Australia: The Land Where Time Began

A biography of the Australian continent 

Younger Dryas - A Classic RCCE

According to the author¹ the Holocene was heralded by the final freezing event of the Pleistocene, in the form of a severe cold snap, the Younger Dryas, named for a white alpine flower, the pollen of which has been found widely distributed during this episode of freezing temperatures.

The snow and ice were largely gone by slightly less than 13,000 years ago and sea levels were rising towards the levels of the present and the climate was warmer and wetter than it had been at any time since the Eemian. There was a sudden change of climate as temperatures dropped almost as low as they had been at the height of the previous glaciation. The average temperature drop in the UK was about 5° C and by 15° C in central Europe. Much of the Northern Hemisphere was in the grip of this freezing period for 1300 years, though no evidence has been found in Antarctica or Oceania of this cold period, and it appears to have started earlier across South America, which the author¹ describes as problematic. The evidence suggests the cooling was less intense in western North America than it was in Europe and the North Atlantic region.

The cooling has been linked to a putative impact, but the consensus holds that it was actually caused by the breaking of the wall of rock and ice holding back a huge lake of glacial meltwater. The large continental ice sheets had been melting for several thousand years by the time of the Younger Dryas and huge volumes of very cold freshwater had accumulated in pro-glacial lakes along the front of the retreating ice sheets. Lake Agassiz was the biggest of all these lakes, covering much of Manitoba, Saskatchewan and western Ontario, in Canada, and in the US, parts of Minnesota and North Dakota. Lake Agassiz merged with Lake Ojibway when it was at its greatest extent to form a prodigious water body that is estimated to have had a volume of more than 160,000 cubic kilometres and with an estimated area of 840,000 square kilometres, nearly 3 times larger than the Caspian Sea, the largest inland body of water on Earth at the present. When part of the impounding wall gave way 9,000 km³ of water poured into the North Atlantic Ocean to the north. The sudden dumping of cold freshwater into the North Atlantic had, according to the prevailing theory, an immediate and cataclysmic effect on the Gulf Stream, either curtailing it or even stopping it. Resulting from stopping the Gulf Stream with its warm water plunged Europe and much of North America into near-Arctic conditions, then the cooling gradually moved out to affect other parts of the world. According to the Author¹ an alternative proposal in a recent paper suggests that it was actually increased rainfall that freshened the North Atlantic, and so reined in the Gulf Stream.

The most extraordinary thing about the Younger Dryas is how quickly it started, whatever its cause turns out to be. It had been believed for some time that it had taken hold in as little as a few centuries, this was later revised down to a few decades, then in 2009 there was a study of mud cores from the bed of Lake Monreagh, County Clare, Ireland. This particular lake was chosen because of the lake sediment extending back as far as the latest Pleistocene and the undisturbed nature of the mud that made it excellent for the preservation of the individual layers. The study found that each layer covered the deposition that took place over a few months and this made possible the determining of the timing and speed of any environmental changes that occurred. The carbon was measured to determine the level of productivity and the oxygen isotopes to determine the temperature of the water. The results that shocked the researchers indicated that both productivity and temperature of the lake had dropped in at most 1-2 years, though possibly in a few months, the time of the sudden change coinciding with the start of the Younger Dryas. The author¹ says this is getting close to the time taken for the freezing conditions to become established in the movie The Day After Tomorrow, that was believed to be pure science fiction. This is one of the findings that are showing just how rapidly climate can change, especially when it is severely stressed as is the case at the present.

During2 the glacial period there were a number of massive RCCEs that occurred, in some of which the temperature dropped almost 30oC (54oF) in the central parts of Greenland. During the last deglaciation of the North Atlantic region, when the ice sheets melted, the Younger Dryas was the most significant RCCE, and it was the first to attract the attention of researchers.  Between about 16,000 BP and about 11,500 BP was the main phase of the deglaciation. A dramatic temperature change is observed in the GISP2 ice core, based on stable isotopes of water, as measured by the instruments of the author's1 team, and, based on calcium, windiness (strength of atmospheric circulation) that occurred at about 14,500 BP. The sea ice decreased significantly over much of the North Atlantic over not more than a few years.

According to the author1 the Younger Dryas makes a good case study for RCCEs in general. The extent reached by the glaciers that advanced  during the Younger Dryas is observed as moraines, debris that was deposited at the margins of the glaciers as they retreated, in sediments deposited in lakes and the ocean, and the remains of plants and animals of cold climates, such as beetles. The focus of earlier studies had been on the termination of the Younger Dryas, which was abrupt, as this marks the end of the last major reorganisation of climate that occurred during the last deglaciation. It is now known that the Younger Dryas also began abruptly. Precision measurements of the GISP2 ice core dated the Younger Dryas as an event with 1300 ± 70 years duration (Alley et al., 1993)  that terminated abruptly as indicated by a temperature rise (Grootes et al., 1993), an increase in the rate of accumulation at 11,640 BP (Meece et al., 1994), and the lower amount of dust sourced from continents (Mayewski, et al., 1993).

The palaeoenvironment during the Younger Dryas was reconstructed by the author's1 team using high-resolution , with a mean 3.48 years per sample, continuous measurements of the chemical species contained in the GISP2 ice cores as these series record the history of the major soluble constituents that are transported to central Greenland where they are deposited. A dramatic example is has been provided by the calcium series from about the last 18,000 - 10,000 with prominent periods of increased dust in the atmosphere that peak about every 400-500 years, which demonstrates the inherent variability of RCCEs. According to the author1 their results indicate that the Younger Dryas both began and ended in 10 years or less, the end marking the start of the Holocene, the current interglacial.

According to the author1 if there were only a few known instances of rapid climate change events such as Younger Dryas, or if they were only known from the ice cores in Greenland they would probably be considered to be isolated anomalies, but they are found in many different records.

During the Younger Dryas the accompanying climate change also occurred elsewhere, not only in Greenland. According to the GRIP data CH4 concentration variations in the ice core indicate a colder, drier climate for tropical and sub-tropical regions during the Younger Dryas, as well as in earlier RCCEs (Chappellaz et al., 1993). Wet-lands in low latitudes are the major natural source regions of CH4, the larger area being covered by them in the tropics result in higher concentrations in the atmosphere. It is suggested by these results that similar events may have occurred in other regions as  it appears the CH4 source in the last glacial period may have been in the lower and middle latitudes (Chappellaz et al., 1993). RCCEs have been found in other records as well as those from Greenland.

Sources & Further reading

  1. McGuire, Prof. Bill, 2012, Waking the Giant: How a changing climate triggers earthquakes, tsunamis, and volcanoes, Oxford University Press.
  2. Mayewski, Paul Andrew @ White, frank, 2002, The Ice Chronicles: The Quest to Understand Global Climate Change, University Press of New England.


Author: M. H. Monroe
Last updated 25/01/2013

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