On the edge of chaos in the Younger Dryas

Around 13 thousand years ago, the world was warming rapidly and the great northern ice sheets in retreat. Plants, animals and humans in Europe were able to and did migrate northwards. Sea level still being low, there was nothing to stop decolonisation of Britain by crossing the huge fluviatile plain of Doggerland where the southern North Sea now stands.(see Return to ‘Doggerland’ in September 2008 issue of EPN). At 12.9 ka there came the shock of a rapid temperature fall at the start of the Younger Dryas episode, when ice sheets began to re-establish themselves in the upland areas of Britain and Scandinavia. What happened to those intrepid migrants we may never know, but what they would have faced had they chosen to remain in the game-teeming NW Europe of that episode has become clearer with detailed investigations in sediments at the bottom of a Norwegian lake supplied by melt water from glaciers (Bakke, J. et al. 2009. Rapid oceanic and atmospheric changes during the Younger Dryas cold period. Nature Geoscience, v. 2, p. 202-205).

The research by Norwegian and German scientists used two interesting proxies for glacial advance and retreat: the amount of sedimentary titanium and the density of the sediment, both of which would have varied with the rate of glacial erosion. The data were calibrated to time by 96 14C dates, and the sampling frequency (every 0.06 mm for Ti and 5 mm for density through a 1.4m core that represents 1700 years) was sufficient potentially to resolve events to a few days and 6 months respectively. Allowing for background ‘noise’ effects, certainly monthly and annual changes should show up, and indeed they do. The pattern is one of rapidly changing conditions between warm and frigid, which the authors interpret as a result of repeated ‘boom and bust’. At 12.18 ka, further cooling occurred and the late Younger Dryas is the more chaotic part of the record. The hypothesis is that the fluctuations reflect growth and shrinkage of sea ice in the North Atlantic, matched by growth and melting of glaciers. Brief warming during periods of prevailing westerly winds melted glaciers, but fed vast amounts of fresh water to the North Atlantic that in turn encouraged surface waters to freeze. Sea-ice formation and the build-up of a polar high pressure area drove weather systems conducive to westerlies southwards, when glaciers grew. Something suddenly stopped this chaotic behaviour and high latitudes rapidly emerged from frigidity at 11.7 ka: the Holocene had begun and, soon, so would humanity in an equally chaotic manner.

 

Climate at the Eocene-Oligocene (E-O) boundary

Oxygen isotopes from benthic foraminifera in deep-sea sediment cores show an abrupt increase in δ18O at around 34 Ma, which spanned a mere 300 ka. This is taken to indicate withdrawal of ocean water to polar ice caps on land that diamictites from high southern latitudes link to the beginning of glaciation of Antarctic. Then as now, the south polar region was thermally isolated, probably as a result of its having become surrounded by seaways and development of the Antarctic Circumpolar Current from the Palaeocene onwards as a result of the final break-up of Gondwana when it became separated from Australia and South America. Other factors at the E-O boundary seem to have been decreasing atmospheric CO2 and low solar heating as a result of the Milankovich effect. Cooling due to such factors was disrupted and delayed by the spectacular global warming at the Palaeocene-Eocene boundary (55.8 Ma) as a result of massive methane release to the atmosphere. Detailed proxy records from both high- and low-latitude sea-floor sediment cores now resolve fine detail of the shifts in sea-surface temperature (SST) at the E-O boundary (Liu, Z.  et al. 2009. Global cooling during the Eocene-Oligocene climate transition. Science, v. 323, p. 1187-1190). The most profound shift in SST took place at high latitudes (in both Northern and Southern Hemispheres) with a drop of around 5 to 9ºC between 34 to 33.5 Ma. This was followed by slight rise to about 3ºC below pre-E-O conditions. Surprisingly, data from low latitudes ‘flat-lined’ at around 28ºC across the transition, suggesting steady evaporation of seawater, more of which would have precipitated as snow at high latitudes. The ‘hothouse’ conditions of the Cretaceous and early Cenozoic saw estimated high-latitude sea-surface temperatures rise from about 7ºC to 12ºC by the Early Eocene. The protracted global cooling that followed reached about 7ºC by about 42 Ma, which stabilised until 40 Ma when SST fell to about 4ºC just before the E-O boundary (see http://www.learner.org/courses/envsci/visual/visual.php?shortname=cenozoic).

The sudden start of Antarctic glaciation at 34 Ma looks increasing like an example of a chaos-like ‘flip’ in global climatic conditions brought on by a blend of factors that collectively reached a threshold, which once crossed permitted no escape, at least not over the last 30 Ma or so (Kump, L.R. 2009. Tipping pointedly colder. Science, v. 323, p. 1175-1176). That is a point that should not be lost at a time when anthropogenic global warming continues unabated, despite so much hype by the G20 leaders at their London meeting in early April 2009. Climatic ‘flips’ can go either way.

 See also: Documenting the Palaeogene transition from ‘hothouse’ to ‘icehouse’ in EPN for August 2005, and Magmatic link to the Palaeocene-Eocene warming in EPN for July 2007

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