A look at diminishing atmospheric pressure

Guest post by Andi Cockroft

In an unrelated article of mine on Isostacy and Mean Sea Level posted here, I mentioned in passing a thesis paper by Theresa Cole (here) and here: ColeTheresaN2011MSc – which included this graph depicting an observed fall in global annual mean atmospheric pressure since 1916 (from NOAA I believe)

A recent exchange with Theresa, has caused me to revisit this apparent anomaly, and wonder where this is all heading – and indeed how long this has been going on !

But why the heading – So Dinosaurs Could Fly ?

Well, seems that engineers are of the opinion that the pterosaurs were just too heavy to get off the ground given today’s environment, and they must have been helped by far denser air.

Denser air of course means a higher pressure – I have seen estimates ranging from about 3.5 to 8 times that of today. Let’s pick a mid-point of say 5 for the purpose of this post. (I trust these are not the same engineers who state categorically that a Bumble-Bee is incapable of flight)

So from 100Mya to today, how has air pressure gone from a possible 5000 mbar to 1013 mbar of today? And why is it still (possibly) continuing to fall?

Questions that spring to mind are:-

· Is our atmosphere being sucked out in to space?

· Is the composition of the atmosphere changing and so getting lighter?

· Change in water vapour?

· Increasing CO 2

· Burning hydrocarbons + O 2 -> CO & CO 2

· Volcanic eruptions

· Release/Uptake of gases from/to the ocean

· O 3 -> O 2

· Is an increase in temperature causing a somehow related increase in pressure?

For those who might not remember, I remind readers I do not have strong scientific qualifications in meteorology, hydrology chemistry etc., just an enquiring mind – so feel free to disagree with my arguments here.

In researching this post, I came across many conundrums. Many contradictions or seemingly incongruent theories. But hey, let’s look at what is out there starting with young Earth and work forwards to see what we shall reveal.

I also found myself using those well used weasel words such as could, may, might, suppose etc. Sorry, but given the nature of the discussion – this is just what it is a discussion of some possibilities – not proven fact!

So, just looking at the graph in figure 1 of the past 90 years:- Temperature may have localised effects, but in general, global mean atmospheric pressure at sea level is directly proportional to the mass of the entire atmosphere – the current accepted mean value is around 1013.25 Mbar. So any warming observed over the past 90 or so years should be ruled out as causation – warm or cold the air weighs the same (within reason)

A drop of 1 Mbar may seem trivial over 90 years, but at that rate mother Earth may run out of atmosphere altogether in just 100,000 years !!

Going back 100 million years, for a pressure equivalent to 5000 Mbar, there would have to be either (a) a lot more air, or (b) different composition – or a combination of each.

And of course the raging question – how has a 5000 Mbar atmosphere reduced to todays 1013.25 Mbar?

The Levenspiel et al 2000 paper is well worth a read, and has been cited indirectly here as part of 450 Peer-Reviewed Papers Supporting Skepticism of AGW caused Global Warming here, and referred to at WUWT here.

What was the air pressure for the 97% of Earth’s life before the age of dinosaurs? Levenspiel et alhave three possible alternatives, as shown in Figure 3.

The pressure could have been at 1 bar throughout Earth’s earlier life, risen to 4–5 bar ~100 Mya (just at the time when the giant fliers needed it), and then returned to 1 bar (curve A).

The pressure could have been ~4–5 bar from Earth’s beginning, 4600 Mya; and ~65 Mya, it could have begun to come down to today’s 1 bar (curve B).

The atmosphere could have started at higher pressure and then decreased continuously through Earth’s life to ~4–5 bar ~100 Mya and down to 1 bar today (curve C).

The third alternative seems to be the most reasonable, so let us pursue it. We will also look into the composition of Earth’s atmosphere, but we will first discuss Earth’s surface and see how it affects the atmosphere.

From http://www.engineeringtoolbox.com, the specific gravity of some common gases can be found in the table below:

Gas Specific Gravity Acetylene (ethyne) – C 2 H 2 0.90 Air1) 1.000 Alcohol vapour 1.601 Ammonia – NH 3 0.59 Argon – Ar 1.38 Arsine 2.69 Benzene – C 6 H 6 2.6961 Blast Furnace gas 1.02 Butadiene – C 4 H 6 1.87 Butane – C 4 H 10 2.0061 1-Butene (Butylene)- C 4 H 8 1.94 Isobutene – C 4 H 8 1.94 Carbon dioxide – CO 2 1.5189 Carbon monoxide – CO 0.9667 Carbureted Water Gas 0.63 Chlorine – Cl 2 2.486 Coke Oven Gas 0.44 Cyclobutane 1.938 Cyclopentane 2.422 Cyclopropane 1.451 Decane 4.915 Deutrium – D 2 0.070 Digestive Gas (Sewage or Biogas) 0.8 Ethane – C 2 H 6 1.0378 Ether vapour 2.586 Ethyl Chloride – C 2 H 5 Cl 2.23 Ethylene (Ethene) – C 2 H 4 0.9683 Fluorine 1.31 Helium – He 0.138 Heptanes 3.459 Hexane 2.973 Hydrogen 0.0696 Hydrogen chloride – HCl 1.268 Hydrogen sulfide – H 2 S 1.1763 Hydrofluoric acid 2.370 Hydrochloric acid 1.261 Illuminating gas 0.4 Isobutane 2.01 Isopentane 2.48 Krypton 2.89 Marsh gas 0.555 Mercury vapour 6.940 Methane – CH 4 0.5537 Methyl Chloride 1.74 Natural Gas (typical) 0.60 – 0.70 Neon 0.697 Nitric oxide – NO 1.037 Nitrogen – N 2 (pure) 0.9669 Nitrogen – N 2 (atmospheric) 0.9723 Nitrous oxide – N 2 O 1.530 Nonane 4.428 Octane 3.944 Oxygen – O 2 1.1044 Ozone 1.660 Pentane 2.487 Phosgene 1.39 Propane – C 3 H 8 1.5219 Propene (Propylene) – C 3 H 6 1.4523 R-11 4.742 R-12 4.174 R-22 2.985 R-114 5.9 R-123 5.279 R-134a 3.522 Sasol 0.42 Silane 1.11 Sulfur Dioxide – SO 2 2.264 Toluene-Methylbenzene 3.1082 Water gas (bituminous) 0.71 Water vapor 0.6218 Xenon 4.53

1) NTP – Normal Temperature and Pressure – is defined as air at 20oC (293.15 K, 68oF) and 1 atm ( 101.325 kN/m2, 101.325 kPa, 14.7 psia, 0 psig, 30 in Hg, 760 torr)

Since specific gravity is the ratio between the density (mass per unit volume) of the actual gas and the density of air, specific gravity has no dimension. The density of air at NTP is 1.205 kg/m3

To change the “mass” of the atmosphere to any meaningful way would require say a 75% mercury vapour composition – something not altogether conducive to life as we know it. The alternative is of course just a lot more atmosphere.

Turning our attention for a moment to Earth’s twin, Venus, formed in probably very similar environs, yet Venus retains an atmosphere composed of CO 2 and Nitrogen, with a pressure equivalent of around 90 Bar. Venus is closer to the Sun, so receives greater energy, but that cannot in itself account for the very significant differences in today’s environments.

Levenspiel postulates that the creation of Earth’s companion Moon stripped off much of Earth’s mantle, leaving it a rather fluid lithosphere compared to Venus. It is this fluid lithosphere that has allowed continental drift to rearrange and directly affect the planet’s atmosphere. Couple that with a slightly cooler Earth (less sunlight), allowing liquid water to form, and the basis for removal of CO 2 is formed.

If say 4 Bya, Earth did have an atmosphere with a 90% CO 2 concentration, with a high atmospheric pressure, Levenspiel proposes that simple dissolution in water would see a 50% reduction in nett CO 2 atmospheric concentrations.

But it doesn’t stop there

Several cycles take place to remove CO 2 from the atmosphere, not least by dissolution in rain, combination with minerals on land and ultimately flowing into the oceans and deposit as sedimentation.

True, some subduction at plate boundaries would recycle carbonates through volcanisms and back into the atmosphere, but over time a gradual reduction of CO 2 takes place.

As carbon life-forms take up even more carbonates to build homes for themselves, then die and bequeath these homes to the sea floor as sediment, more and more carbon is tied up as rock.

In Potential Errors in Estimates of Carbonate Rock Accumulating through Geologic Time (pay walled here), Hay calculates that today the continents contain at least 2.82 × 106 km3 of limestone, which are the remains of deposits over the past 570 million years that have not been washed to sea or subducted back into Earth’s interior. This is equivalent to a CO 2 atmospheric pressure of 38 bar. If we add the carbonates found on the ocean floor, the equivalent CO 2 atmospheric pressure rises to 55 bar.

Adding all this together more than accounts for a 90% CO 2 concentration at 90 Bar being reduced over time to a much lower say 20% CO 2 and 4 or 5 bar – just right for the pterosaurs to take wing.

Whilst all this was going on, plant life took a turn all of its own.

Evolving from the primordial soup, cyanobacteria initially removed Iron from the oceans and created Oxygen. It was this oxygen that then led to multi-celled life-forms and ultimately diverging between the plants and animals such as protozoa, fish, land animals and dinosaurs

Above: A laminated rock formed by the growth of blue-green algae (i.e., cyanobacteria)

So, if we now accept that 100Mya, there was an atmosphere with about 20% CO2 and say 5 Bar pressure, would plant and animal life have thrived under such conditions? Do we even know that these values were anywhere near accurate?

If we believe the aeronautical engineers, pterosaurs needed a denser air to succeed – that estimate is between 3.5 and 8 times current density (=pressure). So that part of our assumption looks OK on the face of it – yes air would have had to have been more dense.

And what of O 2 ?

Well perhaps it comes down to some type of proxies – yes our old friends !

We do know that there were some pretty impressive flying insects around back then, and it seems well known that insects breath through their “tracheae” – narrow tubes – rather than having lungs or gills. These tracheae transfer O 2 directly from the surface of the skin into the organs of the body. The ability to uptake O 2 is governed by the length of the tracheae. Big insects naturally have longer tracheae, so uptake less O 2 – that is unless O 2 is served at higher concentrations and/or pressure so the body can get all the O2 it needs.

Since we know there were huge dragonflies and cockroaches around during the Carboniferous and Permian (300-250Mya), it seems to support a postulation that O 2 concentrations were of the order 35% back then, compared to today’s 20%.

Meganeura, a genus of dragonfly from about 300Mya had a wingspan of up to 65cm (2’1”), and Meganeuropsis Permiana from about 250Mya grew even larger – up to 71cm (2’4”).

Neither survived to compete alongside the pterosaurs however. Many believe the concentrations of O2 dropped too low to allow such mega fauna to survive beyond the Permian.

In Part II, I will pick up on your suggestions from comments here, and look to what has happened to reduce Atmospheric Pressure from 5 Bar to 1 Bar, and why it continues to drop today.

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