Want to stay on top of all the space news? Follow @universetoday on TwitterThe Younger Dryas was a short cold period that is sometimes referred to as the Big Freeze. The period only lasted 1300 years between 12,800 and 11,500 years ago. The period is sandwiched between the end of the Pleistocene and the beginning of the Holocene.
The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere. This period was in sharp contrast to the warming of the preceding deglaciation. The climate change is thought to have occurred quickly(over the course of a single decade or less). Evidence of the period has been found in thermally fractionated nitrogen and argon isotopes from Greenland ice cores. GISP2 indicate that the summit of Greenland was ~15°C colder during the Younger Dryas than today. In the UK, coleopteran fossil evidence from beetles suggests that the average annual temperature was a chilly 5°C. Peri-galcial conditions dominated lowland areas, while icefields and glaciers formed in upland. The Younger Dryas is thought to be an event that is isolated to the Northern Hemispehre, but exacting data is difficult to collect. The remainder of the planet had begun to cool much earlier.
There are several theories about the causes of the Younger Dryas: a shifting of the jet stream, the expansion of freshwater into the ocean(both caused by global warming and the melting of ice sheets), and an impact event.
The theory most widely believed holds that the period was caused by a reduction or shutdown of the North Atlantic thermohaline circulation caused by a sudden influx of fresh water from Lake Agassiz and deglaciation. The global climate would have been like this until freezing removed the fresh water from the north Atlantic.
An alternative theory suggests instead that the jet stream shifted northward in response to the melting North American ice sheet. That brought more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation. Neither of these theories even attempts to explain why South America cooled first. Previous glacial terminations probably did not have Younger Dryas-like events, suggesting that its cause has a random component.
Additionally, some scientists have proposed an impact event to explain the Younger Dryas. This impact event is thought to have caused the period and the near extinction of the Clovis tool culture. A recent examination of carbon-rich materials from sediments dated 15,818 years ago. to present did not find nanodiamonds(hexagonal diamond structures). Instead, graphene- and graphene/graphane-oxide aggregates were found in all specimens examined, which suggests that previous studies misidentified graphene/graphane-oxide aggregates. This seems to cast doubt upon the validity of an impact event as an explanation for the Younger Dryas.
The Earth has seen many periods of the warming/cooling cycle and will again in the future. The Younger Dryas is considered to be unique because of the rapid onset of the climate changes.
We have written many articles about the Younger Dryas for Universe Today. Here’s an article about abrupt climate change, and here’s an article about Big Freeze.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, http://en.wikipedia.org/wiki/Younger_Dryas