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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

The Ordovician glaciation was a brief excursion to coldness during an otherwise warm era, due to a coincidence of conditions. It is completely consistent with climate science.

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)

Geologists refer to ancient ice-cap formations and ice-ages as "glaciations." One such glaciation that occurred during the Late Ordovician era, some 444 million years ago has captured the attention of climate scientists and skeptics alike. To get some perspective on timing, that's just over 200 million years before dinosaurs began to roam the Earth.

Unlike other glaciations in the last 500 million years, this one was exceptionally brief (lasting perhaps only a million years or so) but the main reason for generating so much interest recently is because it took place when CO2 levels were apparently sky-high. As Ian Plimer notes in his book, "Heaven and Earth", pp165:

"The proof that CO2 does not drive climate is shown by previous glaciations...If the popular catastrophist view is accepted, then there should have been a runaway greenhouse when CO2 was more than 4000 ppmv. Instead there was glaciation. Clearly a high atmospheric CO2 does not drive global warming and there is no correlation between global temperature and atmospheric CO2."

On the surface, Plimer does seem to have a point: if ice-caps managed to exist back then in an ultra-high CO2 environment, why are the vast majority of climate scientists worrying so much about keeping CO2 levels piddlingly low?

To answer this, we have to fill in some parts of the puzzle that are missing. Let's start with the CO2.

Plimer's stated value of 4000 ppmv or greater is taken from Robert Berner's GEOCARB, a well-known geochemical model of ancient CO2. As the Ordovician was so long ago, there are huge uncertainties for that time period (according to the model, CO2 was between an incredible 2400 and 9000 ppmv.) Crucially, GEOCARB has a 10 million year timestep, leading Berner to explicitly advise against using his model to estimate Late Ordovician CO2 levels due its inability to account for short-term CO2 fluctuations. He noted that "exact values of CO2... should not be taken literally."

What about evidence for any of these short-term CO2 fluctuations? Recent research has uncovered evidence for lower ocean temperatures during the Ordovician than previously thought, creating ideal conditions for a huge spurt in marine  biodiversity and correspondingly large drawdown of CO2 from the atmosphere through carbon burial in the ocean. A  period of mountain-building was also underway (the so-called Taconic orogeny) increasing the amount of rock weathering taking place and subsequently lowering CO2 levels even further. The evidence is definitely there for a short-term disruption of the carbon cycle.

Another important factor is the sun. During the Ordovician, it would have been several percent dimmer according to established nuclear models of main sequence stars. Surprisingly, this raises the CO2 threshold for glaciation to a staggering 3000 ppmv or so. This also explains (along with the logarithmic forcing effect of CO2) why a runaway greenhouse didn't occur: with a dimmer sun, high CO2 is necessary to stop the Earth freezing over.

In summary, we know CO2 was probably very high coming into the Late Ordovician period, however the subsequent dip in CO2 was brief enough not to register in the GEOCARB model, yet low enough (with the help of a dimmer sun) to trigger permanent ice-formation. Effectively it was a brief excursion to coldness during an otherwise warm era, due to a coincidence of conditions.

The following (somewhat simplified) diagram may make this easier to understand:

Ordovician Glaciation

When looking at events such as these from the deep geological past, it is vital to keep in mind that there are many uncertainties, and generally speaking, the further back we look, the more there are. As our paleo techniques improve and other discoveries emerge this story will no doubt be refined. Also, although CO2 is a key factor in controlling the climate, it would be a mistake to think it's the only factor; ignore the other elements and you'll most likely get the story wrong.

Basic rebuttal written by steve.oconnor

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 6 July 2015 by pattimer. View Archives

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Further reading

Science magazine have published The Mountains That Froze the World by Phil Berardelli. It explains the work of Seth Young, explaining how the world fell into glacial conditions during the Late Ordovician.

This outstanding lecture by geologist Richard Alley is IMHO considered must-viewing for anyone seeking to understand the role of carbon dioxide throughout Earth's history. The lecture is The Biggest Control Knob: Carbon Dioxide in Earth's Climate History


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Comments 101 to 103 out of 103:

  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 ?

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