“What we have to understand is that free will is our capacity to see probable futures, futures which seem like they’re gonna happen, in time to take steps so that something else happens instead.” – Daniel Dennett
Astronomers have discovered thousands of exo-planets, some quite like the Earth, orbiting other stars. To determine if any of these are candidates for Earth-like carbon based life we need to determine their climates. As on Earth, this will be a function of the strength of the star’s intensity at the orbital location of the planet and the constitution of the atmosphere. If we want to be warmer on cold winter nights we can either turn up the thermostat on the furnace or pile more blankets onto the bed.
In my last article, I discussed the eighty-million-year period comprising the end-Permian, the Triassic and the early Jurassic as described by Paleontologists Paul Wignall in his new book “The Worst of Times, how life on Earth survived eighty million years of extinction”. Wignall describes how extreme volcanism during this time caused six extinction events including the most disastrous event in Earth history, the End-Permian. About 95% of all plants and animals went extinct. Wignall discusses the blankets, carbon dioxide and other greenhouse gases, volcanism belched into the atmosphere but he does not discuss the furnace, i.e., solar radiation. To make a useful comparison between the Triassic climate and today’s climate we need to include the variation in solar intensity. Our discussion here follows that given by Gavin Foster, Dana Royer and Daniel Lunt [Foster]. As it turns out, our solar furnace output, total solar intensity (TSI), has been gradually increasing since the sun formed 4.6 billion years ago as shown in Figure 1. This phenomenon is well-known since the 70s as the “faint young sun” paradox.
When the Earth had cooled sufficiently to allow liquid water on the surface and the formation of life between 3.5 and 4 billion years ago the sun was only 75% as luminous as it is today [Feulner] (the red dot in Figure 1). The Earth’s oceans would have been frozen to the equator for the first two billion years of Earth history without atmospheric greenhouse gases, but they were not, thus the “paradox’. And 250 million years ago, the period described by Wignall, the sun was less than 98% as luminous as today using a well-known formula [Gough] derived by astrophysicist Douglas Gough [Wiki Gough] and as shown by the blue dot in Figure 1. If we assume that the pre-extinction climate was about the same as today, then there would have needed to be more blankets of carbon dioxide and other greenhouse gases to make up for a reduction of about 5 Watts per meter squared in the amount of solar radiation absorbed by the Earth surface. Using equations given in [Foster] the cooler sun would have resulted in Earth surface about 6 degrees C colder than the twentieth century climate. This is colder than the Earth surface was during the Last Glacial Maximum Ice Age 22,000 years ago when New York was covered in a mile of glacial ice, yet there is no evidence the climate was that cold at that time. To make up for the reduced TSI, using a value of Equilibrium Climate Sensitivity (ECS) of 3 degrees C per doubling of atmospheric carbon dioxide, the atmospheric concentration of the gas would have had to have been about 1,200 parts per million by Volume (ppmV) or 4 times the twentieth century value of 300 ppmV. Paleoclimatologists Frode Stordal, Henrik H. Svensen, Ingrid Aarnes, and Marco Roscher estimate [Stordal] that the pre-extinction event carbon dioxide was indeed about 1,300 in good agreement. This would have been an Earth with polar ice just like today.
During the early Triassic extinction event Stordal et al. [Stordal] estimate that the Earth surface temperature warmed on average between 7 to 10 degrees. If due entirely to carbon dioxide, its concentration would have had to rise to a value between 5,000 and 10,000 ppmV in the atmosphere again assuming an ECS of 3 degrees C per doubling of carbon dioxide. Many paleoclimatologists make the case that the greenhouse gas methane may have also risen and contributed substantially [Beerling]. Because of the increase in TSI during the last 250 million years, we will only have to increase the atmospheric carbon dioxide load to between 1,200 and 2,500 ppmV in order to warm the planet 7 to 10 degrees and create a climate similar to that of the End-Permian extinction event. Since our annual emissions are the same as the worst-case emissions during this event as described by Wignall, and since we are also emitting other greenhouse gases such as carbon monoxide, methane, and nitrous oxide we are likely to create the conditions favorable to a major extinction event unless we decide to do something else instead.
There is a denier excuse [SS] which goes: “The killer proof that CO2 does not drive climate is to be found during the Ordovician- Silurian and the Jurassic-Cretaceous periods when CO2 levels were greater than 4000 ppmv (parts per million by volume) and about 2000 ppmv respectively. If the IPCC theory is correct there should have been runaway greenhouse induced global warming during these periods but instead there was glaciation.” This quote is from a short unpublished paper by Ian C McClintock [McClintock]. You now know what is wrong with this excuse and why it could never be published in a peer-reviewed journal. There is no mention or consideration of solar luminosity. Obviously, carbon dioxide levels had to be higher to make up for the lower solar irradiation. Since a runaway greenhouse such as occurred on Venus is not possible on Earth during the Phanerozoic nor does science claim it is, the suggestion that the IPCC theory would have predicted it is a red herring and any reviewer would have required it be removed. What is possible and in fact did occur, possibly a result of climate change, was a major extinction event at the End-Ordovician, 442 million years ago. The solar irradiance was 9 Watt per meter squared less than today requiring even more atmospheric carbon dioxide than what was required at the End-Permian to avoid an ice age.
As regards the Jurassic, according to Anthony Hallam, Wignall’s thesis advisor, there is no evidence for glaciation and instead this period experienced a hot house climate as we would expect given the increased greenhouse gas forcing and reduced solar intensity [Hallam]. According to Gregory Price [Price] in a later paper there is modest evidence for sporadic polar glaciation during the Jurassic. Using Gough’s formula, the solar intensity, at the Earth surface at the Jurassic-Cretaceous boundary, 145 million years ago, was about 3 Watts per meter squared less than today which would have required about 600 or 700 ppmV atmospheric carbon for the same climate, i.e. one with polar ice. According to Foster et al. atmospheric carbon was on average about 1,000 ppmV during the Jurassic, rather than 2,000, which indicates a warmer climate than today but not one far removed from the possibility of polar glaciation. This value extends over 55 million years of Earth history and a period involving major tectonic plate movement, including sea level changes and mountain building, associated with the breakup of Pangea. Therefore, atmospheric carbon likely varied considerably and dipped below 600 ppmV often. Earth’s orbital variations, as we’ve experienced throughout the Pleistocene and Holocene would likely have caused polar glaciation.
What is amusing about McClintock’s paper is his remark that: “the IPCC theory is in fact only supported by computer models.” This is not true but what is remarkable is that McClintock’s source for the atmospheric CO2 values is a paper by geochemist Robert Berner [Berner]. Berner was addressing the faint young sun problem by showing that a least during the Phanerozoic, 570 million years ago to present, atmospheric carbon dioxide may have been high enough to make up for the reduced solar influence. So it is weird that McClintock was not aware of this unless he didn’t read the paper. The amusing thing is that Berner’s atmospheric carbon dioxide values, which McClintock takes at face value, are derived from a model. Berner concludes “This type of modeling [Berner’s] is incapable of delimiting shorter-term CO2 ﬂuctuations (Paleocene-Eocene boundary, late Ordovician glaciation) because of the nature of the input data which is added to the model as 10 my or longer averages. Thus, exact values of CO2, as shown by the standard curve, should not be taken literally and are always susceptible to modiﬁcation. Nevertheless, the overall trend remains. This means that over the long term there is indeed a correlation between CO2 and paleotemperature, as manifested by the atmospheric greenhouse effect.”
It is worth reminding ourselves that there are no stupid questions and certainly asking about higher values of atmospheric carbon dioxide in the past and the implications is a good important question. However, there are stupid assumptions which we should remind ourselves “make an ass of you and me” and assuming that high values of carbon dioxide in the past is killer proof of anything other than one’s own ignorance is kind of stupid. A good question is: why do deniers believe so gullibly in so many soap bubble excuses to avoid addressing such serious issues as climate change?
[Bahcall] Bahcall, J. N., M. H. Pinsonneault, and S. Basu (2001), Solar models: Current epoch and time dependences, neutrinos, and helioseismological properties, Astrophys. J., 555, 990–1012, doi:10.1086/321493.
[Beerling] Beerling, D., R. A. Berner, F. T. Mackenzie, M. B. Harfoot, and J. A. Pyle (2009), Methane and the CH4-related greenhouse effect over the past 400 million years, Am. J. Sci., 309,97–113, doi:10.2475/02.2009.01.
[Berner] Berner, R.A. and Kothavala, Z. 2001: Geocarb III: A revised model of atmospheric CO2 over
Phanerozoic time. American Journal of Science 301: 182-20
[Feulner] Georg Feulner, The faint young Sun problem, Rev. Geophys., 50, RG2006, doi:10.1029/2011RG000375, 25 May 2012.
[Foster] Gavin L. Foster, Dana L. Royer & Daniel J. Lunt, Future climate forcing potentially without precedent in the last 420 million years, Nature Communications, DOI: 10.1038/ncomms14845 www.nature.com/naturecommunications 4 April, 2017
[Gough] Gough, D. O. Solar interior structure and luminosity variations. Solar Phys. 74,
[Hallam] STUDIES IN GEOPHYSICS, Chapter 17 Studies in Geophysics, Climate in Earth History, National Research Council, National Academy Press, Washington, D.C., 1982, https://www.nap.edu/read/11798/chapter/19
[Price] Gregory Price, The evidence and implications of polar ice during the Mesozoic, Earth-Science Reviews, Volume 48, Issue 3, November 1999, Pages 183-210 http://www.sciencedirect.com/science/article/pii/S0012825299000483
[SS] Skeptical Science https://skepticalscience.com/co2-higher-in-past.htm
[Stordal] Frode Stordal, Henrik H. Svensen, Ingrid Aarnes, Marco Roscher, Global temperature response to century-scale degassing from the Siberian Traps Large igneous province, Palaeogeography, Palaeoclimatology, Palaeoecology 471 (2017) 96–107.
[wiki Gough] https://en.wikipedia.org/wiki/Douglas_Gough