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Do high levels of CO2 in the past contradict the warming effect of CO2?

What the science says...

Select a level... Basic Intermediate

Climate and CO2 levels have always varied together. During past ice ages CO2 levels were low, and during warm periods CO2 was higher.

Climate Myth...

CO2 was higher in the past

"The killer proof that CO2 does not drive climate is to be found during the Ordovician- Silurian and the Jurassic-Cretaceous periods when CO2 levels were greater than 4000 ppmv (parts per million by volume) and about 2000 ppmv respectively. If the IPCC theory is correct there should have been runaway greenhouse induced global warming during these periods but instead there was glaciation."
(The Lavoisier Group)

Climate and CO2 levels have always varied together. During ice ages CO2 levels were low, and during warm periods CO2 was higher. In the Eocene (56-34 million years ago) there were no polar ice caps, temperatures were about 10ºC hotter than the 20th Century, and CO2 was about 1,500ppm (Westerhold et al. 2020, Rae et al. 2021). During the last Ice Age, CO2 varied between about 180 and 300ppm as ice sheets waxed and waned with orbital wobbles (Rae et al. 2021). CO2 was also about that level during the Paleozoic Ice Age, 340-290 million years ago (Foster et al. 2017).

Early attempts to estimate CO2 for that long ago in Earth’s past were broad-brush and very uncertain (eg Royer 2006), leading to the high CO2 estimates referred to in the myth. New data and refined techniques have since clarified the picture considerably. The 2006 estimates, for example, averaged data across 10-million-year timesteps, the 2017 data in the figure below used 0.5-million-year timesteps, and newer compilations don’t average across timesteps. At the same time, CO2 and temperature uncertainties have reduced considerably so that climates from the geological past are now a useful reality check for climate models (Tierney et al. 2020, IPCC 2021, see the intermediate version for more detail).

Data for the Ordovician are still quite uncertain, but they indicate CO2 was about 2,400ppm and falling before the end-Ordovician glaciation (Pancost et al. 2013). Glaciation at higher CO2 levels than today was possible at that time for a variety of reasons including a less-bright Sun back then (see the intermediate version). The Jurassic and Cretaceous span 134 million years with several hothouse episodes and several cooler episodes, with CO2 varying from about 600ppm to about 1500ppm accordingly (Witkowski et al. 2018), but there was no glaciation in that time.

Earth’s long-term climate (over millions of years) is governed by the balance between CO2 emitted into the atmosphere by volcanoes and CO2 removed from the atmosphere by weathering of rocks (Joel 2017). This has prevented runaway climates and kept Earth’s climate generally habitable for about 4 billion years, but it can be outpaced by abrupt greenhouse gas releases (e.g. at the end-Permian mass extinction), or removals (e.g. “Snowball Earth” periods).

CO2 for the last 420 Million Years

CO2 levels for the last 420 million years, showing periods with ice ages. Note this curve is smoothed and too low resolution to show spikes in CO2, eg at the end-Permian, end-Cretaceous, PETM, etc. Data from Foster et al. nature communications 2017. Late Paleozoic Ice Age per Rolland et al. EPSL 2019. Preindustrial CO2 278 ppm, 2021 CO2 420ppm (CO2.Earth). Newer data zooming in on the last 66 million years can be found on the intermediate tab.

Last updated on 1 September 2021 by howardlee. View Archives

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Comments 1 to 50 out of 105:

  1. The lecture by Richard Alley is good! Very convincing piece of work. I'm wondering a little about why the "skeptic argument" above claims that there was glaciation in the "Jurassic-Cretaceous period". It would indeed be bad for the connection to CO2 if there was widespread glaciation during this period, but I can't find anything about that.
  2. This article seems to acknowledge that the skeptics are completely right. Skeptics do not say that CO2 has no effect, but that it is not the driving force. It is acknowledged in this article that there have been other driving forces in the past that far outweighed the influence of CO2, and it is not a large leap to conclude that there are driving forces today that outweigh CO2.
    Response: The best way to determine whether other driving forces outweigh CO2 forcing is to actually examine all the forcings that drive climate today. This analysis has been done and it's found that CO2 is the greatest forcing and also the fastest rising.
  3. Sorry, but you need to keep in mind the timescales involved. Total solar irradiance has been slowly increasing for a very long time. So it has a large effect on the overall long-term temperature trend of the planet, meaning hundreds of millions of years. That's not really relevant to the timescale of the 21st century. Likewise, the glacial/interglacial cycle plays out on a 26000 - 100000 year timescale. In contrast, we're doubling CO2 on a timescale of a century or so. We're also pumping out CH4, N2O, halocarbons, and other greenhouse gases. Thus, if you look at the actual magnitude of the radiative forcings, over the course of the 21st century the increase in greenhouse gases has a much larger forcing than any changes in TSI, Milankovich, etc.
  4. More a question than a comment. What is the science behind the statement "solar output was about 4% less than current levels"? Is the sun's output increasing over time?
  5. thatnumber5> Yes, the radiation from the sun is increasing. But as Ned says, it increases very slowly. I don't have a good reference for this right now, but try this wiki article on the faint young sun paradox. The general idea is that astronomers think that they have good models for the evolution of stars like the sun, so in particular, they can compute solar output from the age of the sun.
  6. Wow. This topic just came up in the current Greenland melting discussion (#52) so I spent a few minutes looking at denial sites. Widespread indeed is the notion that very high CO2 in geologic past coincided with glaciation and that somehow negates today's relatively paltry 370 ppm CO2. Graphs like this abound: — from the "Frontiers of Freedom" website. There are a couple of very straight-forward holes in these denialist arguments. 1. Ordovician CO2 over 4000 ppm and glaciation proves CO2 doesn't matter! Nope: Look at the distribution of continental landmasses of the Ordovician (~450 MY). Those "glaciers" were the south-polar ice cap. There wasn't much in the way of land in the northern hemisphere. 2. Warming and cooling is purely cyclical! CO2 variation is natural! Sure, there are natural cycles. But something very important and very obvious changed over the geologic time scales involved that makes such simple comparison irrelevant: Plants. Lots of plants. Gymnosperms (conifers etc) originated in the late Devonian-early Carboniferous (380-300 Mya) and angiosperms (flowering plants) in the Cretaceous (100 Mya). All that carbon in the Carboniferous coalbeds? Dead plants that took CO2 out of the atmosphere. The downward trend apparent in the graph above from the Cretaceous forward? More plants. And now we've turned the downward CO2 trend around despite a world rich in plants... maybe we can hope that a whole new class of plant life comes to our rescue... but that would require evolution and the science is still uncertain on that too.

  7. muoncounter, we have a continent at the south pole now, but I suspect that if CO2 were to go over 2000 ppm today most of that ice would (eventually) be gone. As a rough calculation, an increase in solar irradiance by 4% over the past 400 million years would yield something like +9 w/m2 forcing. Compare that to the anthropogenic CO2 forcing of something like +1.5 w/m2 ...
  8. Ned, "if CO2 were to go over 2000 ppm today most of that ice would (eventually) be gone." Agreed. And I certainly am not questioning the role of solar irradiance. But the geological proof that ice once existed at our South Pole -- striated bedrock among other unmistakable features -- would still be there. So any future scientific inquiry -- if there is such an enlightened future -- would say "see, they had 'glaciers' in a time of high CO2!" and conclude that CO2 is unimportant. "increase in solar irradiance by 4% over the past 400 million years" ... "Compare that to the anthropogenic CO2 forcing" 400MY is time enough for evolutionary changes on the grand scale. Isn't anthropogenic forcing is on a time scale of 100s of years? Not enough time for many organisms to get ready for a warmer environment.
  9. Oh, yes, you're quite right .... I'm not at all minimizing the problems resulting from doubling CO2 on short timescales. Just pointing out that when people refer to the very high CO2 in the Paleozoic, 400 million years ago, they need to realize that it was countered by what was a much lower solar irradiance. If CO2 hadn't dropped over time, the world would be more or less uninhabitable today. Or, another way of putting it is that a much smaller increase in CO2 today will produce a climate that would have required much higher CO2 to achieve in the Paleozoic.
  10. Ned, "a much smaller increase in CO2 today will produce a climate that would have required much higher CO2 to achieve in the Paleozoic." That's an excellent way of putting it. The Ordovician's big dropoff in CO2 is usually explained by the massive, continent-wide carbonate banks (Trenton, Knox, Arbuckle, Delaware Basin, etc in the US) deposited in warm, restricted shallow seas. "These carbonate rocks constitute part of the “Great American Bank” (Ginsburg, 1982) that extended more than 3,000 km (1,864 mi) along nearly the entire length of what was the southern seaboard of the Laurentian continental mass" -- Pennsyvania Geological Survey The deposition of carbonates (Ca0+CO2->CaCO3, calcite) is linked to climatic change in this paper: "The accumulation of great volumes of carbonates during pre-Hirnantian late Ordovician, in regions where these deposits were previously absent, is suggested as a major sink of atmospheric CO2. This would have caused an important lowering of the average temperature". We don't see such massive carbonates deposited today.
  11. muoncounter: Thanks for the link to that Villas et al. 2002 paper. That's really neat. They claim that marine carbonate deposition sequestered a mass of carbon equivalent to 350 times the current quantity of atmospheric CO2! I like their explanation of the mechanisms for both the onset and termination of glaciation.
  12. Ned, Those mechanisms are critical to the argument over "high CO2 and glaciation=No". It is certainly clear that widespread carbonate deposition takes up lots of atmospheric CO2, but whether that alone causes an ice age isn't clearly established. It is also clear that the graph of CO2 levels taken from a denialist website, posted above (#6), doesn't take a short-term drop in CO2 due to perfectly valid geological mechanism into account. I have some difficulty with the mechanisms in the "Mountains that froze the world" article John references at the top of this thread. For one thing, the Appalachians weren't all done in the late Ordovician -- it took another 100 MY or so until the Alleghenian Orogeny was complete. The image below is the mid-Ordovician southern ocean: -- source All that light blue is shallow sea -- mostly between 10N and 30S latitude -- perfect environment for carbonate deposition from marine organisms. For another, the idea that Sr86 in Nevada is runoff from the proto-Appalachians just doesn't seem right -- on the map above, Nevada is on the 'north coast' of Laurentia, while the emerging Appalachians are on the 'south coast'. Other mechanisms abound in the literature, from a mega-volcano to a gamma-ray burst. From another key paper on this subject, "the waxing and waning of ice sheets during the Late Ordovician were very sensitive to changes in atmospheric pCO2 and orbital forcing at the obliquity time scale (30–40 k.y.)" I've even seen one author who suggests that the concentration of continental land masses at the south pole would perturb the earth's orbit -- but that's a much longer-time scale event. Please note that I accidentally italicized the last sentence ("We don't see...") in #10. That was my statement and not part of the referenced article.
  13. Thanks, muoncounter. Also, re: Please note that I accidentally italicized the last sentence ("We don't see...") in #10. That was my statement and not part of the referenced article. Yes ... and I solved that by inserting a "/i" tag (in brackets) at the beginning of my comment. :-)
  14. How is solar heat output determined for periods before direct measurement? I ask because the article says solar output was 4% lower during the Ordovician but I can't tell how the number was arrived at.
  15. Here's an excellent writeup on main sequence stars rcglinksi.
  16. Watts has just posted a new article It refers to a new study in PNAS Hope you can comment on this soon.
  17. Robert I don't see anything unusual there. WUWT folks are angry because some poor scientist found out something boxing them in a little bit more.
  18. Thanks Doug.
  19. I think the argument about CO2 levels in the deep past is a bit of a red herring. Yes, CO2 levels were FAR higher in the Ordovician but, correct me if I'm wrong, there were also no land based life forms. Not even land based plant life. Doesn't that make it a little pointless what the CO2 levels were 500 mya?
  20. Just reading through the posts and I noticed your comment. There were land based life forms during the Ordovician. Plant and animal life. I don't know where you heard that there weren't. There was a major extinction event but this just led to a reduction of biodiversity.
  21. Good article.. very insightfull.. One question, how was the percentage of solar output derived. You said that "solar output was about 4% less than current levels.", but there are no sources and no further information as to how the number came about
  22. mmckinstrie, calculations of past solar output come from solar physicists. By studying stars of various sizes and ages they've been able to get a very detailed picture of how stars change throughout their lifetime. See info on the Standard Solar Model for details.
  23. Quoting from the beginning of this post: "When CO2 levels were higher in the past, solar levels were also lower" Can anyone point to a source for the this? And a nice graph showing solar levels in the past?
  24. SRJ - for sun levels in deep time, you dont have measured proxy but rather the calculation based on sun being a main sequence star. See for instance: faint young sun paradox
  25. Shouldn't the title of the article read "DO high levels of CO2 in the past contradict..." (instead of does)?
  26. I think everyone needs to be careful about inferred data based on models that are unproven, even if peer reviewed. Discussing the possible increase in Solar output is fraught with problems. Solar models are not complete, our understandings of the inner workings of stars is far from ideal and certainly not complete. There are problems with the SSM (Standard Solar Model) and this may or may not impact our model of the evolution of Stars in general, but especially those with similar properties to our sun. Many papers have been written on this subject in recent years. I would direct anyone interested to this article, Problems for the standard solar model arising from the new solar mixture. by J.A.Guzik 2008 Whilst I think it is important and helpful to look at climate data in the past, 400My is taking it to extremes as anything we say about that time is largely guesswork based on assumptions and statistical modelling. Anything more than about 5 million years old, in which we have lots of inter-related indicators of climate in the real world is largely pointless, and I would aim that at both sides of this debate. Wasting time on what may or may not have happened 400My ago is not helpful to anyone IMHO.
    Response: [muoncounter] Before you issue a general, unsupported 'be careful' about models, see the debunked argument Models are unreliable; read and digest the content, further comments go there.

  27. I've seen this graphic come up a few times to refute this argument and similar ones. Here's the original source: The page's author, Monte Hieb, is listed at the bottom. Poking around a little more on Google will give you a sense of his paleoclimate qualifications.
  28. Uncle Marc: Please take some time to get acquainted with SkS. See the newcomers guide, browse the 'Skeptic arguments.' There's a lot to learn; it will take some reading, but if you want to understand what's happening, it's well worthwhile. As far as the geocraft graph, see prior discussion starting with comment #6 on this thread, in which this graph gets debunked.
  29. Dr Easterbrook recently uses GISP2 data to show that over the past 25,000 years there have been more extreme fluctuations in temperature than that of the past 200 years. These changes are clearly not AGW. He also shows that there is no relationship to CO2 levels and that over the past 100 years CO2 have shown periods of inverse relationship. Easterbrook GISP2
  30. alecpiper: Dr Easterbrook concludes: "If CO2 is indeed the cause of global warming, then global temperatures should mirror the rise in CO2" No, that would only be true if CO2 were the only thing that affects global temperatures. Nobody would claim that is the case. "In 1945, CO2 emission began to rise sharply and by 1980 atmospheric CO2. had risen to just under 340 ppm. During this time, however, global temperatures fell about 0.9°F (0.5° C) in the Northern Hemisphere and about 0.4°F (0.2° C) globally." Sulphate aerosols (which have a cooling effect) also rose in the 1940s, but began to be phased out from the early 70s. Dr Easterbrook is just demonstrating his ignorance of the work that has been done on attribution of climate change in the 20th century. It isn't hard to find, there is a whole chapter on it in the IPCC WG1 scientific basis report. Being skeptical is fine, but you do need to know what it is you are skeptical about. These two errors ought to be enough to make anyone skeptical of Dr Easterbrooks article, I suspect there are others.
  31. alecpiper, what is it about "Dr Easterbrook" that makes you believe him above all others ?
  32. alecpiper @29 Easterbrook treats a local temperature record as if it were a global temperature record, which is obviously a fallacious method. What is more, he treats the last data point in the ice core record as though it were very recent, whereas it is in fact 1855. Comparison with modern Greenland temperatures show that for most of the ice core record, temperatures have been below modern temperatures (and may have been below for all of it). Further discussion on this point should be taken here where they are already discussed in detail.
  33. alecpiper. You trust Easterbrook? Have a look at: here and particularly here. Of course, dont take a warmist blog word for it. Pull the data, check the references (especially the metadata) and see for yourself.
  34. How is it that 4% less TSI in the Ordovician due to a younger sun results in a sixfold increase (500ppm now 3000ppm then) in the supposed CO2 tipping point for glaciation?
  35. Even with 0ppmv CO2 now the sun is bright enough that a snowball Earth is all but impossible, so the direct comparison wouldn't be valid. The direct comparison would only be valid if we were at the global glaciation threshold now. 500ppmv is clearly not the threshold for global glaciation now, if it were, we would be under a glacier! The drop in TSI of 4% is about 54 W/m^2, three doublings of CO2 would be about 12 W/m^2, so that presumably means that for a global glaciation to happen now, the sun would have to dim by about 42W/m^2 or about 3%. To put that into context, the variation of the 11 year solar cycle is about 2W/m^2 and the difference between glacial and interglacial conditions is apparently about 7W/m^2.
  36. We are currently in an interglacial interlude within an overall "snowball" period. We are at a bit less than 400ppm. My sense is that someone thought once we pass 500ppm we would exit the snowball regime altogether. I doubt this is exactly right but it is at least reasonable. To say that the same tipping point in the Ordovician was 3000ppm is extraordinary. Eccentricity is good for a couple watts as well.
  37. trunkmonkey@36 We are not in an interglacial within an overall "snowball" period. In a snowball period glaciation extends so far towards the tropics that albedo feedback means that the glaciation doesn't stop and continues to the equator and stays there (untill forcings change by 40ish W/m^2!). That hasn't happened for seveal hundreds of millions of years. If you mean the "tipping point" where glaciation would not happen at all, that is not the same "tipping point" at which a global glaciation (the whole of the Earth under ice) can no longer ocurr. The second of those "tipping points" as I said above is below 0ppmv already due to solar brightening.
  38. Try another "back of the envelope" calculation. According to Wikipedia (yes, I know ;o), the difference in solar forcing between a glacial and an interglacial is about 7W/m^2. The pre-industrial CO2 concentration was about C0 = 280ppmV. The radiative forcing for CO2 is given by DeltaF = 5.35*ln(C/C0) which implies that C = exp(DeltaF/5.35 + log(C0)) so substituting the figures, we get C = exp(7/5.35 + 5.6384) = 1000ppmv (ish) That calculation ignores any feedback etc, so if it was within a factor of two of the real answer from a climatologist (who unlike me knows what they are talking about ;o), I would be pleasantly surprised. A value of 500ppmv sounds plausible to me.
  39. Ok,ok I'm stumbling over nomenclature here. I never liked the icehouse/hothouse, snowball thing anyway. We are DEFINITELY in a glacial period. Glacial periods have their ocillations. We are currently in a warm phase. Our glacial period is called the Pleistocene. It has been with us for a couple million years, and happens to coincide, generally, with the evolution of the brains that allow us to carry on this discussion. The last glacial period of any consequence was the so called KT about 230 mya at the Permian-Triassic boundary. This one coincided with the greatest extinctions in the history of life. The notable one before that was at the end of the Ordovician about 450 mya. There were extinctions but only a few liverworts and mosses and possibly insects had made it on to land. The really wierd thing is that there are glacial tillites with a carbonate cap in Australia, and if we can believe the apparent polar wander paths Australia was pretty close to the equator then. There was another glacial period about 650 million years ago in the Proterozoic. Everynoe starts getting really grumpy and calling each other names and the apparent polar wander paths diverge before this. You can see that glacial periods are rare in earth history, ocurring roughly every 200 my. Between hese periods Gloval Average Temperature and CO2 are thought to be higher than now. Where dies the 5.35 come from in DeltaF = 5.35*ln(C/C0)?
  40. 5.35 W/m^2 is the measured radiative forcing constant for carbon dioxide. As for terminology... we are currently in an interglacial (i.e. relatively warm) period of the ongoing ice age (i.e. geological period where large ice caps are present). What you have probably heard people saying is that raising CO2 to ~500 ppm might prevent the next glacial period entirely. That is, normally we would expect the current interglacial period to end some time in the next 15,000 years or so and then be followed by a long period of increasing cold which would cause glaciers to spread out from the poles for ~90,000 years and then retreat as the next warming cycle comes around. However, if CO2 were raised to 500 ppm then it would likely take more than 100,000 years to return to pre-industrial levels (barring some new technology to sequester it faster than would happen naturally) and could thus keep the planet warm enough that we skip the next glacial cycle entirely. That'd actually be a good thing... but given that it is thousands of years away not quite as pressing as dealing with the warming we will see over the next two centuries.
  41. RE: CO2/AveTemp - mya Diagram This diagram could not prove anything. 1. It is a moving average - and of how many values, nobody knows. 2. The values themselves used in the Moving Average are also averaged values. 3. It is not even accurately calculated. Does anyone have any vague idea why the temperature saturation in the Jurassic and Cretaceous in the upper version of the Diagram is 22°C and in the lower version is 23° C, and how does this average temperature look like as distinct values. 3. This trend in the end of the diagram (in the last 10 million years, for example) is a masterpiece of misrepresentation: - What part of this period is with Homo sapiens and what without it (how much is 200 000 of 10 mln)? - What part of this period is with use of fossil fuels and what without? This 'prediction' is for another system and for another world (without humans and vehicles, and their fresh ideas of how to control the world). The guys that put back into the air the carbon (in the form of carbon dioxide) should have any idea of what they are doing and how they will clean up the air and the ocean back in case of 'emergency'. In the past Nature 'regulated' the concentration of CO2 by extinction of species. Who, how, and when will regulate the CO2 produced by the vehicles, for example and which species will extinct first - humans or their cars. The dinosaurs 'ruled over' the Earth for 160 mln years by virtually doing nothing 'as regulation'. We, with our fresh ideas of wasting natural resources, mania to control everything, and dealing with things that we don't fully understand will hardly make a million - seriously.
  42. carbonado wrote "In the past Nature 'regulated' the concentration of CO2 by extinction of species." Care to give a reference to back up that assertion? On a timescale of thousands of years CO2 levels are regulated by ocean-atmosphere transfers, over timescales of tens of thousands of years plus by the chemical weathering thermostat. See e.g. David Archers global carbon cycle primer published by Princeton University Press.
  43. Carbonado#41 Presumably you are referring to the graph posted in comment #6 and at larger scale in #27? This graph is a cartoon; it is not from an authoritative source and is not taken very seriously.
  44. RE: The species I am not specialist in the field, and yet according to Craeme Lloyd, Natural History Museum, London, UK more than 99% of all species ever lived on the Earth are extinct at present ... by one reason or another. RE: The two versions of the Graph I cannot dispute that both of the versions are absolute cartoons, but they are presented all over the Internet as 'Evidence No.1' that the CO2 and the global temperature 'are falling'.
  45. Carbonado#44: "they are presented all over the Internet as 'Evidence No.1'" That should tell you a lot about the quality of those arguments -- and the folks that present them. I'd say the science of using cartoon graphs in place of real data and observation is the real 'climastrology.'
  46. "I am not specialist in the field, and yet according to Craeme Lloyd, Natural History Museum, London, UK more than 99% of all species ever lived on the Earth are extinct at present ... by one reason or another." And that is true, except it says nothing at all about CO2 levels.
  47. This is really great discussion. A question I'd like to pose. As we know, our planet's core is cooling. So presumably, 400 MYA there would have been a lot more volcanic activity then there is today. This volcanic activity, of course, is what likely led to the high atmospheric CO2 levels in the past but my question is this - volcanoes spew alot more then just greenhouse gases. They will also spew dust and other such particles that would have a cooling effect on the earth. As such, could that also explain the reason for high CO2 levels during a period of glaciation?
  48. adesbarats, dust and other particulates from volcanic eruptions definitely have a cooling effect, but since these are solid matter (however small) they tend to settle out of the atmosphere within a few years. Indeed, this effect can be seen in climate records where one or two year temperature drops follow major volcanic eruptions. Thus, I don't think they make a good candidate for the cause of longer term 'low' temperatures alongside 'high' CO2 levels. The usual explanation for such past incidents is that the radiation output of the Sun is increasing as time goes by... 400 MYA the Sun was much 'cooler' than it is now. There are many other factors, but solar output, atmospheric CO2 levels, and surface ice albedo seem to be some of the most significant variables.
  49. adesbarats... Atmospheric CO2 levels are also part of a long term process called the "CO2 Rock Weathering Thermostat." Here is a good article about it. I'm not clear on how much volcanic activity has changed over the past 500 million years but what's really fascinating is you can see in the geologic record almost exactly where the Indian continent started bumping up against the Asian continent to start forming the Himalayas and started a long process where CO2 was pulled out of the atmosphere through rock weathering. And along with that you see the global temperature start a long slow decline from the days where you had crocodiles in the Arctic to modern glacial cycles in the Arctic. All of it a function of the amount of CO2 in the atmosphere. This all fits well with deep glaciation events (Snowball Earth) where the almost complete ice cover of the planet would prevent any rock weathering and thus cause CO2 to build up to very high levels before raising the temperature enough to melt the ice.
  50. CBDunkerson - thanks for your input. Makes good sense. Rob Honeycutt - thanks for the article link!

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