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 CO2
· Burning hydrocarbons + O2 -> CO & CO2
· Volcanic eruptions
· Release/Uptake of gases from/to the ocean
· O3 -> O2
· 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) – C2H2 | 0.90 |
| Air1) | 1.000 |
| Alcohol vapour | 1.601 |
| Ammonia – NH3 | 0.59 |
| Argon – Ar | 1.38 |
| Arsine | 2.69 |
| Benzene – C6H6 | 2.6961 |
| Blast Furnace gas | 1.02 |
| Butadiene – C4H6 | 1.87 |
| Butane – C4H10 | 2.0061 |
| 1-Butene (Butylene)- C4H8 | 1.94 |
| Isobutene – C4H8 | 1.94 |
| Carbon dioxide – CO2 | 1.5189 |
| Carbon monoxide – CO | 0.9667 |
| Carbureted Water Gas | 0.63 |
| Chlorine – Cl2 | 2.486 |
| Coke Oven Gas | 0.44 |
| Cyclobutane | 1.938 |
| Cyclopentane | 2.422 |
| Cyclopropane | 1.451 |
| Decane | 4.915 |
| Deutrium – D2 | 0.070 |
| Digestive Gas (Sewage or Biogas) | 0.8 |
| Ethane – C2H6 | 1.0378 |
| Ether vapour | 2.586 |
| Ethyl Chloride – C2H5Cl | 2.23 |
| Ethylene (Ethene) – C2H4 | 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 – H2S | 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 – CH4 | 0.5537 |
| Methyl Chloride | 1.74 |
| Natural Gas (typical) | 0.60 – 0.70 |
| Neon | 0.697 |
| Nitric oxide – NO | 1.037 |
| Nitrogen – N2 (pure) | 0.9669 |
| Nitrogen – N2 (atmospheric) | 0.9723 |
| Nitrous oxide – N2O | 1.530 |
| Nonane | 4.428 |
| Octane | 3.944 |
| Oxygen – O2 | 1.1044 |
| Ozone | 1.660 |
| Pentane | 2.487 |
| Phosgene | 1.39 |
| Propane – C3H8 | 1.5219 |
| Propene (Propylene) – C3H6 | 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 – SO2 | 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 CO2 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 CO2 is formed.
If say 4 Bya, Earth did have an atmosphere with a 90% CO2 concentration, with a high atmospheric pressure, Levenspiel proposes that simple dissolution in water would see a 50% reduction in nett CO2 atmospheric concentrations.
But it doesn’t stop there
Several cycles take place to remove CO2 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 CO2 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 CO2 atmospheric pressure of 38 bar. If we add the carbonates found on the ocean floor, the equivalent CO2 atmospheric pressure rises to 55 bar.
Adding all this together more than accounts for a 90% CO2 concentration at 90 Bar being reduced over time to a much lower say 20% CO2 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 O2?
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 O2 directly from the surface of the skin into the organs of the body. The ability to uptake O2 is governed by the length of the tracheae. Big insects naturally have longer tracheae, so uptake less O2 – that is unless O2 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 O2 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.
Fascinating topic. The atmospheric pressure argument is far more compelling than the gravity story (magic anyone?).
Raindrop studies however seem to refute it to some extent, but, perhaps twice modern levels was enough?
Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprint Som etal Nature 484, 359–362 (19 April 2012)
http://www.nature.com/nature/journal/v484/n7394/full/nature10890.html
The ‘engineers say bumblebees can’t fly’ thing is one of those urban legends of science. Bumblebee flight is understood quite well. It has great appeal, however, to the ‘those scientists think they’re so smart’ crowd.
agwnot says:
June 2, 2012 at 2:17 am
Pilots know when humidity increases, lift decreases, so yes, water vapor affects the density of the air. That observation is true when water vapor displaces air locally, i.e. when otherwise the air pressure is the same, higher humidity results in less lift. What you attempted to argue was the contribution of more or less water vapor in the total atmosphere. If there is more water vapor, that means all your p’s need to add up to more than we have now. And since all the other p’s don’t change, the total pressure goes up with more water vapor when the other components are constant.
Fascinating. If Levenspiel et al’s arguments for a very thick atmosphere high in CO2 is correct, it implies that rather than not having enough CO2 in the ancient atmosphere to account for the Faint Sun, we have way too much. The Earth had a liquid ocean. At that amount of CO2, that thick of an atmosphere, I would guess that without something to counteract the warming effect, the ocean would evaporate away. Evidence of a thermostatic control mechanism on the Earth’s temperature?
Paging Willis?
Or less formally, sea-levels have been rising not because of AGW, but because the earth’s atmospheric pressure has been decreasing. True or false?
As an exercise in citizen-science, WUWT readers are invited to post their personal assessment as to whether the Sea-Expansion Hypothesis is tenable.
Are we sure the pterosaurs were flying in the atmosphere? Might they have been something like proto-penguins, or air breathing manta rays?
ggm says:
June 2, 2012 at 4:58 am
While many people laugh at the “expanding earth” theory (just like some laugh at AGW skeptics) – simple logic says this…. each day, tonnes of dust and micro meteroites fall to earth. I`ve seen some estimates that claim up to 200,000kg per day. If it`s 100 tonnes per day (100,000Kg), then in just 1 million years, this equals 36 trillion Kg. Now if the “100 tonnes” per day is actually more like 1000 or 10,000 tonnes per day ( we have no accurate measure)…. then the expanding earth theory has some validity.
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So 1,000 tonnes/day gives 365 billion tonnes in 1 million years. The Earth has a [current] surface area of about 500 million square kilometres (very roughly). So that’s about 730 tonnes per square kilometre. Or 730 grammes/square meter/million years. Over 200 million yeras that’s a non-negligible 146 kilograms/square meter; with a density of twice that of water [and it might be more] that will increase the diameter of the earth by about 146 milimetres over those 200 million years.
Moving three inches further from the centre of mass [and an increased mass, too!] will not reduce gravity all that much, I suggest.
Have a wonderful Jubilee Weekend!
Auto.
Legatus says:
June 2, 2012 at 7:31 am
In ancient times, we know that there were giant flying critters. Such could not exist unless the atmosphere was thicker back then.
[—]
We don’t actually know this. We presume it by comparison to modern proxies. We can calculate probable weight to area ratios based on the anatomy of the species as we know it, and with the use of well established and tested ratio ranges for a wide variety of aerodynamic and partially aerodynamic surfaces, deduce a likelihood of flight, but we will never actually know it. Therefore, to conclude denser atmosphere from the anatomy of a sparse number of ancient critter samples is simply a wild-eyed flight of fancy.
“Neither survived to compete alongside the pterosaurs however. Many believe the concentrations of O2 dropped too low to allow such mega flora to survive beyond the Permian.”
Did you really mean “megaflora?” Insects aren’t flora. Perhaps “insect megafauna”?
Are those pressure data from 1916 to present good data? Were stations added or subtracted? Is there any impact from smog in urban areas? Oh, wait, sorry, I’d forgotten that such questions just aren’t to be asked in polite company.
These gentlemen think pterosaurs could fly quite well at todays atmospheric densities. A very good paper. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0013982
I used to make my living as an aviator so I have some familiarity with the requirements for flight. their analysis and the program they used to do the aero computations make sense. 1/2 rho V2, the energy of air flowing past a wing, doesn’t require an adjusted rho from increased air density to enable pterosaur flight capability by their calculations, although it would certainly facilitate flight.
Human/Mammalian physiology would seem to confirm this –
http://sun025.sun.ac.za/portal/page/portal/Health_Sciences/English/Departments/Biomedical_Sciences/MEDICAL_PHYSIOLOGY/Essays/What_is_life
http://sun025.sun.ac.za/portal/page/portal/Health_Sciences/English/Departments/Biomedical_Sciences/MEDICAL_PHYSIOLOGY/Essays/pulmonary_physiology
Venus may be the odd one out due to thermal overturn of the crust “recently”. That might have resulted in a tremendous CO2 release into the atmosphere (as well as the SO2 which we also observe. Mars, too, with the massive Hellas, Argyre and mostly-submerged impact scars on Mare Borealis, might well have lost most of its atmosphere, with its present atmosphere, being regenerated from vulcanism (such as Tharsis and Elesium).
The high percentages of CO2 being suggested for the time of the pterosaurs would be toxic to animal life as we know it.
Too many assumptions, too many variables, too many unknowns.
I made the observation that we often see CO2 concentrations in ppm or ppmv, but seldom in partial pressure.
I think it matters a great deal to the study of climate history whether a given CO2 concentration occurs in a 1013 milibar atmosphere or one of 5000 milibar. I think the question of atmospheric pressure in the geologic record is addressed too little.
http://wattsupwiththat.com/2012/03/03/new-questions-on-isostacy-and-mean-sea-level/
As for the question of what is the shape of the atmospheric decline curve, it really ought to be exponential.
As for replenishment, we have heard of the huge volumes of basalt erupted in the Deccan Traps and Siberian Traps. What might be the range of gas volume errupted with the extruded basalts? Is it a significant amount? If insignificant, then it is difficult to imagine vulcanism adding much to the atmosphere in the past billion years.
So about 2.8 million km3 of calcium carbonate estimated to be in today’s continents. Rough calcs suggest this equates to 780 giga tonnes of C and 3140 x giga tonnes of O locked up as CaCO3. If this deposit was originally all in gaseous form – Co2 and the balance as O2 – it would nearly double the mass of the atmosphere ( est as 5000 giga tonnes).
O2 levels would be about x2 what they are now…..35-40%, and CO2 level would be about double the current…..around 6gigatonnes.
Not nearly enough to explain the much higher concentrations of 2000-4000ppm we see from paleo records.
RogerJ says:
June 2, 2012 at 5:23 am
Hmm, the human body seems to adequately compensate for far lower air pressure (475 millibar, or so), so we should be good for another 50000 years. Obviously then, you should learn to relax and breathe deeply and slowly, if you’re planning on living 50000 years.
If air is “stripped” from earth, would it not also be “stripped” from giant gas balls like Jupiter?
Then where does it all go? Might it not “condence” out in the Ort belt, as mini-comets?
Then might it not “rain” back onto the planets, as shooting stars? This would complete a nice, neat cycle.
Furthermore, once you have this cycle, (a bit like evaporation-leading-to-clouds-leading-to-rain,) you might have times of cosmic “drought,” with few comets, and times of cosmic “rain,” when on any given night there would be several comets in the sky.
What convinces comets to leave the Ort belt and plunge towards our sun? A passing brown dwarf? The shock wave from a super nova? Whatever it is, it seems possible such a nudge might send a thousand comets towards the sun, rather than just one.
None of these comets need to hit the earth. They just need to cross the earth’s orbit, leaving a trail of dry ice and dust.
I remember one night when I looked up into the sky as the earth passed through the orbit of a comet that zipped around the sun nearly a century earlier. The entire sky was filled with shooting stars. There was no time when two or three were not falling at the same time.
Suppose this went on for years? Would the amount of gas our atmosphere held increase?
Oxygen is dissociated from the oceans and injected into the atmosphere by cyanobacteria. The reverse of the sequestration of CO2 as PH and carbonate as regards the mass of the atmosphere. Maybe we should be worrying about oxygen. Oxygen concentration fell like a rock in the major extinctions. Cause or effect? And then there are the banded iron formations to demonstrate ancient fluctuations…
The idea that pterosaurs shouldn’t have been able to fly seems to actually be somewhat controversial. It may matter a great deal how they flew.
See the references here:
http://en.wikipedia.org/wiki/Pterosaur#Flight
However, the other arguments for a more dense atmosphere stand on their own. They may well be correct.
[forgive any redundancy of above comments, have not reviewed all yet]
There are four parameters for flight…gravity, thurst, lift and drag….if we assume gravity constant, then consider ‘thrust’….it is impossible to envision a magnitude changes, since the Jurassic, of muscle or bone, strength or weight….both ‘lift and drag’ are proportional to atmospheric pressure, giving slow moving, but double wingspan flight….this from ‘Slaying the Sky Dragon’…
“Compare the 65 million year old Pterodactyal Quezalcoatus Northopi and it’s eleven meter (thirty-six foot) wingspan with todays largers Peurvian Condor wingspan of five meters (sixteen feet). In addition compare the Meganeura Dragonfly with half meter (twenty-two inch) wing span and today’s Atlas Moth and it’s quater meter (eleven inch) wingspan.”….page 253….
The atmosphere is being continually eroded by solar wind, lunar tidal pulls, meteor impacts, volcanic outbusts that pull gases beyond exit velocity and by molecular decay, as in N14 to C14. But, the Earth is being constantly replenished by ‘elemental’ gases. Helium, Radon, Argon and the other formerly called “Inert Gases” provide a useful measure of production and extinction rates. [may post another reply after reading all comments, thanks]
As already mentioned, but worth getting the point across: Pterosaurs are flying reptiles, not dinosaurs. Not all big reptiles from the past are dinosaurs – in fact many were not. Dinosaurs were just a very successful branch (OK, really two branches) of big reptiles. You guys pride yourselves on being scientifically correct – so this ought to matter – you may someday be involved in “The Great Dinosaur Cover-Up” where government scientists prove dinosaurs never existed using highly massage satellite data (you will not be allowed to see the raw data because it would just confuse you).
Birds are in effect flying dinosaurs… Birds can fly – I know this because I have seen it. I am sure some government organization has a carefully normalized database showing this is impossible and its all my fault for burning logs in my fireplace, but I am sticking to my observations – Birds Fly.
Pterosaurs may have been gliders more than active flyers – there may have been higher winds where they thrived (or the scientists could just be wrong). There are other ways to provide additional lift without reverting to super-heavy atmospheres. Say 20% more pressure and a higher wind average and you might get enough lift to make the possible-problem to go away.
Also, a higher oxygen content would lead to more efficient muscles – maybe the muscle strength for a pterosaur is calculated incorrectly.
(Higher oxygen would also have led to some pretty mean run-away fires – I wonder if there is evidence of this?)
mfo;
The second marries raindrop formation theory (Lorenz, 1993, 1995) with empirical studies of raindrop impact-crater volume (Ghadiri, 2004). Since atmospheric density controls the final size and velocity of raindrops, fossil raindrop imprints on sedimentary surfaces can be used to extract atmospheric pressure.
>>>>>>>>>
It assumes that rain falls at terminal velocity only. Wind conditions can result in rain hitting the ground well below as well as well above terminal velocity and frequently do. Without knowing wind conditions at the time the impression was made, how could they possibly know with any degree of certainty that the rain drop was at terminal velocity?
Bill Tuttle;
Radio-controlled models, actually. Aircraft designer Paul MacCready built and flew a half-scale Quetzalcoatlus in 1986.
>>>>>>>>>>>
Yeah, but the key here is to note that is was “half scale”. The “square cube law” applies here. Double each dimension and the surface area (which provides the lift) would be four times as large, but the mass would be EIGHT times as much.
Besides, he built his model out of wood and all the fossils we’ve found so far were made out of rock…
I think I’ve got the answer. The moon used to orbit far closer to the earth than it does now. So, when the moon was very close to earth and overhead, the gravity experienced by the giant flying reptiles would have been substantially reduced, enabling them to fly.
The other three weeks of the month they sat around and whined about how their offspring of the time were so much more disrepectful than when they were hatchlings.
Wait a minute. R-12 has a specific gravity of 4.174 ? Then how the hell can it destroy ozone in the upper atmosphere?