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The cause of the greatest mass-extinctions of all? Pollution (Part 1)

Posted on 19 March 2015 by John Mason

Part One: Large Igneous Provinces and their global effects


A mass extinction is an event in the fossil record, a fossilised disaster if you like, in which a massive, globally widespread and geologically rapid loss of species occurred from numerous environments. The “Big five” extinctions of the Phanerozoic (that time since the beginning of the Cambrian period, 541 million years ago) are those in which, in each instance, over half of known species disappeared from the fossil record.

How did they happen? The causes of such events, with a truly global reach, have been a well-known bone of contention within the Earth Sciences community over many decades. The popular media likes to portray such things as Hollywood-style disasters, in which everything gets wiped out in an instant. But in the realms of science, things have changed. The critically important development has been the refinement of radiometric dating, allowing us to age-constrain events down to much narrower windows of time. We can now, in some cases, talk about the start and end of an event in terms of tens of thousands (rather than millions) of years.

Such dating, coupled with the other time-tools of palaeomagnetism and the fossil record, have made it possible to develop a much clearer picture of how mass-extinctions occur. That picture is one of periods of global-scale pollution and environmental stress associated with large perturbations to the carbon cycle, lasting for thousands of years. Such upheavals are related to unusual episodes of volcanic activity with an intensity that is almost impossible to imagine. The geological calling-cards of such events are known as Large Igneous Provinces (LIPs). Bringing environmental and climatic changes at rates similar to the ones we have been creating, they have been repeat-offenders down the geological timeline. This introductory piece examines LIPs in the framework of more familiar volcanic activity: it is the only way to get a handle on their vastness.

For those readers already familiar with LIPs, you may want to skip this and go straight to Part Two, which covers the biggest extinction of them all, at the end of the Permian period, 252 million years ago (Ma). With more than 90% of all species wiped out, it was the most severe biotic crisis in Phanerozoic history. The extinction was global: almost all animals and plants in almost all environmental settings were affected. An idea of the severity can be visualised by considering that the time afterwards was marked by the beginning of a coal gap lasting for ten million years: coal-forming ecosystems simply did not exist for that time. Likewise, Howard Lee has recently considered the relationship between the end-Cretaceous extinction - the one that got the dinosaurs - and LIP volcanism here. But for those who are new to LIPs, it is recommended that you read this post first.

A sense of scale

Let's start by contextualising that volcanicity, starting with an especially well-known example. Mount St. Helens is one of a number of volcanoes in the Cascade Range of the north-western United States. In early 1980, it began a period of activity with earthquakes and clouds of steam billowing forth: by the middle of spring its northern side was starting to bulge ominously, a sure sign of magma and pressure build-up. On May 18th, following another earthquake, its entire northern side collapsed, depressuring the magma and volatiles beneath in an instant. The resulting blast destroyed everything in a 600 square kilometres zone around the northern flank of the volcano. A huge cloud of hot ash shot skywards, reaching over twenty kilometres in height. Ash and debris, mixed with great volumes of meltwater, brought major flash-flooding and mudflows into local rivers. The energy released has been estimated to be equivalent to a 24-megaton nuke and in total this nine-hour eruption spewed out some 2.79 cubic kilometres of felsic lava, ash, gases and debris. Remember that last figure.

The famous eruption of Krakatoa on August 26th-27th 1883 reached its climax on the 27th: the largest explosion, at 10:02 A.M, was heard 3,110 km (1,930 mi) away in Perth, Western Australia. The eruption and the tsunamis associated with it killed over 36,000 people according to official figures. This incredibly violent and destructive eruption, with an energy-release likened to a 200-megaton nuke, produced an estimated 21 cubic kilometres of eruptive products. Again, remember that figure.

Now, contrast those deadly eruptions with the mostly late Permian Siberian Traps LIP. The province contains what may be the largest known volume of terrestrial flood basalt (dark-coloured, iron and magnesium-rich lava) in the world. How much? At least three million cubic kilometres. That's enough to bury an area the size of the United Kingdom beneath a layer of basalt some 12 kilometres thick.

Scale of volcanicity

Fig. 1: We're gonna need a bigger graph! Volumes of well-known volcanic eruptions compared to LIPs. Geologists may argue that comparing single eruptions of various standard volcanoes and LIPs is like comparing apples to oranges. Actually, that's the point!


A large igneous province is defined as a vast accumulation, covering an area of at least a hundred thousand square kilometres, of igneous rocks episodically erupted or intruded within a few million years. The majority of erupted products may in some cases accumulate within much shorter time-spans of tens of thousands of years or less. Total eruption volumes are at least a hundred thousand cubic kilometres. Erupted products are dominated by repeated flows of basaltic lava ("flood-basalts"): weathering and erosion of these stacked basalt sheets often gives the countryside where they occur a hilly, stepped topography. Such areas are often referred to today as "Traps" because of this distinctive landscape: the term, as used in "Siberian Traps" or "Deccan Traps" is based on a Swedish word for stairs. Rocks intruded beneath the surface in LIPs include ultramafic (dense, iron and magnesium rich) and alkaline (sodium and potassium-rich) bodies, plus uncommon types such as the carbonate-rich carbonatites. LIP events are infrequent along the geological timeline, with an average of one such event every twenty million years.

Plate tectonics and the long term carbon cycle

Fig. 2: Plate tectonics 101: oceanic crust is erupted at mid-ocean ridges and tens of millions of years later it is consumed at subduction zones. Graphic: jg.

Plate tectonics has over the years been particularly involved with what goes on at existing plate boundaries such as subduction zones and mid-ocean ridges (fig. 2), where magmatism is highly focussed. However, LIPs reflect another set of processes altogether, where vast amounts of mantle-derived magma make it to the surface within plates. They have played a significant role in the development of the hypothesis of great plumes of hot rock and magma occurring deep in Earth's mantle, which create localised "hotspots" that occur irrespective of tectonic plate boundaries and are the sites of major, within-plate eruptions over millions of years. That there is still much lively (and at times acrimonious) debate concerning the Plumes Hypothesis, including postulated alternative formation-mechanisms for LIPs, need not concern us here. That LIP events occurred and how they affected the biosphere is our focus.

Pollution from volcanoes

The Oxford English Dictionary definition of pollution is as follows: the presence in or introduction into the environment of a substance which has harmful or poisonous effects.

Harmful or poisonous effects depend on the physical and chemical properties of any one substance. Substances are widely variable in their toxicity in terms of concentration. Carbon dioxide, essential to photosynthetic plantlife, has other properties which, at higher concentrations, make it dangerous. As a strong greenhouse gas, any substantial increase in its atmospheric levels over a matter of a few centuries make it a pollutant because of the impacts of rapid climate change. At much higher levels it becomes an asphyxiant - a gas that kills by displacing air, thereby causing suffocation, as tragically evidenced in 1986 at Lake Nyos, in Cameroon. Here, the magma underlying the floor of an old volcanic crater-lake gives off carbon dioxide, with which the lake water becomes super-charged. At depth, the pressure of the water-column above keeps the gas stably dissolved in the water. However, any triggering mechanism that suddenly forces a lot of that deep water upwards to shallow levels where that confining pressure is absent can cause it to explosively degas. In the 1986 event, a large cloud of carbon dioxide burst forth from the lake. Due to its relative density, it rolled along the ground, displacing the air as it did so. Over 1,700 people and 3,500 livestock died from asphyxiation in nearby communities. Like many substances, carbon dioxide is best taken in moderation.

All subaerial volcanic eruptions blast out gases and ash into the troposphere and in some cases the stratosphere. The most important volcanogenic gases are water vapour, carbon dioxide, sulphur dioxide and halogen compounds such as hydrogen chloride and hydrogen fluoride.

Of these, only carbon dioxide can contribute to global warming over a geological timescale because of its centuries-long atmospheric residence time (the time it takes natural processes to remove most of it again). At present, global volcanogenic carbon dioxide emissions are calculated to be up to 440 million tonnes a year. This can usefully be compared to human carbon dioxide emissions of (in 2014) 32.3 billion tonnes a year – ours are presently two orders of magnitude greater than those from volcanoes. LIP eruptions are another matter: the entire Siberian Traps LIP eruptive cycle is estimated to have produced thirty thousand billion tonnes of carbon dioxide. Bearing in mind the residence time of carbon dioxide, if eruptive events are continuous or closely-spaced enough to keep recharging the atmosphere with it, a long-lived warming effect would occur.

Sulphur dioxide's greenhouse gas abilities are somewhat stunted as it tends to form sun-blocking sulphate aerosols (suspensions of fine solid particles or liquid droplets in a gas) that have a net cooling effect. Unless an eruption is powerful enough to inject a lot of the gas up into the stratosphere (where sulphur compounds may also cause damage to the ozone layer), the cooling effect is short-term – just a year or two, by which time the sulphate has mostly returned to the surface, dissolved in rainwater and thereby giving a short-term acid rain effect where that rain falls. Stratospheric sulphate aerosols have effects lasting for a few more years, but unless they are continuously supplied then the system recovers to its pre-eruption state. Additionally, because of the way that Earth's airmasses interact with one another as a result of the planet's rotation, gases have to be injected into the stratosphere from a relatively low latitude if they are to be spread on a truly global basis. So a pattern emerges of a steady global warming due to increasing carbon dioxide with shorter, often more regional punctuations along the way in the form of sulphate-induced cooling.

Water vapour quickly cycles back to the surface as rain, bringing with it (in addition to the sulphate) the volcanic ash out of the troposphere. The halogen compounds likewise acidify that rainfall and at higher local concentrations make it directly toxic. Halogen compounds injected into the stratosphere also cause ozone layer damage.

Historically, there are several good examples of problems caused by major eruptions causing short-term atmospheric pollution. A good example is the eight-month long fissure-eruption of Laki, which began in June 1783 in Iceland (a mere 15 cubic kilometres event). Apart from vast amounts of lava, Laki released an estimated 122 million tonnes of sulphur dioxide, fifteen million tonnes of hydrogen fluoride and seven million tonnes of hydrogen chloride. The effect was to leave parts of the Northern Hemisphere shrouded in an unpleasant fog for several months. The acidic, halogen-rich haze and resulting toxic rains were highly damaging to terrestrial life in Iceland, Europe and North America. Livestock mortality in Iceland was over 50% and a quarter of the island's population perished in the resultant famine.

large igneous provinces and the extinction rate

Fig. 3: Extinction magnitude through the past 400 million years plotted against the age and estimated original volume of large igneous provinces. Continental flood basalt LIPs are shown as black bars, while oceanic plateau basalt provinces are shown as gray bars. Abbreviations: D = Devonian; C = Carboniferous; P = Permian; Tr = Triassic; J = Jurassic; K = Cretaceous; T = Tertiary; CAMP = Central Atlantic magmatic province. Figure is adapted from Bond & Wignall, 2014.

Killers and non-killers

The geological timeline of the Phanerozoic (part of which is shown in fig. 3) is marked by a number of LIP events. A few seem to have had little impact on planetary life, especially the oceanic plateau basalt provinces (perhaps underwater eruptions have different outcomes?); some are linked to moderate extinctions and some are linked to major mass-extinctions. Why this variability and what makes a LIP event a killer?

large igneous provinces: kill-mechanisms

Fig. 4: potential kill-mechanisms associated with Large Igneous Provinces. Graphic: jg.

Several factors are clearly critical in determining the outcome of a LIP event. The state of the biosphere and climate prior to an eruption - how stressed the systems are - must be important. Any occurrence of other global-scale events coincident with a LIP eruption - such as a large asteroid impact - would only make things worse. But the most important factor must surely relate to the "three D's" - the distribution, duration and degree of pollution.

Distribution, duration and degree of a pollution event depends on frequency and intensity of eruptive events (the pollutant supply) and the residence times of the pollutants involved. Continuous or very frequent intense events over tens of thousands of years would not only provide sufficient pollutants but give them adequate time to be spread globally at dangerous levels. On the other hand, continuous low-intensity eruption over a similar time may not raise levels of pollutants to harmful values, or perhaps only do so on a regional basis. Low frequency LIP eruptions occurring over longer timespans may still yield vast volumes of lava, but the low frequency allows ecosystem recovery in between eruptions. Therefore, it is possible for some LIPs to have had little more than local effects whereas in others the global ecosystem has been almost completely overwhelmed.

Now we have had an overview of LIPs and their effects, we can look at a specific example in Part Two with the end-Permian mass-extinction, how it occurred and its links to the Siberian Traps LIP - and its significance compared to the pollution caused by modern-day human activities.


The following paper, available online, is an excellent overview of LIPs and their role in specific extinction events - it also has an exhaustive list of references for further reading.

Bond, D.P.G. and Wignall, P.B. (2014): Large igneous provinces and mass extinctions: An update. In: Keller, G., and Kerr, A.C., eds., Volcanism, Impacts, and Mass Extinctions: Causes and Effects: Geological Society of America Special Paper 505.



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Comments 1 to 35:

  1. Speaking as a geologist, this is a very well-written and readable account. Kudos to you for assembling it all in such an easily-understood essay.

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  2. And props too for accurately including this caveat, usually skipped in popular geology:

    That there is still much lively (and at times acrimonious) debate concerning the Plumes Hypothesis, including postulated alternative formation-mechanisms for LIPs, need not concern us here.

    There is indeed a lively debate, including many recent papers (and presentations at last year's AGU) arguing that contrary to every textbook, deep-mantle plumes do not exist. Some of that work is cited in this more colorful piece describing the plume hypothesis as "zombie science."

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    Moderator Response:

    [PS] Fixed link

  3. Could it be that these gigantic outpourings of lava are a result of a bunch of continents coming together and forming an insullating layer on a big chunk of the earth's surface.  Heat from radioactive sources builds and builds under these mega-continents until the melt is too much for the strength of the overlying, thick, continental rock.  If this is so, one would expect such eruptions to occur around the middle of super continents and possibly lead to the splitting of the mega continent and the creation of a new ocean.

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  4. William, I havent run a specific model on that but based on lots of models for "normal" heatflows, I dont think can get remotely get to melt temperature. If there is an effect, I think it is dwarfed by variations in heat flow from mantle convection.

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  5. Speaking as a genuine geologist, this is a high school essay cut and pasted from somewhere, with a bunch of politicised carbon dioxide semi-truths jammed in every so often.

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  6. Chris - start with the Bond & Wignall paper referenced here. Then read the numerous references cited below part two. We'll hear your piece in a few days after you have done that.

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  7. Speaking as a member of the general public, Chris, I find your arguments less convincing than John's.  Maybe it's the lack of evidence on your part.  

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  8. What about nitric acid being converted into tropospheric ozone, which is highly toxic to plants?  Could that have played a role in mass extinctions where volcanic emissions are the initial event?

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    Moderator Response:

    [JH] Link activated.

  9. Chris:

    You are arguing from authority but we do not know who you are.  Can you provide some information about your background and degree in geology?  BA, MS or PhD?  What was your specialty?  Have you published any papers about this era?  Are you currently employed as a geologist?  Can you provide evidence that your claims are true?

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  10. I can certainly state a BSc, M.Phil, time spent in the minerals industry, a number of papers and involvement in more than one book. Plus a penchant for  - if I am going to write about a topic - obtaining the latest peer-reviewed research. So come on Chris, whoever you are. Speak up!

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  11. Chris Geo - I have just received a copy of "Volcanism and Global Environmental Change" by Schmidt, Fristad, and Elkins-Tanton (Cambridge University Press). As a genuine geologist you will find this interesting - and it goes into further detail about the topics John Mason has written about, with chapters from 53 eminent scientists including a number of genuine geologists.

    You may also enjoy Geological Society of America Special Paper 505, by a bunch more geologists, which you will find entirely consistent with this post.

    I have spent the last few years following the research on LIPs and global climate change and I also connsider myself a genuine geologist. Many of us geologists were educated  some time ago with the idea that LIPs were slow-burn phenomena, but the latest high-resolution dating shows us they were far more rapid than thought. Lake all scientists we must follow the data, even if they overturn old ideas.

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  12. Interesting related paper just came to my attention today in Geology by Lindstrom et al, concering the Central Atlantic Magmatic Province (CAMP) associated with the end-Triassic Mass extinction. They say:

    "magmatic intrusions into sedimentary strata during early stages of CAMP formation caused emission of gases (SO2, halocarbons, polycyclic aromatic hydrocarbons) that may have played a major part in the biotic crisis"

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  13. L. Hamilton @ 2 - I made a point of attending as many of the sessions concerning plumes vs no plumes as I could. The geochemistry of plume-related magmas does argue for a base-mantle source and the fact that plumes have been imaged down to the D'' layer seems a clincher to me. Plus if you have residue of subducted slabs sinking down to the teepest mantle (as a number of studies have imaged) you must, as even Don Anderson agreed, have a compensating mass movement upwards. Agreed, plumes may not be universally accepted, but they are increasingly the consensus and about as accepted today as Plate Tectonics was in the mid 1970s.

    Did you catch Barbara Romanowicz's AGU presentation "Of Mantle Plumes, Their Existence, and Their Nature: Insights from Whole Mantle SEM-Based Seismic Waveform Tomography"? Some clear imagery of ~400km wide plumes extending from the CMB there.

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  14. The article you reference is open access:

    Bond, David PG, and Paul B. Wignall. "Large igneous provinces and mass extinctions: an update." Geological Society of America Special Papers 505 (2014): SPE505-02.

    It is part of a special collection:

    Keller, Gerta, and Andrew C. Kerr. Volcanism, Impacts, and Mass Extinctions: Causes and Effects. Vol. 505. Geological Society of America, 2014.

    ... which has a total of 24 articles, three others being open access:

    Font, Eric, et al. "Atmospheric halogen and acid rains during the main phase of Deccan eruptions: Magnetic and mineral evidence." Geological Society of America Special Papers 505 (2014): SPE505-18.

    Self, S., A. Schmidt, and T. A. Mather. "Emplacement characteristics, time scales, and volcanic gas release rates of continental flood basalt eruptions on Earth." Geological Society of America Special Papers 505 (2014): SPE505-16.

    Miller, Steve. "The public impact of impacts: How the media play in the mass extinction debates." Geological Society of America Special Papers 505 (2014): 439-455.

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  15. Michael Sweet  @9 Why have you singled out Chris to provide information about his professional background but do not ask other geologists, for example vroomie @1, to provide similar information?  

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  16. @ John Mason (or moderator)

    Speaking as a genuine non-geologist...      ;)

    I'd just like to add my thanks for a well-written and informative piece, but I'd like to point out that the embedded link to Part 2 appers to be broken.

    Please see para 4, beginning...

    "For those readers already familiar with LIPs, you may want to skip this and go straight to Part Two"

    cheers    bill f

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  17. Thanks Bill - it was a .htm suffix instead of the correct .html one. Fixed!

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  18. Ryland,

    Chris's entire argument was that he is an expert and we should believe him because he is so smart.  He provides no data or citations of research to support his wild claims.  Vroomie was making a personal comment and not a claim of fact.  The other commentors were citing peer reviewed papers to support their claims.  Since Chris is making an argument from authority I wondered what his authority is.  Here Chris is an unknown internet poster with no credentials.  When he was asked for his experience, he has declined to provide any support for his claims that he is an authority.  It appears he does not feel that he is qualified as an authority when he is asked.

    By contrast, John Mason has posted many times here about geological subjects and has a reputation for being informed.  He cites peer reviewed data to support his claims, not his personal authority as Chris did.  He provides citations to the original research to suport his claims.  I can check them if I doubt John.  When there was a question of qualifications he provided evidence that he is qualified, although a simple search of this site would provide background about him.

    What evidence do you have that Chris really has a geology degree and is not just a troll who falsely claimed that he knew about geology?  What evidence do you have that Chris's specialty in Geology is related to the OP?  Why should I accept Chris' argument from  authority?

    "Skeptics" rely on the unsupported authority of false experts like Monfort and Watts.  At SkS we have scientific discussions and people are required to support their claims.  I am not impressed with Tom Curtis college degrees (the only qualification Chris claims).  His long, detailed posts with multiple cites of recent research are difficult to argue with.  Tom has authority here because he makes strong, scientificly based arguments.

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  19. Michael Sweet @18  Thank you for answering my question.  With regard to your question "What evidence do you have that Chris really has a geology degree?" etc.  The answer is none.  But then, I don't have evidence of the background of many of those who comment here and equally those who comment here have no evidence of my background.  Is it a pre-requisite to provide details?  Not sure who Montfort is.  Anyway, thanks again for your courtesy.

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  20. Ryland,

    I frequently ask posters here to provide evidence (preferably peer reviewed) to support their positions.  Since Chris had no data, only his claim of personal authority, I asked for evidence of that.

    If other posters want to be taken seriously they have to provide evidence to support their positions.  That is the scientific method.  Everyone is required to provide citations to support their position if asked (better posters like Tom provide evidence when they make the claim).  All claims are evaluated based on the supporting evidence, not on the authority of the poster. I have an MS in chemistry.  If I make a chemical based claim I am still required to provide citations if asked, even if the other poster has no chemical experience.

    Kevin Cowtan is a reknowned world authority on temperature records who posts at this site.  If questioned by skeptics who have no qualifications (which happens frequently), he provides citations to support his positions.  I do  not ask Dr. Cowtan for citations because I know his reputation and am comfortable that he can support his claims.  Likewise mpelto is an occasional poster on this site who is a world authority on glaciers.  If questioned by novices he provides citations to support his positions.

    I am sorry, I meant Andrew Montford the well known denier.

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  21. angela @22:

    "I speak for the genral public..."

    Really?  Who appointed you to that position?  And can we see the documentation showing that you have been appointed to "speak for the general public".  Personally I only ever speak for myself.  That is probably because I am flabberghasted at the arrogance shown by people who claim to speak for the "the general public" (or whoever else they want to drag in to give their questions unwarrented authority).

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    Moderator Response:

    [TD] I believe Angela is a bot, so I've deleted her/its comments.  Angela is quite welcome to prove me wrong by posting a comment having more substance.

  22. At what level does CO2 become an asphyxiant?

    Please keep in mind that even at 40,000 ppm CO2 is not harmful to humans and probably to mammals.

    Also keep in mind that as water precipitates it is readily replaced by evaporation, and it appears that water vapor varies over a wide range at a given location and time....

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    Moderator Response:

    [PS] Welcome to Skeptical Science.

    Thank you for taking the time to share with us.  Skeptical Science is a user forum wherein the science of climate change can be discussed from the standpoint of the science itself.  Ideology and politics get checked at the keyboard.

    Please take the time to review the Comments Policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

    In particular, note that comments must be on-topic. No one is suggesting CO2 caused extinction by aphyxiation. Claims about water vapour should be in the appropriate place and backed by references. Unsubstantiated claims are sloganneering and banned.

  23. To Moderator Response,

    Please tell me what policies did I not comply with?

    My comment about water vapor was in response to the satement "Water vapour quickly cycles back to the surface as rain".

    I did not suggest or imply that CO2 caused extinction by asphyxiation; my question was in response to "Harmful or poisonous effects depend on the physical and chemical properties of any one substance. Substances are widely variable in their toxicity in terms of concentration. Carbon dioxide, essential to photosynthetic plantlife, has other properties which, at higher concentrations, make it dangerous. As a strong greenhouse gas, any substantial increase in its atmospheric levels over a matter of a few centuries make it a pollutant because of the impacts of rapid climate change. At much higher levels it becomes an asphyxiant".

    Please note that National Safety Council's "Fundamentals of Industrial Hygiene" [4th Edition] states in the first paragraph in page 42: "Carbon dioxide is always present in the atmosphere, but the proporetion of carbon dioxide in air exhaled from the lungs is 100 times greater"; thus my statement about the 40,000 ppm of CO2.

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  24. dudo39...  I can't see that you proposed any actual question(s).

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  25. dudo39 @23:

    1)  CO2 toxicity appears to be a complicated subject.  To begin with, the consequence of CO2 toxicity is not asphyxiation, which requires reducing oxygen concentrations to about 4%, but from increasing acidity in body tissues.  The effects of this increased acidity include hamorrhaging of lungs, spleen, intestines and kidneys.  It also includes loss of body mass, and the mass of various organs, although the rate of loss varies by organ and by species.  Finally, at 15% concentration, it results in the complete loss of spermatogenesis.  15% concentration sustained over several weeks also resulted in the deaths of 30 to 50% of guinea pigs, but not of rats.

    At lower levels of CO2, sustained exposure can be effectively buffered but at a cost.  Specifically, at sustained 1% CO2 concentrations humans buffer against CO2 by loss of bone mass.  This also is associated with replacement of calcium carbonate with calcium bicarbonate in bones as part of the buffering process.  I am not sure what effect this has on the strength or brittlness of bone.  The buffering is not continuous in time, but appears to go through stages.

    The upshot is that while humans cannot increase CO2 concentrations in the atmosphere to levels that are immediately fatal, or which will result in permanent sterillity (15%), at the upper limit we can increase atmospheric CO2 concentrations to levels that are toxic and have harmful effects on humans.  Other species, however, are far more susceptible to CO2 concentration, particularly some marine species.

    2)  Yes, atmospheric H2O is replaced by evaporation, but that just means the concentration of H2O is controlled by temperature.  Total human emissions of H2O does not increase the atmospheric concentration appreciably over the concentration due to that evaporation. 

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  26. Rob Honeycutt @24,

    First lines of my comments 22 & 23 are "actual" questions.

    Tom Curtis @25,

    Thanks for the detailed information. I would then amend my implied statement that CO2 is not toxic, or pollutant, to humans in concentrations lower than about say 45,000 ppm.

    Does your statement "at sustained 1% CO2 concentrations humans buffer against CO2 by loss of bone mass" imply then that us humans  are undergoing loss of bone mass since birth [since it appears that  the background CO2 concentration in our lungs is about 4%]?

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  27. dudo39 @26, fairly obviously, a 1% CO2 concentration in respired air will result in an increase in pCO2 in the pulmonary system, and presumably also in the end-tidal CO2 concentration.  That the end-tidal CO2 concentration in breath is around 4% is therefore irrelevant to the result.  In other words, prolonged exposure to just 1% CO2 concentration is physiologically harmful to humans, if mildly so.

    Never-the-less, the concern about anthropogenic CO2 has nothing to do with direct physiological impacts.  It has to do with climate impacts, and the impacts of ocean acidification.  Discussion of physiological impacts (direct toxicity) is raised as a red herring by climate change deniers.  Even on the red herring, however, they get their facts wrong in that levels of CO2 concentration that could be reached in a determined BAU scenario do in fact have negative physiological impacts.  Not impacts worth considering given the other harms that would result from such a high CO2 concentration, but impacts none-the-less.

    Note:  the "background" CO2 concentration you refer to is the end-tidal CO2 concentration in respired air, ie, "the level of (partial pressure of) carbon dioxide released at end of expiration".  It represents the peak CO2 concentration in respired air, not the average concentration.  It is not even the average concentration in the alveoli, which start with a slightly lower level even durring expiration, and are likely to have a substantially lower level of CO2 at the end of inspiration, given that the inspired air has only the atmospheric concentration.

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  28. dudo39... The first "question" is completely irrelevant to the main topic of mass extinctions, the second question is merely asking why your irrelevant question was moderated.

    I'm still not seeing a question that requires a response.

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  29. Tom Curtis @ 27,
    Please note that I mentioned that the relatively high CO2 concentration in exhaled air as an indication that CO2 in ambient air is not harmful or toxic to humans.
    While the inspired air currently has about 400 ppm of CO2, the residual air in the lungs has about 4% of CO2: as the inspired and residual air volumes readily mix the resulting CO2 concentration may be in excess of 1%.
    I did not say, nor imply, that the anthropogenic CO2 has something to do with direct physiological impacts.

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  30. The first question is based on the comment "at much higher levels it [CO2] becomes an asphyxiant": thus it is valid question that should be answered.

    Please don't cherry pick: My entire comment was "moderated".

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  31. Dudo39,

    This study found cognative decreases that were sigificant at 1000 ppm CO2.  when the ventelating air increases to 500 ppm, it will become common to have air in buildings that is 1000 ppm or greater, some buildings already exceed those amounts.  I am not as sanguine as you that increasing CO2 does not affect human mental performance.  Think how sleepy people get when they do not ventelate the air enough in airplanes.

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  32. michael sweet,

    Your comment and the study you cited bring up an interesting point: People in "Closed" enclosures will change the air composition within by increasing CO2 concentration at the expense of lowering the oxygen concentration [which the study does not mention]. The point, or question, is what is really affecting human performance? Too much CO2, too little O2 or both to various degrees? The chicken or the egg?

    In my opinion, lack of O2 is the main problem in buildings and airplanes.

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    Moderator Response:

    [PS] As interesting as the toxicity level of CO2 to humans is, I am failing to see how this has relevance to the argument or conclusions of this article. If there is some relevance to the article rather than intellectual curiosity, then can you please outline your argument? Otherwise, this seems rather offtopic.

  33. dudo39 @32, increasing CO2 content to 1000 ppmv decreases O2 content from 209,460 to 208,860 ppmv.  The effect on respiration is less than that of going from sea level to 30 meters above sea level - ie, unnoticable.

    Michael Sweet @31, the effect of moderate CO2 in ventilation is not a direct effect of CO2.  Rather "high carbon dioxide concentrations in offices" is "an indirect indication of poor ventilation and contaminant build-up".  The list of other contaminants in office (and school) air is quite extensive.  It is not known which of these, or which combinations of these lead to the loss of cognative function you mention - but it is not attributable to CO2 alone.

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  34. dudo39 - there are plenty of detailed accounts of the Lake Nyos incident, if you want to do some online searching. That was a clear-cut case of asphyxiation due to the gas displacing the air. CO2 can also be a serious problem underground, in badly ventilated mineworkings, such as blind raises or winzes. Again, some searching will provide details. I recall collecting a pocket of pyromorphite specimens in the 1990s, up a 5m winze accessed by a ladder, and spending too long up there working in a strenuous position. Got breathless and very light-headed, with symptoms coming on suddenly. Descending down into the main workings, recovery was swift.

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    Moderator Response:

    [PS] The problems with C02 levels in Apollo 13 and with Biosphere 2 would be other examples of adequate O2 but excess CO2. But the relevance is??

  35. Tom,

    The paper I linked (only the press release, but the paper can be Googled) only varied the CO2 content of the air.  They had a small sample size but the data was striking that performance decreased even at 1000 ppm CO2.  Many schools have CO2 that high from respiration.  As the atmosphere increases in CO2, interior spaces will increase a lot.  The jury  is still out on how much effect this will have on function.  Even a small effect will be on the entire human population.

    Airplanes are probably due to lower O2 concentration at flying altitude.

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