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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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)

At a glance

Before diving headlong into this myth, one key thing needs to be pointed out. The confidence expressed in the above statement. Phrases like 'killer proof' should be enough to ring alarm bells warning the statement is not the work of any credible scientist. Scientific writing is a relatively sober business.

That aside, this myth is about the nature of Earth's atmosphere and climate through deep time. We know quite a bit about that atmosphere now - but far from everything. Through geological studies, we know a lot more about how the planetary climate evolved over time. But far from everything. It's work in progress.

Evolution of climate over deep time was governed by several factors. Fluctuations in the carbon cycle were driven by changes in the balance between CO2 sources and sinks. In those pre-human times, the key CO2 source was volcanic and the key sink was, as now, weathering. Volcanism is still a significant CO2 source - but about a hundredfold less than human emissions.

Weathering is a chemical reaction that involves the breakdown of the minerals making up the rocks of Earth's surface. Its key agent is carbon dioxide dissolved in rainwater, this being a weak acid. Since rainwater delivers the CO2, the intensity of weathering will partly depend upon rainfall. In turn, that depends on how wet - or dry - the climate happens to be in any given place at any given time.

That leads us into palaeogeography - the science of how the layout of the globe has changed through time. Slow changes in layout are driven by plate tectonics and continental drift. Geological evidence tells us that most of the continents on Earth were sometimes gathered together, to form 'supercontinents'. At other times, they were widely dispersed. Such changes in layout through deep time had implications for both the climate and intensity of weathering.

In general terms, the dry interior of a supercontinent reduces weathering, allowing CO2 levels to rise because the sink cannot keep up with the source. A dispersed pattern is a lot better for weathering and the sink can outpace the source, allowing CO2 levels to fall.

Over geological timescales, changes in Solar brightness matter, too. Solar brightness is considered to have increased steadily by about 10% per billion years of Earth's history. In the late Ordovician, 445 million years ago, there would therefore have been 4-5% less sunshine reaching the Earth. That's a big difference and enough to change what is known as the 'ice-threshold' - the point beyond which perennial ice-sheets can exist on Earth's surface.

There was certainly an ice-age in the late Ordovician. There are multiple lines of evidence that lend support to that statement. Dispersed continents favoured weathering and CO2 drawdown. Because of the dimmer Sun, the ice-threshold was set at a much higher CO2 level than in more recent times. Something else happened too. The late Ordovician ice-age was accompanied by the second-greatest mass-extinction in the fossil record. Neither the quote nor its parent document mention that. One wonders why.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Although our understanding of atmospheric composition through deep time is still a work in progress, we now know enough to state that 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 300 ppm as ice sheets waxed and waned with orbital wobbles (Rae et al. op.cit.). CO2 was also about that level during the late 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 (e.g., 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 (e.g., Fig. 1) 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 less certain, but they suggest that 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 600 ppm to about 1500 ppm 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. 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

Figure 1. 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. (2017). Late Paleozoic Ice Age per Rolland et al. (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.

Nevertheless, Earth's climate system has, for the most part, maintained a near-balance in terms of the overall habitability of the planet. This is despite periodic shocks of an internal (e.g. supervolcanic eruptions) or external (e.g. giant impacts) nature. That the key pre-human source of CO2 was volcanic activity and that volcanic activity is largely driven by plate tectonics is likely to be the key to this stability. Plate tectonics is a constant, ongoing process and probably has been for much of Earth history. That CO2 sources and sinks mostly do not stray too far apart - with the unique exception of human emissions - is very likely to be down to plate tectonics and its vital role in the Slow Carbon Cycle through geological time.

Last updated on 7 October 2023 by John Mason. View Archives

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Comments 51 to 75 out of 105:

  1. In my naivistic view of the things science is something that should serve to the people, should improve their quality of life, and should reduce the risks of getting into 'a dead end street'. CO2 is a dead end street from any point of view. Isn't it better to avoid entering this street at all instead of wondering what to do when we come to the 'dead end'. What kind of a science would teach us to close our eye when the disaster of climate change strikes on us? What kind of a science would teach us not to pay attention to the raising acidity of the oceans? What kind of a science would encourage us to expose to risk the only breaks we have so far against CO2 (the plants) and what kind of a science will have no idea of how to proceed (to hinder the processes at least)? Believe it or not but the increase of CO2 in the air (and in the ocean) could not improve our quality of life from any point of view. RE: This 500 MYA-CO2/Temp. Diagram If I am going to trade at the stock exchange with a million moving average in how many nanoseconds I would be Dead on Arrival? BTW: How does the planet look like at 24-25 deg.C ave temp?! What happens with the climate? Why does the temperature stop rising?
  2. Responding to a comment here: The geocraft cartoon graph rises again, with its ominous "... consternation of global warming proponents". And once again, there is no such consternation. The timing of high CO2 and glacial stage onset is critical - and that was always very tough to do on samples that are 450 M yrs old. So one has to wonder about the accuracy of the chronology shown on the geocraft cartoon. Especially in light of Saltzman 2005: ... the evidence suggests that the ice began to build up some 10 million years earlier than when volcanoes began pumping the atmosphere full of the CO2 that ended the Ordovician ice age. Abstract here. Young et al 2010 adds more to this consternation-busting line of research: The integrated datasets are consistent with increasing pCO2 levels in response to ice-sheet expansion that reduced silicate weathering. Ultimately, the time period of elevated pCO2 levels is followed by geologic evidence of deglaciation. -- emphasis added Full paper pdf here.
  3. I want to make sure I have this right. In a statement such as, "Doubling the CO2 increases the temperature 3 degree C (eventually)" are we talking about doubling the CO2 from pre - industrial levels, doubling the CO2 from what we have now, or what?
  4. curiousd @53, across a wide range of CO2 concentrations, including all those that have been experienced on Earth in the last 600,000 years or are projected under anthropogenic emissions, doubling CO2 results in a 2-4 degree increase in temperature if we ignore slow feedbacks such as melting of ice sheets. The IPCC best estimate for that figure is 3 degrees C.
  5. curiousd, The short answer is "both." The way it works (and remember that 3˚C is an estimate that doesn't apply exactly in all cases, but each case could be a little more or less) any doubling will increase temps by 3˚C. So if temps were at 288˚K at 280 ppm (the pre-industrial level), then if we double that to 560 ppm then we should expect temps to be at 291˚K (add 3˚C). CO2 levels are currently at 400 ppm. This implies that we have already committed ourselves to an equilibrium temperature increase of 1.54˚C, or a new "setting" of 289.54˚C. The planet hasn't reached this temperature yet, but if we held CO2 levels constant starting now, that is the temperature we'd expect the planet to reach. Given that, if we then doubled CO2 levels from the current 400 ppm up to 800 ppm, we should expect to add another 3˚C when the planet reaches equilibrium, for a final temperature of 292.54˚C (289.54˚C + 3˚C)... a total increase since pre-industrial levels of 4.54˚C.
  6. Curiousd, There is a nice article on climate sensitivity here by Dana1981. As Spherica said, short term (perhaps 50-100 years) climate sensitivity is about 3C per doubling. Long term (hundreds of years) is estimated at double that. Usually only the short term sensitivity is discussed. If you care about life in 300 years the picture is worse.
  7. There is an excellent article in Science News about new research to figure out exactly what mix of factors was responsible for the greenhouse gas mix being sufficient to overcome the really faint Sun 2.5 to 3.8 billion years ago (the Archean period).  It even involves fossil evidence of raindrops, to infer raindrop shape and speed!

  8. There is a man who lives close to me who is in the Guiness Book of Records for growing the world's largest vegetables.

    He pumps CO2 into his greenhouse.


    [TD] Please read the post "CO2 is Plant Food," and comment there, not here.

  9. roscoe:

    Your point being?

  10. If high CO2 levels of the past ages can be discounted by claiming the sun was 4% cooler back then, couldn't the slightly higher temps today be attributed to the sun being that much warmer than in the past? And not necessarily due to the CO2 levels? To discount levels in one scenario and not the current one? That seems like making the science fit a desired result rather than analysing the science to find the results...just an observation...

  11. Eric...  One simple question. Do you honestly think the entire scientific community is fitting science to a desired result? Or, perhaps, might it be that you haven't yet grasped what the science says relative to your question?

    The first act of skepticism should be to ask a question, as you've done, and then see if you can find the answer. 

    Your comment seems more leading than "just an observation." It sounds more like you're headed off into territory where you are forming conclusions based on a lack of knowledge.

  12. eric...  Next, you need to consider what 4% actually means relative to solar output.

    Our sun is a relatively stable star which varies only ~0.1% over the 11 year solar cycle. Solar irradiance is measured at around 1361W/m^2. Four percent of that is 55W/m^2, which is a significant difference of forcing on the climate system.

    The change in solar forcing since ~1900 is on the order of 0.5W/m^2. So, we're talking about 2 orders of magnitude difference.

    Others can check, but I think I have my figures correct.

  13. eric7866:

    First, solar output, on the largest scale, changes extremely slowly over geological timescales, but is extremely significant when it changes. A few % increase in solar output will, in a billion years or so, render the Earth uninhabitable. So it's no surprise that a few % decrease in solar output going back into the geological past requires an immense amount of atmospheric greenhouse gases to compensate. That said, geologically-significant changes in solar output are not something that has the effects consistent with what we have seen of global warming over the last 150 years.

    Second, today's global mean surface temperatures are cold compared to most of the Phanerozoic. (Although given sufficient unabated warming our species may yet have the opportunity to experience surface temperatures consistent with, say, the Late Cretaceous).

    Third, if you think that scientists studying present and paleoclimates are "discounting" any forcing agent in any situation, you are simply mistaken.

  14. "Periods of low CO2 coincide with periods of geographically widespread ice ... This leads to the concept of the CO2-ice threshold - the CO2 level required to initiate a glaciation."

    Sorry for my ignorance, CO2-ice threshold... my understanding is we are around 490 CO2 equivalent right now... Is the converse of deglaciation the same 'number'? Am I also correct that the current 'number' is 500?

  15. Current CO2 is 400ppm, not 490ppm. 500ppm for sufficiently long period of time (1000s of years) is postulated to be enough for it to be more or less ice free at poles, (though Antarctica would likely retain some ice as much higher in altitude than north pole). Note that in Pleistocene we have ice age cycle driven by Milankovich cycles. In Pliocene, there was no such ice age cycle although the Milankovich cycle was almost certainly present (driven by earth orbital mechanics) and CO2 was around 400ppm and solar input roughly the same.

  16. Should have checked before commenting. There is evidence of Milankovich cycles operating in Pliocene and moderating the size of the West Antarctica ice sheet. See here. The milankovich forcing seems to have been operating since at least Oligocene (see here) but in the without low CO2, they did not precipatate ice ages. At 500ppm, our climate would likely be similar to the warmer Miocene.

  17. @ 65

    I'm interested in this comment:

    500ppm for sufficiently long period of time (1000s of years) is postulated to be enough for it to be more or less ice free at poles, (though Antarctica would likely retain some ice as much higher in altitude than north pole).

    How did you come across that idea may i ask?

  18. ooh, also how does comment 66 alter things?

  19. @ 54,

     What happens to the sensitivity predictions if we include the slow feedback of melting ice sheets?

  20. Ravenken @64.

    I feel scaddenp @65/66 is a little quick converting your measure ppm CO2(e) into ppm CO2.

    The use of CO2(e) as a measure of climate forcing also has to take on board the anthroprogenic negative forcings which are less well defined but which will (indeed, can only) reduce the CO2(e) figure.

    The net forcing equivalent to that during time past when CO2=500ppm is a good start point for considering when the globe would lose the southern ice cap. The northern one is more suseptable to increases in temperature. The IPCC suggest in this SLR graphic that a temperature rise above pre-industrial of ≈1.5ºC would see Greenland melt down. (Mind, the ≈ is a worry.) And as the summit drops to warmer altitudes, it would become a tipping point and irreversable outside a renewed ice age. So if Antarctica is 500ppm, Greenland would be 400ppm.

    Of course these are very slow multi-millenia processes. The forcing from methane etc and also the negative forcings are short-lived compared with CO2. And even CO2 will drop over such long time periods. But then melting the very last snow flake on the planet (or just in the NH) is not some target we should be taking as something to avoid. We will be in deep deep do-do far far earlier than the arrival of an ice-free hemisphere or two.

  21. MA Rodgers. Whoops! I did indeed miss the CO2(e). Good point. On that basis, we would indeed be heading for Miocene-type climate (eventually).

    Bozza - based on past climate with assumed similar net forcings. It takes a very long time for icecaps for melt and change in albedo would be very slow till nearly gone.

  22. Until the Faint Young Sun Paradox is resolved we cannot claim that the geological record agrees with holocene climate models.


    [PS] You need to tell us more about what you mean. The faint young sun paradox concerns solar output billions of years ago and irrelevant to the holocene. Greenhouse gases give us a good resolution to the paradox from what can be construed about past atmosphere so what is that you think is unresolved of relevance here?


    What sceptics think it shows:

    There is no correlation between temperatures and C02 and that c02 levels was higher in the past, like hundreds of millions of years ago (thus arguing that todays c02 level is nothing to worry about).


    But why is there no correlation between c02 and temperature on the graph? Are there another major driver of temperature than C02? And this is so ironic; Deniers favorite mantra is "Its the sun", but what do they do? LOL..

    They use a graph which do not include the sun.

    The assertion that only CO2 drives temperature it's as much a logical falacy as the sun being the only driver of temperature though science considers both. Its amusing that deniers, who say it's the sun which drives the climate, do not consider the sun when they try to demonstrate there is no correlation between CO2 and temperature.

    (The effects of today's global warming are felt by societies and existing ecosystems adapted to the Holocene climate in OUR TIME - NOT the climate and CO2 levels that existed hundreds of millions of years ago.)

    But anyway, can you see the name in the bottom left corner?

    It says C02 after Robert A Berner 2001. refers to a study.

    Lets check this study and see what it says about c02 and temperatures:


    On page 201:

    "Thus, exact values of CO2, as shown by the standard curve, should not be taken literally and are always susceptible to modification. Nevertheless, the overall trend remains. This means that over the long term there is indeed a correlation between CO2 and paleotemperature, as manifested by the atmospheric greenhouse effect"

    WOW. The man behind the C02 graph says there IS a correlation between temps and c02, which is the opposite of what Patrick Moore claims and what deniers thinks of the graph. Berner also confirms the greenhouse effect, which is basic physics many deniers refuse to believe in.

    Full debunk of the graph:

    Can we make better graphs of global temperature history?



  24. Poholer54 discusses this from about 3:32.

  25. First of all, I do not believe "CO2 does not drive climate". Beside, the word "drive" is way too vague in this context and can easily be used to smuggle in a whole host of possible interpretations the author possibly didn't have in mind at all.

    I prefer classical terms like "correlation" and causation". Let's rephrase the question now:

    We know this planet has experienced CO2 concentrations in the past that exceed the current levels by a magnitude without triggering a runaway greenhouse effect. Now you're making 2 claims:

    1) those, much smaller concentrations will cause a runaway greenhouse effect, and

    2) you can be certain you can understand all the processes and factors involved to such a degree that you can model the mechanism of triggering the runaway effect (I'm not talking about the model predicting the avreage temperatures).

    Those are really, really extraordinary claims. I hope you agree with that assessment. And if so, you already know what I will say about proof levels required.

    If you say, we can't be certain, but the risks involved are too high, thus worth taking seriously, I have no problem with that... but that's not my question.

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