Australia: The Land Where Time Began

A biography of the Australian continent 

Climate Change - "all part of a natural cycle" See Climate models agree things will get bad. Capturing just how bad is tricky

To say the current warming trend is "all part of a natural cycle" is no consolation, as can seen by studying what has happened at earlier times in the history of the Earth, as at the PETM and a number of earlier such events, such as occurred in each of the Triassic, Jurassic and the Cretaceous, and at other times in the history of the Earth. The author¹ compares the current warming trend to what appear to be similar events that have happened in the distant past such as the PETM (aka LPTM) and the Mid-Miocene Climate Optimum (MMCO) in his book (source 1).


The author¹ suggests the PETM provides a laboratory for studying the effects and consequences of the addition of large amounts of carbon to the atmosphere in very short time periods, and also suggests that the hothouse world of the PETM may be what we have to look forward to over the coming centuries. About ¹/8 of the carbon that drove the PETM has been released by human activity over the past 200 years, and coupled with the current deforestation rate and the estimate that known conventional fuel reserves would provide about 5,000 billion tonnes of carbon, it seems reasonable that the carbon released by human activities will reach the amount that triggered the PETM. And this is on track to be achieved in a much shorter time than it took 55-56 Ma. The author¹ suggests that it might not be necessary to match, by human activities, the release of carbon at the PETM that triggered the sudden, rapid and continuous temperature rise. The conditions are predicted to become less favourable for life as the temperature of the world rises in response to the increasingly high amounts of carbon in the atmosphere, though some of the excess carbon is taken up by the land and the oceans. An example of a positive feedback effect is that as the world warms the carbon sinks of the present take out progressively less of the atmospheric carbon which leads to an increasingly more rapid rise in atmospheric carbon which causes the temperatures to rise even faster. Another possibility that has been proposed is that as the temperature rises the carbon sinks of the present could stop taking up carbon or even become carbon sources. This could be a problem as it is estimated there is currently more than 2,000 billion tonnes of carbon stored in plants and soils and 40,000 billion tonnes in the world's oceans and the organisms that live in them.

Research is showing that some carbon sinks are becoming less effective. The oceans have absorbed between ¹/5 and ¹/3 of the carbon produced by human activities, but this may change, as is suggested by the work of Samar Khatiwala et al. of Lamont-Doherty Earth Observatory, Columbia University, that found that after 1950 the amount of anthropogenic carbon absorbed by the oceans rose sharply but in recent decades this uptake has started to decline, the implication being that the amount of carbon that is put into the atmosphere is already showing signs of overwhelming the oceanic carbon sink, probably as it is more difficult for carbon to be dissolved in the increasingly warmer water of the oceans, which also become more acidic as the amount of carbon taken up increases. The result of the oceanic carbon sink becoming ineffective would be more carbon remaining in the atmosphere leading to even higher temperatures.

According to the author¹ in a dedicated issue of the Royal Society's Philosophical Transactions (A) published in late 2010 Richard Betts et al. of the Met Office Hadley Research Centre presented a paper in which they summarised how soon the world's average temperature might rise to 4° C, taking into account feedback effects in the carbon cycle. Assuming the fossil fuel-dominated SRES scenario, A1FI, the choice being justified by the rate at which emissions are climbing, their best estimate suggests average global temperatures may reach 4° C by the 2070s, and if positive feedbacks are particularly strong this could be reached by the 2060s, an average temperature that is double that which  it is widely believed must not be crossed if the worst effects of climate change are to be avoided. The author¹ suggests that if action is not taken in the very near future to limit emissions it is likely the threshold of dangerous climate change, 2° C, could possibly be reached by 2030 or not long after. If the temperature increase reaches as high as 5-6° C by the end of this century, that is comparable to the temperatures reached in the PETM, the warmer water could reach to progressively greater depths in the ocean and lead to the breakdown of the submarine gas clathrate deposits. This would release vast quantities of carbon that would increase the warming even more. It should be a worry to everyone, not just climate scientists, that in the shallow seas off the coast of Siberia there are already large volumes of methane venting to the atmosphere as the permafrost on the ocean bed thaws. If a rerun of the PETM conditions does occur it would be necessary to plan for the long haul, as the conditions of the PETM lasted for about 170,000 years.

Another period that may provide clues to future climate is the Mid-Miocene Climate Optimum (MMCO). According to Aradhna Tripati et al. of UCLA, atmospheric CO2 levels were at about 400-450 ppm at that time, around the levels they are expected to reach in the next decade if emissions are not cut drastically very soon. It is expected that atmospheric carbon dioxide levels will need to rise only slightly higher than their present levels to eventually lead to a rise of 3-6° C above the pre-industrial levels, assuming the link between atmospheric carbon dioxide levels and temperature is the same now as it was in the MMCO, obviously higher than the 2° C that is widely believed to be the point that must not be exceeded by keeping the CO2 levels below 450 ppm.

Some side effects that were associated with warming episodes of the past, and that can be expected if the present warming trend continues include:

Sources & Further reading

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


  1. Understanding climate change
  2. The Smithian - Lethally Hot Temperatures in the Early Triassic


Author: M. H. Monroe
Last updated 22/03/2021

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