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Climate Hustle

Do volcanoes emit more CO2 than humans?

What the science says...

Select a level... Basic Intermediate

Humans emit 100 times more CO2 than volcanoes.

Climate Myth...

Volcanoes emit more CO2 than humans

"Human additions of CO2 to the atmosphere must be taken into perspective.

Over the past 250 years, humans have added just one part of CO2 in 10,000 to the atmosphere. One volcanic cough can do this in a day." (Ian Plimer)

The solid Earth contains a huge quantity of carbon, far more than is present in the atmosphere or oceans.  Some of this carbon is slowly released from the rocks in the form of carbon dioxide, through vents at volcanoes and hot springs. Volcanic emissions are a small but important part of the global carbon cycle. Published reviews of the scientific literature by Mörner and Etiope (2002) and Kerrick (2001) report a range of emission of 65 to 319 million tonnes of CO2 per year. Counter claims that volcanoes, especially submarine volcanoes, produce vastly greater amounts of CO2 than these estimates are not supported by any papers published by the scientists who study the subject. 

The burning of fossil fuels and changes in land use results in the emission into the atmosphere of approximately 34 billion tonnes of carbon dioxide per year worldwide, according to the U.S. Energy Information Administration (EIA). The fossil fuels emissions numbers are about 100 times bigger than even the maximum estimated volcanic CO2 fluxes. Our understanding of volcanic discharges would have to be shown to be very mistaken before volcanic CO2 discharges could be considered anything but a bit player in contributing to the recent changes observed in the concentration of CO2 in the Earth's atmosphere.

Volcanoes can—and do—influence the global climate over time periods of a few years but this is achieved through the injection of sulfate aerosols into the high reaches of the atmosphere during the very large volcanic eruptions that occur sporadically each century. But that's another story...

Recommended further reading on CO2 and volcanoes can be found here: Terry Gerlach in Earth Magazine ; USGS

Last updated on 2 June 2017 by John Cook. View Archives

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Related Arguments

Further reading

Tamino has posted two examinations of the "volcanoes emit more CO2 than humans" argument by looking at the impact of the 1991 Pinutabo eruption on CO2 levels and the impact of past super volcanoes on the CO2 record.

The Global Volcanism Program have a list of all volcanoes with a Volcanic Explosivity Index (VEI) greater than 4 over the past 10,000 years.

Comments

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Comments 251 to 282 out of 282:

  1. Thanks for that Logan. I see that this study includes outgassing from volcanic lakes which were missing from earlier estimates. However, as the paper notes, the emissions are still insignificant compared to anthropogenic sources.

  2. The article could be updated because "Humans emit 100 times more CO2 than volcanoes" no longer reflects current best estimates. Human emission is about 50 times more than volcanic.

  3. Here's a recent article by a volcanologist, summarizing latest developments in volcanic CO2 emission research, and the significant uncertainties in the field:

    Long Invisible, Research Shows Volcanic CO2 Levels Are Staggering

  4. Logan, the link to the Italian study mentioned in the Op-ed you cite does not lead anywhere, can you reference it otherwise?

  5. PhilippeChantreau @254.

    It was linked @250.  Burton et al (2013) Deep Carbon Emissions from Volcanoes.

  6. Logan, I agree that the article should be updated to include the recent figures.  Those figures indicate 637 Mt per year of CO2 from volcanic sources (including volcanic lakes), and 300 Mt per year from non-volcanic sources (ie, metamorphism), the later derived from Morner and Etiope (2002).  In Burton et al (2013), the 937 Mt CO2 per annum from geophysical sources is compared to 35,000 Mt CO2 per annum as calculated by Friedlingstein et al (2010).  That estimate was for 2010.  A more recent estimate (for 2012), by Le Quere et al (2013), indicates total anthropogenic emissions of CO2 of 38,867 +/- 2,600 Mt CO2 per annum (10.6 +/- 0.71 PgC).  Consequently CO2 from geophysical sources represents 2.4% of anthropogenic CO2.

    The Burton et al estimate is likely to be too high rather than too low, in that it is significantly greater than recent estimates of in gassing of CO2.

    While important to update the figures in the interests of accuracy, it remains clear that geophysically sourced CO2 is emitted far to slow to have been responsible for the recent rapid rise in CO2, which is entirely of anthropogenic origin.

  7. That's also confirmed by carbon isotope measurements as well. So, we have two methods that converge on the same answer. 

    The Robin Wylie article is bizarrely over-the-top with regards to rhetoric, with the title stating that volcanic CO2 levels are "staggering." And, of course, when you google the article title you see that it's been reposted numerous times throughout the denial blogosphere.

  8. Logan, the article has been noted as needing update but it will sit in the queue with many others. As Tom points out, a good literature review of both volcanic and anthropogenic estimate papers is needed for such an update. Note though that while it is good to put emissions in a context, it is worth noting that no revision of estimates is going get humans off the hook. As Rob points out, FF and volcanoes have different isotopic signitures. Almost all of the increase in CO2 since pre-industrial is of human origin.

  9. CO2 degassing rate by volcanoes does have implications on our understanding of long-tail of antropogenic CO2 slug (i.e. the rate of CO2 sequestration by silicate thermostat or rock weathering).

    For example GEOCAB model from UChicago has default parameters of co2 degassing (both Spinup & Simulation) as 7.5E12 mol/y, which is 0.1GtC/y - 1/100 of human emmisions. With such assumption, the GEOCARB simulation of the 1000GtC initial slug ver 10ky, yields the final pCO2 as 319ppm, which is 1.17 times the initial value of 272ppm before Spinup. In other words, 17% of the original C slug remains in the atmosphere after 10ky according to GEOCARB.

    Now, if you keep everything the same but increase the CO2 degassing (both Spinup & Simulation), say twice to 15E12 mol/y closer to the latest figures discussed herein, then the final pCO2 value reported by GEOCARB will be 1.07 times the initial value, i.e. only 7% of the original C slug would remain in the atmosphere after 10ky.

    So, we can see that increased natural degessing rate would signifficantly shorten the 'long tail' of CO2, now commonly tought to be 'at least 100ky', to something less, say 50ky. Which is good news for the possibility of earth's recovery, maybe the sixth mass exctinction will be avoided. But within human timescale, such correction is irrelevant because 50ky as 100ky, is still "essencially forever".

  10. chriskoz @259, I am not sure what you did with the online model.  I increased the volcanic degassing to 27.5 terra moles per annum (the value arrived at in Burton et al), and increased land area by the same factor (3.67) to set the default ingassing level to the same value.  Doing so I arrived at the same result that you did.  I noticed, however, that after 1000 years, the CO2 level was still 338 ppmv (24% increase; 1.15 W/m^2 forcing), and that it was 412 ppmv (51% increas; 2.2 W/m^2 forcing) afer 400 years.  At a time scale where long term feedbacks are starting to be significant, these are still significant forcings.  Further, the 1000 Gtc slug used here is the trillion tonnes of carbon target that is commonly accepted to maintain temperature increases below  2 C.  That is, it is the policy target we are very far from achieving.  Increasing the slug to 2000 Gtc increases the CO2 concentration to 554 ppmv (100% increase; 3.7 W/m^2 forcing) in 400 years, to 417 ppmv in 1000 years (53% increase; 2.3 W/m^2 forcing), and 322 ppmv in 10,000 years (18% increase; 0.9 W/m^2 forcing).  Clearly if we overshoot the 1000 Gtc mark, this salvation comes to late and to slowly to be of much use.

    Further, this estimate of geophysical outgassing is significantly greater than current estimates of ingassing.  It may be that those estimates will also be revised upwards in future; but it is as likely that future estimates of geophysical outgassing will be revised down towards the current ingassing estimates of 403 to 515 Mt CO2 per annum (the two figures cited by Burton et al).  That would still represent a substantial increase over over previous estimates of geophysical outgassing, but just half of the Burton et al estimate.

    It may, of course, be that both the ingassing and outgassing estimates are accurate, and that there is an imbalance between the two.  That, however, would be bad news in the long term as it would indicate the current high outgassing levels to be an aberration, not likely to last long, and likely to permanently raise CO2 levels if they do.  It is the presumed increase in ingassing that is the good news, not the increased rate of degassing.  And even that is only good news in the very long term, humanly speaking.  (I am certain you are aware of these nuances, but think they are worth clariffying for others.)

  11. Tom@260,

    Indeed i ran the GEOCARB model just like your did, except I increased the Degas rate just twice (15E12 mol/y) as my own experiment (Burton et al value maybe an overestimate). I also increased land area twice so that degassing and ingassing (shown as "WeatS") be in balanace which was my implicit assumption that I forgot to state in my previous comment (sorry). Then I compared the pCO2 output with the default run.

    Hint for those who play with this model online: in between two runs, you can hit "save model run in background" button and all your graphs will be duplicated (the other run values will be displayed as "pCO2 alt", "WeatS alt" etc.), and you can compare the two runs superimposed.

    So comparing the pCO2 of my two runs, in say 500y timeframe, the difference is 413 (default) vs. 401 (mine), i.e. 12ppm only. In 1000y the difference is 372 vs. 356, therefore more. In 10ky, it is 319 vs. 307 which is still more in terms of climate forcing. As expected, the stronger degassing/ingassing exchange has signifficant influence on atmospheric carbon carbon slug decline in long term (>1000y) only.

  12. This sentence doesn't seem to make any sense:

    ''In fact, the rate of change of CO2 levels actually drops slightly after a volcanic eruption, possibly due to the cooling effect of aerosols.''


    Shouldn't that read ''...the rate of temperature increase actually drops slightly after a volcanic eruption, possibly due to the cooling effect of aerosols.''?

  13. LuisC @262, cooler water absorbs more CO2 than warmer water.  That is why soft drinks give of CO2 as they warm.  The volcanic cooling due to aerosols sufficiently cools the surface ocean that the oceans absorb more CO2 than the volcanoes emit.  Consequently the statement you quote is typically true.  (Coincidence of a strong El Nino with a volcano can cancel this effect.)

    While the statement is true and does emphasize the small amount of total volcanic emissions, not to much should be read into it.  In particular, as the ocean warms with the passing of the volcanic aerosols, the excess CO2 emited by the volcano will be outgassed by the warming oceans (or at least, 55% of it will, as with human emissions).  Therefore the volcanic cooling has no long term effect.  The volcanic emissions, particularly those of a single volcano remain small in annual terms relative to anthropogenic emissions.

  14. I would like to see more about what would have to occur in order for volcanic activity to make humans decreasing carbon emissions not effective. I also want know how much if any volcanic activity has on global warming and compared to humans effects.

  15. Clapper, large volcanoes in this era mostly cool the earth temporarily thanks to aerosols which persist for a few years. In the distant past, gigantic volcanic eruptions sustained for a long time (Large Igneous Provences) are associated with mass extinctions. See here (and the other part) for more detail. Short answer is that you need volcanic activity like hasnt been experienced for 100s of million years to get something comparable to human FF emission rates.

  16. "Counter claims that volcanoes, especially submarine volcanoes, produce vastly greater amounts of CO2 than these estimates are not supported by any papers published by the scientists who study the subject" ...  this statement inferes that the scientists who study the subject know everything there is to know about the subject. The inconvenient truth is that the ocean occupies two thirds of the planet and the ocean floor isn't mapped as nicely as the streets of Manhattan. Just recently a vast area of underwater vents emmitting CO2 like a glass of champagne were accidentally found near sea coral off New Guninea. Science is not based on consensus, it is based on fact. Before we can determine cause and effect as it pertains to global warming we must identify all of the CO2 emmitting sources then measure their variance against the change in global temperature. We are a long way from knowing how many CO2 emmitting sources are under the sea.

  17. Ybnvs @266, it is ironic that somebody trumpeting that "science is based on fact" provides no evidence if the CO2 vents "just recently" found of New Guinea - something I can find no evidence of either by searching google, or google scholar.

    However, science is not founded on 'fact' as you put it, but on fact and reasoning.  It follows that if you are to criticize a scientific finding, you must be at least aware of the scientific reasoning behind the result.  In this case, the total volcanic CO2 flux is determined not just be adding up sources, but by detecting atmospheric (or sea water) concentration of tracer gases from volcanoes, such as H3 - determining the total flux from that concentration, and from that and knowledge of the ratio of CO2 to the tracer gas from volcanic emissions, determining the total CO2 flux.

    A third approach is to determine the rate at which CO2 is naturally sequestered.  Given that CO2 concentrations have been stable for 10 thousand years, and (once temperature fluctuations are accounted for, over millions of years), the total geological flux of CO2 must be very close to the rate at which CO2 is sequestered - given a third method of determining the total geological CO2 flux.

    As the rate of geological CO2 flux has been determined by two methods in addition to the simple inventory method you assume, we have good reason to think that changes in that inventory will not substantially revise the current estimates, and certainly not by two orders of magnitude required for geological flux to equal anthropogenic flux.  That is particularly the case given that your uncited new source consists of a CO2 vent, ie, a type with a much lower overall flux than is typical of direct volcanic sources.

  18. Well Tom that's a long winded way of saying that we're still not sure of exactly what's occuring. Occum's razor has it's place in discussions such as this and we have to be careful not to get caught up in the lure of excessive and obscure data, after all... figures can lie and liars can figure. Sometimes it's not as difficult to find meaningful information as we tend to make it, and learning of the CO2 emitting vents near New Guinea can be as easy as watching a documentary on Nova. In regard to climate change one thing that has remained constant is change. The planet isn't as it was and won't be as it is. My interest in this topic came from hearing environmentalists voice the absurd notion that mankind is responsible for climate change. Decades ago I noticed that the hockey stick graph actually showed temperature increasing before CO2 increased, but it was only recently that I heard someone else point that out, I wonder why it took so long. Anyway... I don't know anyone who wants dirty air or water and I'm suspicious of demagogues whose solution to a perceived problem is monetary. I understand that my thoughts will be criticised as simple (Occums razor) but my assertion that we can't establish cause and effect is rock solid. Time is the best underwriter and in time I'm sure I will be vindicated. Peace.

  19. Ybnvs says "that's a long winded way of saying that we're still not sure of exactly what's occuring." I do not see how Tom's post can possibly be interpreted this way. In fact, it is exactly the opposite. The atmospheric carbon budget is a well understood part of the climate and there are numerous ways to address it. Newly discovered vents would have to spew out amounts of gas ata rate that would defy the laws of physics to make a significant difference.

    The isotopic signatures of volcanic vs organic fossil carbon are well studied. The physics of radiative transfer are also well understood. Occam's razor does not cut the way you seem to think it does in this case. Everything that is well known and easily verifiable about Earth climate indicates that the climate should be changing because of anthropogenic CO2; it would be very strange if observations did not match that expectation. Fortunately, they do.

    I tried to locate a recent NOVA documentary on deep sea vents off New Guinea. I located several short segments intended for teachers, the most recent dated from 2005, 11 years ago. Other ranged between 1977 and 1999. If you are to contribute anything here, you should link the specific new information that you think invalidates some really well established knowledge. Extraordinary claims require extraordinary evidence.

  20. The most recent I could find is a National Geographic story about a hydrothermal field 1300 ft long located 150 miles East of La Paz, Mexico. Not quite a game changer.

  21. Ybnvs @268:

    1)  It is not simple reasoning (except in a perjorative sense) to take William of Ockham's principle that we should not multiply entities beyond necessity and conclude that volcanic seeps and subsurface volcanism exist far in excess of, not just what has been observed, but what would be expected from surveys of the ocean floor.  Rather, it invokes a principle as justification of doing the reverse of what the principle dictates by mulitiplying our estimate of the number of seeps and subsurface volcanoes beyond any necessity justified by the data.  (Note it is Ockham in the English spelling, or Occam from the anglicized latin spelling - not Occum.)

    2)  That a NOVA documentary features a volcanic seep near New Guinea (of which several are known) in no way proves the seep to be newly discovered, or extensive enough to alter in any way estimates of subsurface CO2 emissions.  And FYI, there are smaller seeps than those listed at the link above such as those in Milne bay, but again these are well known.  It remains the case that you have yet to present any evidence for your claims.

    3)  While the uncertainty about volcanic emissions is sufficiently large that they may be up to double current estimates, we would need to be underestimating volcanic emissions by a factor of 50 for volcanic emissions to represent even 50% of anthropogenic emissions.  That scale of error is simply not on the cards, and for you to be certain that the error in current estimates is even greater than that, as it would need to be for volcanic emissions to be the primary cause of the increased CO2 levels, without having become even superficially familiar with the relevant scientific papers shows that your certainty the the scientists who have dedicated their career to studying this issue (and hence who are well familliar with the facts, as you are not; and well familliar with the relevant arguments, as you are not) represents a breath taking arrogance.  The style of reasoning you evidence even has a formal name - invincible ignorance.

    4)  As PhillipeChantreau alludes to, while there is significant uncertainty as to the actual value of volcanic emissions, regardless, other evidence makes as certain as it is possible to be in science that anthropogenic emissions are the cause of the rapid rise in CO2 levels in the twentieth century.

    5)  CO2 emissions and concentrations started rising around 1750, and rose rapidly after 1850:

    In contrast, temperatures did not start rising significantly until 1910:

    Again, whatever your argument with regard to temperatures, it is based on a very selective misinterpretation of the evidence.

  22. Ybnvs

    Just a reaction to several things you have said:

    "My interest in this topic came from hearing environmentalists voice the absurd notion that mankind is responsible for climate change."

    Why absurd? This sounds like an 'Argument from Incredulity'. Surely whether humans are responsible for climate change depends on two factors, both ultimately quantitaive.

    • How big does some influence have to be to impact Climate?
    • How big an influence does humanity have?

    Both these questions can be explored through measurement, observation and quantification. Just using the label 'absurd' is a cop out.

    "Decades ago I noticed that the hockey stick graph actually showed temperature increasing before CO2 increased, but it was only recently that I heard someone else point that out"

    Then you need to check your sources. The 'temperature increased before CO2' argument applies to the ice cores that cover time scales 10 to 80 times longer than the 'hockey stick'. The 'hockey stick' does not reference CO2 levels at all. And the ice core ecord is more complex than that. Different ice cores show different raltionships.

    'and I'm suspicious of demagogues whose solution to a perceived problem is monetary. '

    So what does this have to do with the science? If the science says that we need to reduce CO2 emissions, that is not as as such a monetary question. Implementing it might be but it is a logical fallacy to say that 'Problem A requires a solution that looks like B and since I don't like B, A is not real'.

    "I understand that my thoughts will be criticised as simple (Occums razor) but my assertion that we can't establish cause and effect is rock solid."

    Sorry, this is illogical. If your assertion is rock solid then you are claiming that you have established an alternative cause and effect relationship. Also there is a fundamental distinction between saying we 'haven't' established a cause and effect relationship and saying we 'can't'. Finally, if you think a cause and effect relationship hasn't yet been established, you need to back that up.

  23. Gavin over at RealClimate has a good post up on the "Volcano Gambit" — a look at the twisted history of misinformation on volcanoes over the last few decades.

  24. Hi John C, you might want to use this quote from the United States Geological Survey as regards the amount of CO2 released by volcanoes compared to human emmissions:

    Gas studies at volcanoes worldwide have helped volcanologists tally up a global volcanic CO2 budget in the same way that nations around the globe have cooperated to determine how much CO2 is released by human activity through the burning of fossil fuels. Our studies show that globally, volcanoes on land and under the sea release a total of about 200 million tonnes of CO2 annually.

    This seems like a huge amount of CO2, but a visit to the U.S. Department of Energy's Carbon Dioxide Information Analysis Center (CDIAC) website (http://cdiac.ornl.gov/) helps anyone armed with a handheld calculator and a high school chemistry text put the volcanic CO2 tally into perspective. Because while 200 million tonnes of CO2 is large, the global fossil fuel CO2 emissions for 2003 tipped the scales at 26.8 billion tonnes. Thus, not only does volcanic CO2 not dwarf that of human activity, it actually comprises less than 1 percent of that value.

    This might be a more digestible reference for a basic level audience compared to a quote from two scientific papers.

  25. earthquake tragedy in china was on 2008. there is volcanoes and earthquake zone in ring of fire. see on information of earthquake prone zones.

  26. I enjoy it so when the agenda (funding) driven science of climatology states so much they know that just isn't so... or at least isn't all of the story.   (snip)

    The last twenty years have seen huge steps in our understanding of how, and how much CO2 leaves the deep Earth. But at the same time, a disturbing pattern has been emerging.

    In 1992, it was thought that volcanic degassing released something like 100 million tons of CO2 each year. Around the turn of the millennium, this figure was getting closer to 200. The most recent estimate, released this February, comes from a team led by Mike Burton, of the Italian National Institute of Geophysics and Volcanology – and it’s just shy of 600 million tons. It caps a staggering trend: A six-fold increase in just two decades.

    These inflating figures, I hasten to add, don't mean that our planet is suddenly venting more CO2.

    Humanity certainly is; but any changes to the volcanic background level would occur over generations, not years. The rise we’re seeing now, therefore, must have been there all along: the daunting outline of how little we really know about volcanoes is beginning to loom large.

    Response:

    [RH] Read the comments policy for why you were snipped here.

    For the remainder of your comment, please cite the actual research instead of just recounting. People here will want to check your sources.

    Also, note that annual human emissions of CO2 are 30GT. That's about 30,000 million tons.

  27. Prof X @276 fails to provide his reference for his claim of deep Earth degassing of CO2 of approx 600 million tonnes of CO2 per annum, and nor is a recent discussion of global geophysical degassing rates evidents from Burton's list of publications.  In any event, the 600 million tonnes figure is a reduction from Burton's prior estimate (2013) of 937 million tonnes of CO2 per annum from all deep sources, discussed by me @256 (July, 2014) above.  If Burton indeed has a new estimate of approx 600 million tonnes, that would be a reduction from his prior estimate, which would spoil Prof X's narrative.

    To add slight confusion, Burton does have a 2014 conference paper which estimates a global flux of 1,800 million tonnes of CO2 based on new measurements of CO2 flux (still only 5% of anthropogenic emissions), but that estimate does not appear to have made it into a journal article.  Further, as more recent direct measurements of CO2 flux from a volcano contradict the claims heightened flux from Burton's indirect measurements, the premise of the 2014 conference paper estimate appears to have been falsified.

  28. i came across Wylie's article the other day. Interesting. The other point he covered was diffuse CO2, that is invisible, i.e. not associated with steam plumes, part of the reason for upping the estimates, but more importantly that some we thought extinct are invisibly emitting CO2 adding perhaps another 50%, which would take CO2 to ~1 billion tons a year, 10 times what we thought 20 years ago, and therefore now around 3%, three times what was said at the beginning of the thread.

    it occurred to me that as this gas was bubbling through the magma, the diffusion would seperate the C12 from C13 and that, depending on time, variation etc, could distort the ratio that we use for measuring anthropogenic emissions from fossil fuels, in the same direction, meaning we would overestimate anthropogenic by up to that amount.

    Still small in the scheme of things but not insignificant, with obvious effects in rare of AGW and 2100 levels.

     

    just a thought.

  29. No relation to Roy btw!

  30. Tony Spencer @278, the estimates of geological CO2 emissions are certainly in ferment at the moment.  One factor is that we know that over the long term, CO2 concentrations are essentially stable.  Specifically, the CO2 concentration either at glacial maximums, or interglacial peaks have not varied by more than a few ppmv relative to other glacial maximums or interglacials respectively, for 800,000 years.  It follows that natural emissions are essentially in balance with natural uptake of CO2.  As it stands, however, where estimates of CO2 uptake used to exceed estimates of emissions by about 50%, they are not dwarfed by them.  That means there is a problem with one set of figures, or the other, or both.  My suspicion is that currently the vulcanoligists are over counting, but assume the estimate of natural uptake is too low.  It would remain the case that total geological contribution to atmospheric CO2 increase is essentially zero.

  31. @Tom 280,

    That would make sense Tom, simply because the primary regulator of atmospheric gasses is the biosphere. When glaciation events were the main way the biological function was reduced by covering a significantly large area of land with ice, then the geological emissions would exceed natural uptake. The trend reverses till enough ice melts to allow the natural uptake to reign supreme again. So one would expect this.  It would bracket the atmospheric CO2 in a range. This is what we see for the last 800k years.

    This would support the idea that the degrading biosphere and ecological systems caused by mankind are what has allowed fossil fuel emissions to force the atmosphere to exceed that bracketed range. (very roughly ~170 - 320 ppm +/-) The biological stabilizing feedback function has been degraded simulataneously with increased emissions. Either alone is probably not enough to upset the balance. But both together obvious is since we are watching it happen.

  32. RedBaron @281, if the primary regulator of atmospheric CO2 is the biosphere, as you claim, covering vast swathes of that biosphere with land ice would reduce the fixing of CO2 into soil, and hence result in an increase in atmospheric CO2 durring glacials.  Instead we see the reverse.

    Although it is not yet entirely clear what drives the synchronous changes of pCO2 and GMST, the evidence strongly suggests the deep ocean has a major role.  That role must be at least modulated by change in surface vegetation, which were extensive, even in the tropics.  Specifically, the Sahara was not a desert (and much of the Australian outback was greened as well); but much land now covered with tropical rainforest was covered with grassland.  The greening of the Sahara, however, survived several millenia past the start of the Holocene - so its contribution to pCO2 was minimal relative to the glacial/interglacial cycle.  And total carbon sequestration in rain forest, per meter squared, exceeds that on grassland in every review I have seen, which would make that change, again, counter cyclical. 

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