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

Posted on 24 December 2010 by steve.oconnor

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.

This post is the Basic Version (written by Steve O'Connor) of the skeptic argument "CO2 was higher in the past".

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Comments 1 to 22:

  1. Plimer has done cherry picking. As one goes back in time, one sees less and less detail. Certainly carbon dioxide was higher in the past but many things were different in the Ordovician than they are at present. One example is that all land was in the Southern Hemisphere at that time. Ocean and atmospheric circulations were different and mountain ranges were lower with the possible exception of the Taconic Mountains. The Sun was certainly dimmer. Steve O'Connor's post is a very good one. We need to address the deniers every time they open their mouths or put pen to paper. The science is sound and conclusive.
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  2. You may also be interested in the recent paper by Young et al, which shows a dip in CO2 concentrations coinciding with the Odovician glaciation. To quote their abstract:
    "The Late Ordovician Hirnantian Stage (∼444 million years ago) was one of three time periods during the past half billion years in which large continental glaciers formed over Earth's polar regions. The effects of this glaciation were far-reaching and coincided with one of the largest marine mass extinction events in Earth history. The cause of this ice age is uncertain, and a paradoxical association with evidence for high atmospheric CO2 levels has been debated. Precise linkages between sea level, ice volume, and carbon isotope (δ13Ccarb and δ13Corg) proxy records of pCO2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections. Although correlation difficulties remain, recent Hirnantian biostratigraphic studies now allow for improved correlations. Here we show that consistent trends in both δ13Ccarb and δ13Corg from two well-dated stratigraphic sequences in Estonia and Anticosti Island, Canada coincide with changes in Late Ordovician (Hirnantian) climate as inferred from sea level and the extent of ice sheets. 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.
    Young et al. Did changes in atmospheric CO2 coincide with latest Ordovician glacial–interglacial cycles?, Palaeogeography, Palaeoclimatology, Palaeoecology, 2010
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  3. Given the logarithmic response of CO2 wouldn't the difference in forcing between 3000 and 4000 be quite small? That would then suggest that the "sensitivity" to CO2 warming shown in the figure above is much larger than what has been calculated in detailed-look-at-climate-sensitivity.html. But it also would suggest that paleo sensitivity analysis has lots of confounding factors that are not being considered. If there are such factors above (extremely likely) then they exist in more recent paleo history as well.
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  4. #3: Eric, a doubling is a doubling; ln 2 is still 0.693 regardless of the level. The difference is that there are more ppm between 2000 and 4000 than between 280 and 560, so it may take longer for subsequent doublings to occur (depending, of course, on how fast CO2 is being put into the environment). The sensitivity, ie the constant of proportion between delta T in degrees and delta F in W/m^2, remains the same. If you are familiar with music, an A = 110 hz & A = 220 hz & A = 440 hz & A = 880 hz & A = 1760 hz etc.
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  5. So the more CO2 -> higher temperature part would work, but maybe not the higher temperature -> more CO2 part of the feedback cycle?
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  6. #5: "maybe not the higher temperature -> more CO2 part of the feedback" That part is cryptic; presumably you mean CO2 released from warming oceans? Every time I hear that assumption, I have to ask 'if there is a net increase of CO2 coming from the oceans, why are the oceans acidifying?'. We have an excellent thread on that topic.
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  7. Eric #3 The ratio of forcing between 4000 and 3000 is roughly comparable to the ratio we see between glacials and interglacials in the far more recent Ice Core records - 100's of 1000's of years rather than 100's of millions. So the size of the forcing change is comparable. The reason it occurs at several doublings of CO2 higher in the deep past is that the Sun was several percent weaker 400 million years ago and so the CO2 threshold at which you can slide into glacial conditions is several doublings higher.
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  8. Glenn, I'm not sure that explains how we got out of the glacial conditions 400m years ago. How does warming temperature feed back to create 1000 more ppm of CO2 (i.e. where does the CO2 come from?)
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  9. Going back as far as 450 mya requires that you take plate tectonics into account. Look at this map; the entire land mass was in the Southern Hemisphere and the area affected by Ordovician glaciation was the South Pole at the time. The last ice left in our world, if we ever get that far down the drain, will probably be at the South Pole. The other aspect of the distant past that some people ignore is evolutionary. There weren't any land plants. So it is the most extreme apples and oranges (except there weren't any oranges!) comparison to look at today's CO2 concentration and compare numbers with 'way back when'.
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  10. "Glenn, I'm not sure that explains how we got out of the glacial conditions 400m years ago. How does warming temperature feed back to create 1000 more ppm of CO2 (i.e. where does the CO2 come from?)" If everything's in equilibrium, the oceans ...
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  11. Eric @8, the mechanism postulated by Young et al. is that increased glaciation reduced exposed rock, thus reducing the removal of CO2 from the atmosphere by weathering. With a large background vulcanism, this resulted in elevated CO2 levels.
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  12. ... CO2 is a key factor in controlling the climate ... - how the results from the current post, since the Ordovician glaciation in the Sun may be responsible (it could be as high as 30% weaker than it is now)? Most researchers favor a decisive influence CO2 on the current warming, but when it comes to paleo-warming, opinions are much more diverse. Let's not forget about the possible impact (Ordovician) of galactic: Figure 4., and Fig. 5.
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  13. #8: "how we got out of the glacial conditions 400m years ago." #10: "where does the CO2 come from?" A large quantity of CO2 was removed from the atmosphere/oceans during this point in the geologic past. It's not a question of where it came from (may have been 'primordial'), but where it went: The widespread deposition of Ordovician marine carbonates (CaCO3) in warm shallow seas.
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  14. muoncounter, thanks for the info. My question was narrower, I just wondered if there was a known CO2 source for the increase 445m years ago in the chart above. dhogaza, it could have come from the oceans, but not with today's ocean turnover rate. Since it took millions of years to vent out (although I'm sure the chart above is fairly crude), the ocean turnover would have to be very slow to very slowly warm and thus very slowly release its CO2. Such slow turnover may be possible with the continents arranged the way rockytom pointed out. Tom's explanation in #11 also sounds plausible and simpler.
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  15. #14: "not with today's ocean turnover rate." Why would you use today's anything rate when talking about 450 mya? The continents were in a completely different configuration; oceanic circulation was vastly different. Here's evidence that the Ordovician ice began forming in a much lower CO2 environment: 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. "... sounds plausible and simpler. " The answer to this question will not be found in a simple one-size-fits-all model.
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  16. Re: Eric (skeptic) (14) The biggest obstacle to finding a "smoking gun" (causative mechanism) to end the ice age in question is the granularity of the measurements. Unless we find a datable proxy that catches a mechanism "in the act" we may never know for sure. An example of a postulated "quick" mechanism would be the methane clathrate gun. Recent evidence has been found to support such a mechanism (see free copy here). By no means am I postulating this mechanism is the specific one to end the ice age in question. Just wanted to point out one possible mechanism. Hope this helps, The Yooper
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  17. muoncounter and Daniel Bailey, thanks, I'll look at those some more tomorrow. On first glance though, those factors are not sufficiently quantified into CO2-doubling-type forcings to get a sensitivity number out of the graph above (not to mention poor granularity, not much of a temperature proxy shown on the graph except glaciated and not glaciated, etc). It might speak to the general argument but is not quantifiable.
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  18. Eric (skeptic) @ 14: The CO2 could not have come from the oceans as they were iced over and hence disconnected from the atmosphere. However this means that the oceans were no longer removing CO2 from the atmosphere either*, and mineral weathering on land was also reduced by ice cover (as mentioned earlier in the thread). This means that the relatively small amounts from volcanic eruptions would just accumulate in the atmosphere until the greenh ouse effect was high enough to start melting the ice. Once that happened, there would be feedback from decreasing albedo and from increased water vapour, so it is unlikely that CO2 alone was responsible for the emergence from the snowball Earth. *The exchange of CO2 between the atmosphere is not governed only by temperature, it is also governed by the difference in partial pressures between the atmosphere and the surface waters, so all things being otherwise equal, the oceans would try to remove excess CO2 from the atmosphere (by becoming a net carbon sink), as they are doing now.
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  19. FYI, current astrophysical models put the solar constant increase at around 7% per billion years, which works out to ~40 W/m² since the Ordovician. Meanwhile 4000 ppmv of CO2 would imply about 14 to 18 W/m² without feedbacks. This is again a pretty good indication that CO2 feedbacks are strongly positive, otherwise the entire Ordovician would have been glaciated.
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  20. As I have only recently discovered this site and not spent much time searching for the answer to my question, perhaps someone can enlighten me. It seems to me that there has been no response on this website to the critical argument put forward in "The Great Global Warming Swindle". This was that the build up of CO2 in the atmosphere is a product of global warming and not the cause. The coincidence of the fluctuations in CO2 and GW was the claim made in "An Inconvenient Truth" and was illustrated by the huge stage-encompassing graph showing their correspondence. What the GGWS documentary pointed out the graph didn't take into acount the 800 year lag, which was swallowed by the huge time scale of the graph. To state the obvious, if CO2 is the product of GW then the world is embarking on probably the most expensive mistake in history.
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  21. #20: Consider looking around SkS, you might learn something See this thread on the non-existent lag question. If you have further questions about the lack of lag, use that thread. See this website for thorough debunking of your 'swindle,' which is a swindle of its own. To state the obvious, facts make a better argument.
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  22. To those of you who think Plimer is "cherry picking" and his input ought to be discounted, I would ask you if you think the data collected from the Vostock ice core sample are also suspect? The data used by Al Gore (incorrectly) came from that source. The original tests were taken from 1000 year slices that seemed to back his assumption. The retest, using 100 year slices showed a much different dataset. As the new data was out in print 2 - 3 years prior to the release of "An Inconvenient Truth" Mr Gore was forced to only intimate that increasing CO2 levels were responsible for warming. Of course I'm sure all of you debaters know the data showed just the opposite. Another dataset to consider comes from Temperature after C.R. Scotese CO2 after R.A. Berner, 2001 (GEOCARB III) Particularly the 600 million year chart of both CO2 and temperature, which shows only one other period of time, when both Temp and CO2 levels were as low as they are today. This article also shows a glaciation during the Carboniferous period at a time CO2 levels were just slightly higher (400ppm) than they are today. I really believe the bulk of actual scientific studies, not computer simulations, support the position that IF humans are involved in altering climate... it would be a minuscule involvement at best.
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    Moderator Response: [Daniel Bailey] Then you should have no difficulty providing links to those "bulk of actual" scientific studies then. I assume they were all peer-reviewed, right?

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