Australia: The Land Where Time Began

A biography of the Australian continent 

Cenozoic Climate

Variations of CO2 composition of the Pleistocene atmosphere occur as climate feedback, the exchange of carbon among surface reservoirs - the oceans, soils, atmosphere and biosphere. Increased levels of atmospheric CO2 is the most effective feedback as the climate warms, transfer of CO2 being mainly from the ocean to atmosphere [27' 28]. The total amount of CO2 in the surface reservoirs varies as a result of exchange of carbon with the solid earth over longer time scales, resulting in CO2 becoming a primary agent of climate change on long time scales.

The Cainozoic (Cenozoic) Era that covers the past 65 My is said by the authors¹ to provide a valuable complement to the Pleistocene as a means of exploring climate sensitivity. The larger climate swings of the Cenozoic, that include a period when the Earth was free of ice, puts the changes from glacial to interglacial into a wider perspective, though the data on the Cenozoic atmosphere are not as precise.

The authors¹ suggest the oxygen isotope composition of the shells of deep ocean benthic foraminifera in a global compilation of oceanic sediment cores [26] allow the analysing of climate change in the Cenozoic. The oxygen isotope change δ18O at times when there were negligible ice sheets gives a direct measure of the temperature of the deep ocean (Tdo), thus between 65 Ma and 35 Ma Tdo (°C) is approximately -4 δ18O + 12.

At about 34 Ma δ18O increased rapidly associated with the glaciation of Antarctica [26,40] and cooling on a global scale, indicated by data from North America and Asia [42]. In deep ocean foraminifera 18O is affected by both volume of ice and Tdo from that time until the present, 16O that is lighter and evaporates preferentially from the ocean surface and accumulates in the ice sheets. The change of δ18O was about 3 °/oo between 35 Ma and the last ice age at 20,000 years ago, change of Tdo was about 6° C (from +5 to -1° C) and volume of ice change was about 180 msl (metres of sea level). As a 1 o/oo change is associated with a 6° Tdo change the remaining change of δ18O is assigned to ice volume linearly at the rate of 60 msl per mil δ18O change (thus 180 msl for δ18O between 1.75 and 4.75). When δ18O is divided equally between sea level and temperature it gives a change in sea level in the Late Pleistocene that accords reasonably with the sea level data that is available. When the ice volume portion of δ18O is subtracted it gives the temperature of the deep ocean Tdo (°C) = -2 (δ18O - 4.25  °/oo ) after 35 My.

The large temperature change during the Cenozoic, about 14° C between 50 Ma and the ice age at 20,000 years must have been forced by changes in the atmospheric composition. Alternatively there could be external drives, such as solar irradiance, or the surface of the Earth, the location of continents. In the Cenozoic solar brightness increased by about 0.4 % [43], a linear forcing change of +1 W/m², as well as being of the wrong sign to contribute to the cooling trend. Climate forcing resulting from continental locations was less than 1 W/m², as the continents were already close to the present latitudes by 65 Ma. The timing of glaciation might have been affected by the opening or closing of oceanic gateways, though it would not provide the climate forcing to explain global cooling.

The concentration of CO² in the atmosphere varied from about 180 ppm during glaciations to 1500 ± 500 ppm in the early Cenozoic [44]. This change produces a forcing of more than 10 W/m² [16], that was larger than any other known forcings by an order of magnitude. In the period with accurate data (ice cores) CH4 and N2O correlated positively with CO2 and global temperature, the authors¹ suggest it likely increased the total greenhouse gas forcing, though their forcings are much smaller than that of CO2 [45, 46].

See Source 1 for references used by authors.

From Source 2

According to the author² the climate of the Cenozoic was a roller coaster ride of temperature changes, beginning at a time of very high temperatures and cooling inexorably thereafter, the climate swinging from hothouse to icehouse conditions and back again along the way to the present.


There have been no climate changes that occurred as dramatically as the Younger Dryas since the close of the Pleistocene. Since the opening of the Holocene there have been a number of fairly sudden climate flips but none of them are as dramatic as the Younger Dryas. Following the Younger Dryas flood Lake Agassiz soon refilled, releasing a second flood from the lake in the earliest part of the Holocene that is widely believed to be responsible for the Preboreal Oscillation, a cool period that persisted for a few centuries. Following that flood Lake Agassiz filled once again and again broke through and another flood occurred that caused another sudden cold snap about 8,200 years ago, that has been called the 8.2 ka event. As with the Younger Dryas the effects of the 8.2 ka event appear to have been felt most around the North Atlantic, though cooling has been recognised from around the Earth at this time. The author² suggests this final cataclysmic draining of Lake Agassiz-Ojibway dumped 160,000 km³ of freshwater into the Atlantic Ocean, an event that raised the global sea level by 2-4 m almost overnight. As with earlier flood from Lake Agassiz the currents of the North Atlantic appear to have been disrupted, causing a cooling period that lasted for 400 years and was felt to some degree around the world. The impact of this cooling was not as severe as the global temperatures were significantly higher than at the time of the the Younger Dryas, though the world's climate was affected by temporary changes, such as a drop in temperatures as much as 5° C in some areas, and possibly affecting the course of human history. In parts of Africa and Asia a drought that lasted for a number of centuries is believed responsible for the development of irrigation in ancient Mesopotamia and it has been suggested to have encouraged people in the region to gather in larger numbers in order to cope better with the resulting food shortages. The demographics of this region appear to have undergone a sudden change at this time, the families and small groups of the region congregating to form towns and cities.

The 8.2 ka Event is the largest natural climate blip, though not the only one, being considered to be one of a number of Bond Events that occur about every 1,500 years, involving fluctuations that originate in the North Atlantic, though there hasn't been a climate blip at all predicted times. Such blips have been recorded at 5,900, 4,200, 2,800 and 1,400 years ago, the last of which coincided with the Little Ice Age from the 17-19th centuries. Bond Events have been recognised from the Holocene and North Atlantic fluctuation of climatic have been recognised from the Pleistocene, called Dansgaard-Oeschger Cycles. The author² suggests the cooling associated with Bond Events appears to be related to changes of the currents in the North Atlantic, that were augmented by the catastrophic draining of Lake Agassiz-Ojibway in the 8.2 ka event, though the underlying cause has still to be determined. Gerard Bond has suggested that the cooling associated with Bond Events results from reductions in the Sun's activity. The Little Ice Age coincided with the Maunder Minimum, a period of reduced solar output. In recent times it has been speculated that the cold winters in western Europe and the UK are associated with a period of low activity of the Sun, the lowest activity for about 100 years.

It has been speculated by Mike Lockwood et al., Reading University, that periods of reduced solar output can be correlated with the presence of  persistent winter high-pressure systems across the eastern Atlantic that block the westerly winds, that are milder, and favouring north-easterlies that are much colder. They have calculated that there is about a 10 % chance that Little Ice Age conditions will return in the next 50 years. Though the Sun has some influence on the climate of the Earth, and always has, according to the author² it is not the driving force of the contemporary global climate change, as the skeptics and deniers say it is.

Sources & Further reading

  1. Hansen, James et al., 2008, Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, 2, 217-231.
  2. 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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