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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 101 to 105 out of 105:

  1. Thank you michael sweet and nyood for your enlightening thoughts;

    I knew I was off topic, but I'm new to the forum and I didn't understand how vast the forum is. I also realize my question was more philosophical in nature and does not merit discussion on this thank you for responding. The forum seems filled with intelligent people presenting good data and I shall greatly enjoy delving through its contents.


    [DB] Your question and statements were on-topic and such are always welcome here.  As you note, the site is quite vast (thousands of discussion threads exist, on all of the near-several-hundred most-popular denier mantras and years of blog posts and re-posts).   You can either use the Search function to find threads to review or look at the Taxonomy of denial (or sorted by Popularity).

  2. Livinginawe @98,
    You pose an interesting thought (which I think can be addressed on-topic).

    In essence, I don't think mankind's could ever become saviour of a living world by boosting atmospheric CO2 levels.

    If we look tens-of-thousands of years into the future, there is talk (eg see this Wikithing page, although the reference it makes Berger & Loutre (2002) is not as defininte) that in 50,000 years time the world will face an ice age that will not be dodged by our emissions. By that time the impact of our emissions on the atmosphere would be much diminished. But an ice age does not of itself reduce the amount of carbon about. Rather it sucks it out of the atmosphere into oceans and frozen soils.
    While these dips in CO2 could soon make life for C3 plants very difficult, C4 plants can survive at atmospheric CO2 levels well below 100ppm. And there are also aquatic plants which maybe even benefit from colder waters enriched by CO2.

    Over the longer term, hundreds-of-millions of years, the increase in solar strength will increase rainfall and thus increase rock weathering drawing down carbon into the geology via the slow carbon cycle(as this NASA web-page terms it) while the release of carbon back from geology via volcanic activity is presumably fixed and will not respond enough to compensate. Thus the 60,000Gt(C) on the planet will more-&-more become trapped in the geology (currently about 10,000Gt(C) is in trapped in rocks) and atmospheric CO2 levels will drop.
    The works of man so-far have release ~700Gt(C) from FF with perhaps 1,500Gt(C) of FF reserves and so they don't amount to anything significant in the grand scheme of things.
    Mind, as the sun warms the planet creating a wetter world which draws down more CO2 into the rocks, that loss of CO2 will cool the world and act as a brake on the process. The loss of CO2 would be something like 500My of the strenghening sun, so after that sort of time period the wetter climate would have no CO2 'brake'.

    Over the last 50My we have seen atmospheric CO2 levels drop but that atmospherc loss of perhaps ~5,000Gt(C) was probably driven by the Himalayas being weathered down in the wet climate of the tropics with a feedback of lost CO2 cooling the climate and drawing further CO2 into the oceans (where most of the planet's carbon resides today). So the rate of loss of atmospheric CO2 over the Cenozoic era probably shouldn't be projected into the future with any confidence.

  3. Dear all,

    Life was built here during bilions of years. To preserve life for Human we need specific enviroment /forest+fields + rivers + clean water and air for health/.  Increasing temperature and changed water distributions is threat for this human positive enviroment. find  Reason for negative trend is not highest need , Highest need is to keep human friendly enviroment.


    Plant a trees

    Built artificial lakes and water dam.

    Avoid water and air pollutions.

    Do not concentrate populations to cities only.

    Do we need fear as only motivations to do this ?

  4. Robert Murphy#46 
    "And that is true, except it says nothing at all about CO2 levels." From: 'The Undesigned Universe' - Peter Ward
    “ . . . it>
    62:26 is these ocean state changes that are
    62:28 correlated with the great disasters of
    62:30 the past impact can cause extinction but
    62:35 it did so in our past only once that we
    62:38 can tell whereas this has happened over
    62:40 and over and over again we have
    62:42 fifteen evidences times of mass
    62:45 extinction in the past 500 million years
    62:48 so the implications for the implications
    62:51 the implications of the carbon dioxide
    62:52 is really dangerous if you heat your
    62:55 planet sufficiently to cause your Arctic
    62:58 to melt if you cause the temperature
    63:01 gradient between your tropics and your
    63:03 Arctic to be reduced you risk going back
    63:07 to a state that produces these hydrogen
    63:11 sulfide pulses “


    [BL] You have been warned before about posting comments that consist of little more than quoted material. You are skating on thin ice, approaching violation of the comments policy.

    The comment you claim to be responding to (#46) was posted in 2011. Do you really expect to be engaging in discussion with that participant?


  5. Please note that we published new versions written by Howard Lee of both the basic and the intermediate rebuttal versions today (August 31/September 1 2021 depending on where you are). Comments above this one therefore refer to what are now the archived basic and archived intermediate rebuttal versions respectively.

    Please check the new versions and the many references linked in them!

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