Testing hypotheses for the onset of Northern Hemisphere glaciation

Whereas Antarctica began to develop significant ice caps in the early Oligocene (maybe in late Eocene times) those of the Northern Hemisphere, principally on Greenland, did not arise until about 3 Ma ago. There are several hypotheses for that onset of the Great Ice Age: closure of the Panama seaway and increased poleward heat transport in the North Atlantic; perhaps related development of the El Niño cycle in the East Pacific; uplift of the Himalaya and Rocky Mountains changing atmospheric circulation; lowered atmospheric CO2, and a combination of all four that allowed the Milankovich astronomical forcing to get a grip on Earth’s climate ‘machine’. Testing the hypotheses is somewhat more difficult than find empirical support for them; i.e. coincidences in timing. Climate scientists from Bristol, Cambridge and Leeds universities in the UK have attempted such a test, using a complex climate model involving coupled atmosphere-ocean circulation and ice-sheet models (Lunt, D.J. et al. 2008. Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels. Nature, v. 454, p. 1102-1105). Only a decrease in the greenhouse effect could have transformed climate over Greenland sufficiently to equip it with a large ice sheet, the other three main hypotheses falling a long way short, although each could have led to small ice volumes. Significantly, the study failed to find support for any of the terrestrial processes having been capable of ‘priming’ orbital and rotational forcing to such an extent that they triggered glaciation. Despite the claims by the authors, as computing power goes up and the resolution of feasible climate modelling comes down it is quite likely that within a few years there will be another view ‘supported’ by models.

Climate shock of the Younger Dryas

Between 12,900-11,500 years before the present, high northern latitudes returned to almost full glacial conditions, after about 6000 years of warming since the last glacial maximum. Just prior to the Younger Dryas cooling event, conditions had warmed sufficiently that European people had migrated northwards, some to occupy what are now the British Isles. Temperate grasslands teeming with game were the probable attraction, and still-low sea levels permitted crossing of what became the North Sea. Although it is possible that some people remained in Britain through the thousand-year mini glaciation, conditions would have been at the extremes of winter cold and year-long windiness, judging from the Greenland ice-core records of air temperature and dust. Those records have shown for some time that the transition from warmth to frigidity was rapid, but not how rapid. The cold spell had much in common with sudden, millennial-scale coolings repeated several times during the run-up to the last glacial maximum. Each such event has been linked with interruptions in the shallow and deep circulation of North Atlantic ocean waters, a likely trigger having been reduction in the salinity of surface waters as a result of floods of fresh water, either through collapses of ice caps and melting of icebergs or, in the case of the Younger Dryas, release of massive amounts of fresh water from glacially-blocked lakes in North America. One result would have been failure of cold surface water to sink at high latitudes, thereby shutting down the suction effect that drags warm water northwards to raise temperatures, especially in NW Europe.

There are concerns that unsuspected climate shifts that stem from the Earth System rather than astronomical influences – the Milankovich effect – may characterise the period of global warming caused by human activities. Increased precipitation at high northern latitudes or melting of ice on Greenland could result in falling ocean salinity and slowing or shutdown of the North Atlantic heat conveyor. Two sets of data published in August 2008 highlight potential climate shifts that may arise with virtually no warning. Both rely on the potentially high resolution of cores through ice caps and stagnant lakes that are annually layered, which has hitherto not been fully exploited by climate scientists. European and North American researchers have focussed on the upper part of the latest core through the Greenland ice cap, using two or three samples from each annual layer (Steffensen, J.P. and 19 others 2008. High-resolution Greenland ice core data show abrupt climate change happens in few years. Science, v. 321, p. 680-684). Deuterium and oxygen isotopes during the onset of the Younger Dryas show a marked cooling at the source of moisture precipitated as snow within 1 to 3 years, which the authors ascribe to the Intertropical Convergence Zone migrating northwards through a major change in atmospheric circulation. Temperature over the Greenland ice cap also changed, but over about 50 years [note however, that the sharp warming of the Bolling episode took less than a decade].

The second study uses annually varved lake sediments that accumulated in an isolated lake in central Germany that filled a circular depression formed by explosive volcanism (Brauer, A. et al. 2008. An abrupt wind shift in western Europe at the onset of the Younger Dryas cold period. Nature Geoscience, v. 1, 520-523). The seasonal sediment layers change in thickness, colour and mineralogy as warmth gave way to the frigidity of the Younger Dryas. One of the proxies, the iron content of the sediments deposited under anoxic conditions during winters fell significantly within a year at 12679 BP, along with a 4-5 fold increase in the rate of sediment deposition. Together with shifts in the lake biota, these features suggest to the authors that within a year wind strength increased greatly, probably due to a greater incidence of storm-force westerlies brought on by a change in the position of the jet stream. Today, westerly winds add to warming in northern Europe, around 12.7 ka they added to cooling, which can only be explained by global cooling or a southward excursion of sea ice in the North Atlantic.

Neither abrupt climate shift can be produced by validation of today’s climate models using actual data from the time just before they took place. It follows therefore that similar shifts in the near future could make themselves felt with no warning.

Opinion has drifted back and forth regarding the global effects of the Younger Dryas, evidence for its effects in the Southern Hemisphere being scanty. The best place to look for direct evidence would be in mid-latitude glaciers, especially where they are abundant in South America and New Zealand. A study of the largest of these, the Southern Patagonian Icefield (Ackert, R.P et al. 2008. Patagonian glacier response during the late glacial-Holocene transition. Science, v. 321, p. 392-395) indicates that the ice there advanced around the time of the YD. However, its dating indicates that the advance lay outside the 1300 year span of the cold period in the Northern Hemisphere. It was more likely due to a local response to increased precipitation from air moving from the east.

See also: Flückiger, J. 2008. Did you say “fast”? Science, v. 321, p. 650-651.

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