Australia: The Land Where Time Began

A biography of the Australian continent 

Quaternary Climate

From a bit before 2.7 Ma, in the Late Pliocene, there has been no periods of persistently hot climatic conditions, instead ice sheets were accumulating across the landmasses in the Northern Hemisphere. Starting in the European Arctic and northeast Siberia, followed by Alaska, and eventually across the northeastern parts of North America. By 2.5 Ma the Quaternary began as the greatest extent of the first ice age had been achieved. Most of the time period covered by the Quaternary coincided with the Pleistocene.

At a bit less than 12,000 years ago is the beginning of the Holocene, the most recent epoch of Earth's history, defining post-glacial times and characterised by a climate that is broadly clement. According to the author² what tipped the Earth into a full ice age was a cyclic variation of the Earth's orbit around the Sun. The Earth wobbles on a range of time scales and in a number of different ways, and these have been suspected of being linked in some way with the triggering of the Quaternary Ice Age, as well as to the alternating glacials and interglacials that have occurred throughout the cycle since the start of the Quaternary. In the middle of the 19th century the French mathematician Joseph Adehémar proposed a link between astronomical forcing and the most recent ice age. James Croll, a self-taught Scottish scientist developed the idea further 20 years later, though the precise nature of the connection was established and quantified in the 1930s by Milutin Milankovitch, a Serbian engineer. It is  now known as the Croll-Milankovitch Astronomical Theory of the Ice Ages, in which they argue that the repeated advances and retreats of the ice sheets, characteristic of the Quaternary, are functions of the geometry of the rotation of the Earth and its orbit.

Once all the other factors were in place it appears the obliquity of the Earth's axis to be in the correct position (the tilt of the Earth's axis of rotation with respect to its orbital plane) was all that was required to trigger an ice age The obliquity of the Earth's axis is one of the wobbles demonstrated by the Earth as it moves around the Sun. Milankovitch had recognised that the changes of obliquity follow predictable cycles that play a key role in the workings of the last ice age. The tilt of the Earth's axis changes from slightly over 22° to 24.5° and back again, over a period of 41,000 years, the result being small though significant changes in the amount of sunlight reaching the surface of the Earth. There would be no seasons if the axis of the Earth was not tilted. When it is summer in the Northern Hemisphere the North Pole is tilted towards the Sun, the surface north of the equator receiving direct solar radiation, so the temperatures are higher. During the northern winter the North Pole is tilted away from the Sun resulting in the temperatures being lower and shorter periods of daylight. The Southern Hemisphere experiences the opposite seasons to the Northern Hemisphere, northern summers are southern winters and vice versa.

When the tilt is larger there are extreme winters, though it is not the best way to trigger the formation of ice sheets. The conditions for ice sheet formation are more likely to result from cool summers at high northern latitudes, that occurs when the axis tilt of the Earth is small. The snow and ice from winter are more likely to remain throughout the summer in which case they can grow in thickness with each winter. The accumulation of snow over the years leads to an increase in the albedo (reflectivity) of the surface increases which reduces the melting potential of the solar radiation that reaches the surface, and so the ice sheet can grow. According to the author² during the Late Pliocene there appears to have been such a situation that allowed a series of glacial advances and retreats that have continued for the 2.5 My since then. At the present the tilt value is 23.4°, slightly more than half way between the extremes and has been found to be decreasing. It has been estimated that the minimum tilt angle should be reached in about 8,000 years from the present at which time the conditions would be ideal for the formation of ice sheets, though this time the carbon dioxide levels in the atmosphere could be much higher than levels associated with glacial conditions.

Since the beginning of the Quaternary there have been 50 times of glacial advance and retreat that have been recognised, repeating at about 40,000-year intervals and this has led to a general consensus that the cyclic variation in the axial tilt of the Earth is the main control that determines the timing of glacial and interglacial phases. In the earlier glacial phases since the commencement of the cycle the length of the cycles were not especially large and the temperatures between both the glacials and interglacials not particularly extreme. The length of the glacial-interglacial cycles started increasing between about 1.5 Ma and 600,000 years ago, then after this time their range was between 80,000 and 120,000 years, with an average of about 100,000 years. There was also a change from cold to warm and back again. Another difference with the early cycles was that they were regular and symmetrical, whereas the later cycles had a prolonged cooling period of about 80,000 years then sudden changes to interglacials in periods as short as 4,000 years. A global temperature graph of about the last 1 My shows a distinctive saw-tooth pattern, downward trending they dipped gently then were terminated by upswings that were sudden and short. Over the last 1 My the climate has begun to vacillate more wildly with the glacial phases lasting longer and with lower temperatures and more ice accumulation. In the interglacials over the same period the temperatures were unusually high, even in the depth of the last ice age the temperatures were notably higher than at the present.

The author² suggests the conditions in the PETM and the MMCO may provide some clues on what the future of an anthropogenically warmed world may look like. Over the past 0.5 My the 5 interglacials, including the present one, were also times when the atmospheric carbon dioxide levels were higher than those characteristic of earlier, cooler interglacials. In the case of the latest, in the Holocene, human activity has been increasing the concentrations of the gas that have been forced up to far higher levels.

Climatologists have been studying the interglacial before the present one alongside the PETM and the MMCO, looking for clues to our future climate. Named for the Eem River in the Netherlands, the Eemian began about 130,000 years ago, coinciding with the termination of the penultimate glaciation, lasted for about 14,000 years, though parts of Europe appear to have stayed warm for a few thousand years after other parts had returned to glacial conditions. The rapidity with which the climate changed from icehouse to hothouse was astounding, as indicated by the presence of tropical animals such as hippos in England, as well as other temperate zones, a few thousand years after the end of the previous glacial phase. At about 125,000 years ago, at the peak of the last interglacial, forests were returning to the north of Norway and Baffin Island in Canada, both of which are well inside the Arctic Circle. It has been estimated that temperatures at that time were probably about 1-2° C warmer than at the present, though ice core evidence indicates that Greenland was 5° C warmer, and summer temperatures may have been 4° C higher than at present. The oceans were about 2°-3° C warmer than at present and sea levels were significantly higher. It has been estimated by Jonathan Overpeck et al., a palaeoclimatologist at the University of Arizona, that the sea levels during the Eemian were 4-6 m above present levels. They have linked the sea level heights in the Eemian to partial melting of the Greenland and West Antarctic ice sheets, and they also point out that, assuming a 1%/year increase in the carbon dioxide or equivalent greenhouse gas per year, slightly higher than at present, Greenland could be much hotter than it was during the Eemian, leading to the possibility of even more melting leading to even higher seal levels. The author² suggests that warming of West Antarctica and its surrounding waters, may also see bigger contributions to sea level rises of the future than it appears to have done during the Eemian. During the Eemian the sea level seems to have been rising at as much as 2 cm/year. As well as being twice as fast as the average rise at the end of the last glacial, it is almost 10 times higher than the rate of rise during the 20th century.

Sources & Further reading

  1. McGuire, Prof. Bill, 2012, Waking the Giant: How a changing climate triggers earthquakes, tsunamis, and volcanoes, Oxford University Press.



Author: M. H. Monroe
Last updated 25/08/2012
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