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I was curious about how the Earth's overall air pressure has varied over time, and tried to take a look around the internet. However, Google pops up a lot of sites with questionable science proposing that the air pressure was much higher in the past. The more outrageous claims (such as those by David Esker) are dealt with on Skeptics.SE.

However, there are more reasonable claims that air pressure was perhaps 4 or 5 times higher in the Mesozoic, thus allowing pterosaurs to fly more effectively, etc. The websites making such claims do heavily set off my pseudoscience alarms, but then some are published by the American Chemical Society by reputable sounding scientists with references to papers.

There is evidence that in the far distant past, air pressures were lower than they are currently. But after the changes to the atmosphere wrought by photosynthetic life, is there any evidence supporting the various theories of higher air pressures?

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  • $\begingroup$ A comment in the linked Skeptics Q&A links to this related Nature article. $\endgroup$ – gerrit May 5 '17 at 13:38
  • $\begingroup$ @gerrit Edited that in to the question. That certainly contradicts the link to the ACS webpage. $\endgroup$ – kingledion May 5 '17 at 13:42
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    $\begingroup$ "Chemical Innovations" was an ACS publication, but I'm fairly sure it was a magazine rather than a peer-reviewed journal. Not necessarily bad science of course, but I'd be a little wary, since I've seen fringe theories published in other professional society magazines -- a good tactic for avoiding peer review while getting an apparent imprimatur from a respectable organization. I seem to recall that the Geological Society magazine even published an article against plate tectonics a few years back, but quietly pulled it from their online archive some time later. $\endgroup$ – Pont May 5 '17 at 14:08
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    $\begingroup$ @trondhansen perhaps that should be a separate question, but gravitationally, Earth can hold a tremendous amount of atmosphere. There's no reason why not. Venus, without a magnetic field and smaller and hotter than Earth has an atmosphere about 90 times more massive than Earth's. Earth's gravity is too weak to hold lighter gases like hydrogen and helium, but beyond that, the gravity is sufficient to hold an enormous atmosphere. Hundreds or thousands of times more massive. en.wikipedia.org/wiki/Atmospheric_escape $\endgroup$ – userLTK May 6 '17 at 15:16
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    $\begingroup$ @trondhansen on Oxygen content. That's an interesting question. Generally, the % oxygen between 250 and 65 million years ago is thought to be similar to today, but precise answers aren't know. en.wikipedia.org/wiki/Geological_history_of_oxygen Higher oxygen content might be more efficient, and/or allow for smaller lungs. It could make flying easier due to more oxygen could be taken in through breathing. But there are other factors relating to muscular exertion beyond oxygen content. Your correct that it could be a factor. $\endgroup$ – userLTK May 6 '17 at 15:23
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Yes, there is. But the data is still very sparse and errors are large.

Past atmospherics pressures have been estimated by at least three different methods:

  1. Isotopic composition of fluid inclusions trapped hydrothermal quartz ( Nishizawa ey al 2007; Goldblatt et al 2009; Marty et al, 2013)

enter image description here

Image from Fig. 2 of Nishizawa et al 2007.

  1. Size distribution of gas bubbles in basaltic lava flows (Som et al 2016)

enter image description here

Figure 3c of Som et al (2016): Beasley River geologic context and flow detail (scale bar, 1 cm)

  1. Size distribution of fossilised raindrop imprints (Som et al, 2012; Kavanagh & Goldblat, 2015)

enter image description here

Figure 1 of Som et al (2012) The 2.7-billion-year-old Ventersdorp Supergroup raindrop imprints lithified in tuff at Omdraaivlei, South Africa.

Each study is based on samples that capture the conditions at a fairly specific point in time. Therefore, different results not necessarily contradict each other, but offer a sense of how variable has been the atmospheric pressure over geological time.

The size distribution of fossilised raindrop imprints might have a large range of error due to the many factors that influence drop size beside atmospheric pressure. However, some studies suggest that the Archean (4 to 2.5 billion years ago) atmosphere was almost ten times denser than it is today (Kavanagh & Goldblat, 2015). That figure is based on fossilized raindrop imprints dated about 2.7 billion year ago, that is after the evolution of photosynthesis, but still in its early stages, when most oxygen was absorbed by the oceans and there was very little of it in the atmosphere. On the other hand, studies based in isotopic composition of fluid inclusions and bubbles in basaltic lava have found that the atmosphere was less dense than it is today.

These studies are of great interest, as a denser atmosphere even if poor in greenhouse gasses can produce warmer surface condition, helping to resolve the faint young Sun paradox. In a nutshell, this paradox refers to how could liquid water exist on Earth in the past when the sun was much fainter than it is today.

It is important to note that the additional temperature in a thick atmosphere doesn't come from adiabatic warming as some people have suggested here. A good treatment of the phenomena is presented by Chemke et al (2016) in the paper "The thermodynamic effect of atmospheric mass on early Earth's temperature", there they say:

We find that higher atmospheric mass tends to increase the near-surface temperature mostly due to an increase in the heat capacity of the atmosphere, which decreases the net radiative cooling effect in the lower layers of the atmosphere. Additionally, the vertical advection of heat by eddies decreases with increasing atmospheric mass, resulting in further near-surface warming.

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  • $\begingroup$ One of my colleagues is researching a topic closely related to this. Her opinion is that no one actually knows, and there are lines of evidence both against and for a higher/lower (pick your fav) pressure during the early Earth. $\endgroup$ – Gimelist Apr 7 at 1:03
  • $\begingroup$ @Gimelist Interesting. But I find very likely that both scenarios took place at different points in Earth's history. $\endgroup$ – Camilo Rada Apr 7 at 1:07
  • $\begingroup$ I choose to not hold an opinion. If one of the world's experts on the topic says we have no idea, then who am I (an igneous petrologist) to say anything? :) $\endgroup$ – Gimelist Apr 7 at 1:09
  • $\begingroup$ @Gimelist Are you saying that she thinks that the answer is either higher OR lower? Ruling out the option that both scenarios took place? That would be surprising, as I cannot think of any other atmospheric variable that have changed monotonically throughout Earth's history. $\endgroup$ – Camilo Rada Apr 7 at 1:23
  • $\begingroup$ She says that the evidence both ways is not conclusive. Many contradictory things. $\endgroup$ – Gimelist Apr 7 at 1:43
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All manner of isotopic analyses confirms that the chemical composition of the earth's atmosphere has fluctuated wildly in the past. Besides the Archean hypoxic atmospheres of 2.5 billion years ago, the late Paleozoic saw oxygen concentrations that were much higher than they are today allowing a wide variety of giant insects to thrive (simple diffusion is a large part of insect respiration). Some believe that in addition to much higher oxygen levels, a much higher air density (pressure) may have accelerated the development of flight in these insects:

ATMOSPHERIC OXYGEN, GIANT PALEOZOIC INSECTS AND THE EVOLUTION OF AERIAL LOCOMOTOR PERFORMANCE

By the way, the internets are full of people who don't understand the difference between "high oxygen partial pressure" and "high atmospheric pressure".

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I think there is good evidence that air pressure was 3-5 times higher during hothouse periods like the Mesozoic Era.

It solves how 1 Pterosaurs could fly 2 Sauropods could grow up to 80 metric tons 3 Why arthropods were much larger 4 The climate was so warm and wet, even at the poles.

Someday I plan on getting a team together to investigate this further; not that the atmosphere was denser at the surface, but how it would be possible. The obvious answer will be found in the correlation between ocean chemistry cycles and hothouse/icehouse climate cycles. The correlation suggests that the oceans were significantly warmed via geothermal heat flux but more importantly hydrothermal pumping, causing increased degassing and evaporation, and thus a thicker atmosphere.

The fact that marine chemistry and the major climate cycles correlate very well and marine chemistry is controlled by variable rates in sea floor production suggests some link between the two and it is sad to me that no one has investigated this link beyond invoking that CO2 is the overarching climate control knob, which all evidence suggests to be erroneous. It's basic physics - gravitational potential energy and kinetic theory of gases - the heavier the atmosphere over your head the warmer the surface will be.

http://ftp.earthbyte.org/Resources/Pdf/Muller_etal_2013_Seawater_chemistry_driven_by_Supercontinent_assembly_breakup_dispersal_Geology.pdf

https://s3.amazonaws.com/academia.edu.documents/3468640/Veizer_Nature_2001.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1534442228&Signature=3Pxz67vBnWWnBbh%2BUzIp%2BwnewP4%3D&response-content-disposition=inline%3B%20filename%3DEvidence_for_decoupling_of_atmospheric_C.pdf

Another good comparison would be crocodilia since they are still extant today. They were much larger in the mesozoic (up to 8 tonnes!) but food availability is not limiting the size of these creatures today. The size limitation is due physical limitations as body mass increases. In a denser atmosphere you explain why these physical limitations are at a much larger body size.

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    $\begingroup$ Supersized animals were perhaps enabled by high oxygen partial pressure (which allowed their lungs to be more relatively efficient). Doubling available oxygen would only increase atmospheric mass/pressure by 1.2. The climate factors are controlled by greenhouse gasses like water vapor, carbon dioxide and others. Increasing those by a factor of 10 would barely move the needle on atmospheric mass and pressure. So the 'evidence' you site doesn't necessarily imply a 3x - 5x increase in atmospheric pressure. $\endgroup$ – kingledion Aug 16 '18 at 17:58
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    $\begingroup$ Regarding your papers, you seem to be making an argument that marine chemistry changes drive climate change over time. Your first paper does nothing to support that, being about the mechanism by which mid-ocean ridges alter the Mg/Ca ratios of the ocean. But even conceding the argument of the second letter (not peer reviewed! although I know the author), that has little to nothing to do with total atmospheric pressure. I'm going to have to put this post down as a complete troll job. -2 for wasting my time, but +1 for an interesting paper on mid-ocean ridges. $\endgroup$ – kingledion Aug 16 '18 at 18:07
  • $\begingroup$ Typical cognitive dissonance, if you don't agree with someone then just claim it's a "troll job." It's not just about oxygen, but how something like an 8 tonne crocodile can or an 80 tonne sauropod can physically support their structure in 1 atm, or how very large pterosaurs could fly in 1 atm. $\endgroup$ – Bobby T Aug 16 '18 at 18:43
  • $\begingroup$ "Not peer reviewed" is not a rebuttal, it only suggests that you are using multiple logical fallacy to support your lack of argument. $\endgroup$ – Bobby T Aug 16 '18 at 18:59
  • $\begingroup$ Also, sea level was significantly higher during hothouse periods, possibly contributing to a higher surface pressure via smaller atmospheric volume or higher gravitational energy.journals.ametsoc.org/doi/abs/10.1175/JAS-D-12-059.1 $\endgroup$ – Bobby T Aug 16 '18 at 19:23

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