theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic thermohaline circulation in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America.  The global climate would then have become locked into the new state until freezing removed the fresh water "lid” from the North Atlantic Ocean.  A recent alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, bringing more rain to the North Atlantic which freshened he ocean surface enough to slow the thermohaline circulation.  Neither theory explains why South America cooled first.

 

Previous glacial terminations probably did not have younger Dryas-like events, suggesting that its cause has a random component.  Nevertheless, there is evidence that some previous glacial terminations had post glacial cooling periods somewhat similar to the Younger Dryas.

 

 

The end of the Younger Dryas  

 

Measurements of oxygen isotopes from the GISP2 (Greenland Ice Sheet Project 2) ice core suggest the ending of the Younger Dryas took place over just 40-50 years in three discrete steps, each lasting five years.  Other proxy data, such as dust concentration, and snow accumulation, suggest an even more rapid transition, requiring a ~7ºC warming in just a few years; the total warming was 10º±4º.

 

The end of the Younger Dryas has been dated to around 9620 BC (11,550 calendar years BP (before present), occurring at 10000 radiocarbon years BP, a "radiocarbon plateau”) by a variety of methods, with mostly consistent results:

 

• 11,530±50 BP – GRIP (Greenland Ice Core Project) ice core, Greenland
• 11,530⁺⁴⁰ _-60 BP- Krakenes Lake, western Norway.
• 11,570 BP – Cariaco Basin core, Venezuela
• 11,570 BP – German oak/pine dendrochronology (study of annual tree rings)
• 11,640±280 BP – GISP2 ice core, Greenland