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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)

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 300ppm as ice sheets waxed and waned with orbital wobbles (Rae et al. 2021). CO2 was also about that level during the 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 (eg 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 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 still quite uncertain, but they indicate 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 600ppm to about 1500ppm 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 (Joel 2017). 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

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. nature communications 2017. Late Paleozoic Ice Age per Rolland et al. EPSL 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.

Last updated on 1 September 2021 by howardlee. View Archives

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Comments 1 to 25 out of 105:

  1. The lecture by Richard Alley is good! Very convincing piece of work. I'm wondering a little about why the "skeptic argument" above claims that there was glaciation in the "Jurassic-Cretaceous period". It would indeed be bad for the connection to CO2 if there was widespread glaciation during this period, but I can't find anything about that.
  2. This article seems to acknowledge that the skeptics are completely right. Skeptics do not say that CO2 has no effect, but that it is not the driving force. It is acknowledged in this article that there have been other driving forces in the past that far outweighed the influence of CO2, and it is not a large leap to conclude that there are driving forces today that outweigh CO2.
    Response: The best way to determine whether other driving forces outweigh CO2 forcing is to actually examine all the forcings that drive climate today. This analysis has been done and it's found that CO2 is the greatest forcing and also the fastest rising.
  3. Sorry, but you need to keep in mind the timescales involved. Total solar irradiance has been slowly increasing for a very long time. So it has a large effect on the overall long-term temperature trend of the planet, meaning hundreds of millions of years. That's not really relevant to the timescale of the 21st century. Likewise, the glacial/interglacial cycle plays out on a 26000 - 100000 year timescale. In contrast, we're doubling CO2 on a timescale of a century or so. We're also pumping out CH4, N2O, halocarbons, and other greenhouse gases. Thus, if you look at the actual magnitude of the radiative forcings, over the course of the 21st century the increase in greenhouse gases has a much larger forcing than any changes in TSI, Milankovich, etc.
  4. More a question than a comment. What is the science behind the statement "solar output was about 4% less than current levels"? Is the sun's output increasing over time?
  5. thatnumber5> Yes, the radiation from the sun is increasing. But as Ned says, it increases very slowly. I don't have a good reference for this right now, but try this wiki article on the faint young sun paradox. The general idea is that astronomers think that they have good models for the evolution of stars like the sun, so in particular, they can compute solar output from the age of the sun.
  6. Wow. This topic just came up in the current Greenland melting discussion (#52) so I spent a few minutes looking at denial sites. Widespread indeed is the notion that very high CO2 in geologic past coincided with glaciation and that somehow negates today's relatively paltry 370 ppm CO2. Graphs like this abound: — from the "Frontiers of Freedom" website. There are a couple of very straight-forward holes in these denialist arguments. 1. Ordovician CO2 over 4000 ppm and glaciation proves CO2 doesn't matter! Nope: Look at the distribution of continental landmasses of the Ordovician (~450 MY). Those "glaciers" were the south-polar ice cap. There wasn't much in the way of land in the northern hemisphere. 2. Warming and cooling is purely cyclical! CO2 variation is natural! Sure, there are natural cycles. But something very important and very obvious changed over the geologic time scales involved that makes such simple comparison irrelevant: Plants. Lots of plants. Gymnosperms (conifers etc) originated in the late Devonian-early Carboniferous (380-300 Mya) and angiosperms (flowering plants) in the Cretaceous (100 Mya). All that carbon in the Carboniferous coalbeds? Dead plants that took CO2 out of the atmosphere. The downward trend apparent in the graph above from the Cretaceous forward? More plants. And now we've turned the downward CO2 trend around despite a world rich in plants... maybe we can hope that a whole new class of plant life comes to our rescue... but that would require evolution and the science is still uncertain on that too.

  7. muoncounter, we have a continent at the south pole now, but I suspect that if CO2 were to go over 2000 ppm today most of that ice would (eventually) be gone. As a rough calculation, an increase in solar irradiance by 4% over the past 400 million years would yield something like +9 w/m2 forcing. Compare that to the anthropogenic CO2 forcing of something like +1.5 w/m2 ...
  8. Ned, "if CO2 were to go over 2000 ppm today most of that ice would (eventually) be gone." Agreed. And I certainly am not questioning the role of solar irradiance. But the geological proof that ice once existed at our South Pole -- striated bedrock among other unmistakable features -- would still be there. So any future scientific inquiry -- if there is such an enlightened future -- would say "see, they had 'glaciers' in a time of high CO2!" and conclude that CO2 is unimportant. "increase in solar irradiance by 4% over the past 400 million years" ... "Compare that to the anthropogenic CO2 forcing" 400MY is time enough for evolutionary changes on the grand scale. Isn't anthropogenic forcing is on a time scale of 100s of years? Not enough time for many organisms to get ready for a warmer environment.
  9. Oh, yes, you're quite right .... I'm not at all minimizing the problems resulting from doubling CO2 on short timescales. Just pointing out that when people refer to the very high CO2 in the Paleozoic, 400 million years ago, they need to realize that it was countered by what was a much lower solar irradiance. If CO2 hadn't dropped over time, the world would be more or less uninhabitable today. Or, another way of putting it is that a much smaller increase in CO2 today will produce a climate that would have required much higher CO2 to achieve in the Paleozoic.
  10. Ned, "a much smaller increase in CO2 today will produce a climate that would have required much higher CO2 to achieve in the Paleozoic." That's an excellent way of putting it. The Ordovician's big dropoff in CO2 is usually explained by the massive, continent-wide carbonate banks (Trenton, Knox, Arbuckle, Delaware Basin, etc in the US) deposited in warm, restricted shallow seas. "These carbonate rocks constitute part of the “Great American Bank” (Ginsburg, 1982) that extended more than 3,000 km (1,864 mi) along nearly the entire length of what was the southern seaboard of the Laurentian continental mass" -- Pennsyvania Geological Survey The deposition of carbonates (Ca0+CO2->CaCO3, calcite) is linked to climatic change in this paper: "The accumulation of great volumes of carbonates during pre-Hirnantian late Ordovician, in regions where these deposits were previously absent, is suggested as a major sink of atmospheric CO2. This would have caused an important lowering of the average temperature". We don't see such massive carbonates deposited today.
  11. muoncounter: Thanks for the link to that Villas et al. 2002 paper. That's really neat. They claim that marine carbonate deposition sequestered a mass of carbon equivalent to 350 times the current quantity of atmospheric CO2! I like their explanation of the mechanisms for both the onset and termination of glaciation.
  12. Ned, Those mechanisms are critical to the argument over "high CO2 and glaciation=No". It is certainly clear that widespread carbonate deposition takes up lots of atmospheric CO2, but whether that alone causes an ice age isn't clearly established. It is also clear that the graph of CO2 levels taken from a denialist website, posted above (#6), doesn't take a short-term drop in CO2 due to perfectly valid geological mechanism into account. I have some difficulty with the mechanisms in the "Mountains that froze the world" article John references at the top of this thread. For one thing, the Appalachians weren't all done in the late Ordovician -- it took another 100 MY or so until the Alleghenian Orogeny was complete. The image below is the mid-Ordovician southern ocean: -- source All that light blue is shallow sea -- mostly between 10N and 30S latitude -- perfect environment for carbonate deposition from marine organisms. For another, the idea that Sr86 in Nevada is runoff from the proto-Appalachians just doesn't seem right -- on the map above, Nevada is on the 'north coast' of Laurentia, while the emerging Appalachians are on the 'south coast'. Other mechanisms abound in the literature, from a mega-volcano to a gamma-ray burst. From another key paper on this subject, "the waxing and waning of ice sheets during the Late Ordovician were very sensitive to changes in atmospheric pCO2 and orbital forcing at the obliquity time scale (30–40 k.y.)" I've even seen one author who suggests that the concentration of continental land masses at the south pole would perturb the earth's orbit -- but that's a much longer-time scale event. Please note that I accidentally italicized the last sentence ("We don't see...") in #10. That was my statement and not part of the referenced article.
  13. Thanks, muoncounter. Also, re: Please note that I accidentally italicized the last sentence ("We don't see...") in #10. That was my statement and not part of the referenced article. Yes ... and I solved that by inserting a "/i" tag (in brackets) at the beginning of my comment. :-)
  14. How is solar heat output determined for periods before direct measurement? I ask because the article says solar output was 4% lower during the Ordovician but I can't tell how the number was arrived at.
  15. Here's an excellent writeup on main sequence stars rcglinksi.
  16. Watts has just posted a new article It refers to a new study in PNAS Hope you can comment on this soon.
  17. Robert I don't see anything unusual there. WUWT folks are angry because some poor scientist found out something boxing them in a little bit more.
  18. Thanks Doug.
  19. I think the argument about CO2 levels in the deep past is a bit of a red herring. Yes, CO2 levels were FAR higher in the Ordovician but, correct me if I'm wrong, there were also no land based life forms. Not even land based plant life. Doesn't that make it a little pointless what the CO2 levels were 500 mya?
  20. Just reading through the posts and I noticed your comment. There were land based life forms during the Ordovician. Plant and animal life. I don't know where you heard that there weren't. There was a major extinction event but this just led to a reduction of biodiversity.
  21. Good article.. very insightfull.. One question, how was the percentage of solar output derived. You said that "solar output was about 4% less than current levels.", but there are no sources and no further information as to how the number came about
  22. mmckinstrie, calculations of past solar output come from solar physicists. By studying stars of various sizes and ages they've been able to get a very detailed picture of how stars change throughout their lifetime. See info on the Standard Solar Model for details.
  23. Quoting from the beginning of this post: "When CO2 levels were higher in the past, solar levels were also lower" Can anyone point to a source for the this? And a nice graph showing solar levels in the past?
  24. SRJ - for sun levels in deep time, you dont have measured proxy but rather the calculation based on sun being a main sequence star. See for instance: faint young sun paradox
  25. Shouldn't the title of the article read "DO high levels of CO2 in the past contradict..." (instead of does)?

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