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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 26 to 50 out of 105:

  1. 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.

  2. 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.
  3. 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.
  4. 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
  5. 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.
  6. alecpiper, what is it about "Dr Easterbrook" that makes you believe him above all others ?
  7. 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.
  8. 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.
  9. 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?
  10. 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.
  11. 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.
  12. 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.
  13. 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.
  14. 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)?
  15. 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.
  16. 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.
  17. 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.
  18. 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.
  19. 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'.
  20. 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.'
  21. "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.
  22. 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?
  23. 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.
  24. 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.
  25. CBDunkerson - thanks for your input. Makes good sense. Rob Honeycutt - thanks for the article link!

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