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Climate Hustle

CO2 has been higher in the past

Posted on 27 October 2009 by John Cook

We've seen empirical evidence that more CO2 causes an enhanced greenhouse effect. However, when we look back over the Earth's history, we see many periods where CO2 is higher than current levels of 384 ppm. Intriguingly, for some of those periods where CO2 was higher than now, the planet experienced widespread regions of glaciation. Does this contradict the warming effect of CO2?

No, it doesn't, for one simple reason. CO2 is not the only driver of climate. To understand past climate, we need to look at other forcings that drive climate. One paper that does this is CO2-forced climate thresholds during the Phanerozoic (Royer 2006). It pieces together 490 proxy records to construct a 540 million year timeline of CO2 levels. This period is known as the Phanerozoic eon.


Figure 1: Atmospheric CO2 through the Phanerozoic. Comparison of the predictions of GEOCARB carbon cycle model (dashed line) with a smoothed representation of the proxy record (solid line). Source: Royer 2006.

CO2 levels reach some spectacular values in the deep past, possibly topping over 5000 ppm in the late Ordovician, around 440 million years ago. However, solar activity also falls as you go further back. In the early Phanerozoic, the solar constant was about 4% less than current levels. Royer 2006 combined the radiative forcing from CO2 and solar variations to find their net effect on climate. The result is shown in Figure 2. Cooler climate is indicated by shaded areas which are periods of geographically widespread ice.


Figure 2: Combined radiative forcing from CO2 and sun through the Phanerozoic. Values are expressed relative to pre-industrial conditions (CO2 = 280 ppm; solar luminosity = 342 W/m2); a reference line of zero is given for clarity. The dark shaded bands correspond to periods with strong evidence for geographically widespread ice.

They find that periods of low CO2 correlate with long-lived, extensive continental glaciations while periods of high CO2 don't overlap with these glaciations. They also explore the concept of the CO2-ice threshold - the CO2 level required to initiate a glaciation. When the sun is less active, the CO2-ice threshold is much higher. For example, if the CO2-ice threshold for present-day Earth is 500 ppm, the equivalent threshold during the Late Ordovician (450 million years ago) would be 3000 ppm.

A follow up paper Climate sensitivity constrained by CO2 concentrations over the past 420 million years (Royer 2007) offers a more quantitative comparison of global temperatures and CO2 levels over the Phanerozoic. They couple the CO2 record to temperature and find a climate sensitivity of 2.8°C. The results "indicate that a weak radiative forcing by carbon dioxide is highly unlikely" and that a "climate sensitivity greater than 1.5°C has probably been a robust feature of the Earth’s climate system over the past 420 million years"

So we see that comparisons of present day climate to periods 500 million years ago need to take into account the fact that the sun was 4% less active than now. What about times closer to home? The most recent period when CO2 levels were as high as today was around 15 million years ago, during the Middle Miocene. CO2 levels were at about 400 ppm. What was the climate like at the time? Global temperatures were 5 to 10 degrees Fahrenheit higher than they are today. Sea level was approximately 75 to 120 feet higher. There was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland. The close coupling between CO2 and climate led the author to conclude that "geological observations that we now have for the last 20 million years lend strong support to the idea that carbon dioxide is an important agent for driving climate change throughout Earth's history."  (Tripati 2009).

To sum up, Dana Royer says it best: "the geologic record contains a treasure trove of 'alternative Earths' that allow scientists to study how the various components of the Earth system respond to a range of climatic forcings." Past periods of higher CO2 do not contradict the notion that CO2 warms global temperatures. On the contrary, they confirm the close coupling between CO2 and climate.

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Comments 51 to 61 out of 61:

  1. Was curious, so I Googled 'geological CO2 history'.

    Found the graph on the top of page 92 in this paper showing the decline in CO2 over the past 160 million years. Am I correct in assuming that this is showing a decrease in atmospheric CO2 over that time period? Where is the CO2 going over geologic time?

    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692169/pdf/YAKVJWBFM62NRFN4_353_83.pdf

    Thank you,
    Chris Shaker
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  2. cjshaker wrote "Where is the CO2 going over geologic time?". On geological time scales, the climate appears to be regulated by the "chemical weathering thermostat", where weathering of rocks sequesters carbon dioxide from that atmosphere as carbonates in the lithosphere. There is a very good primer on the carbon cycle by David Archer that does a good job of explaining the behaviour of the carbon cycle on geological timescales and shorter, well worth reading.

    I note that you have been very active today posting a large number of papers for discussion on a wide range of topics. Some constructive criticism: this gives the impression that you are not really interested in the answers, because very few people would be able to hold a worthwhile discussion on so many topics simultaneously. I would advise in future that you stick to a small number of topics at any one time so that you can have an in-depth discussion that science demands.
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  3. I posted articles that I found while discussing climate issues with my friends on Facebook. I don't have time to read this website every day, and posted the sources and questions that seemed like they would be interesting to others.

    I've had one person call me a 'link bomber' before, and he also falsely accused me of not having time to read all of the articles I posted. I read them before, and argued about some of them before, on Facebook

    Chris Shaker
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    Response:

    [DB] Chris, it's not the posting of links and questions that is at issue.  It's the not bothering to respond to the answers you get which is the real issue in play.

  4. 53, cjshaker,

    If I may, this is not your blog. The sort of comments you are posting are the things someone might do on twitter or their own blog. To simply use SkS as a conduit for making your own drive by posts is not commenting, it's spamming.

    You also do ask questions. What is the point of questions if you never look for or pursue the answers? The appearance is that you want to give the illusion of engaging in discussion without actually doing so. That lends to the idea that you are using the popularity of SkS to push your own message without contributing in any way to the dialogue of the site.

    Which, again, is equivalent to spamming.
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  5. So a graph showing the trend of declining CO2 over the past 160 million years is not applicable to 'CO2 has been higher in the past'? And asking why the CO2 declines over time is not a question I should ask?

    It appears that Geologic processes bind the CO2 into rock, if I understand the previous answer correctly.

    Is there some way to subscribe to updates to only the questions that I have posted questions or links on? I do not have time to wade through emails of all updates to all questions on this website, but I am interested in updates to the questions in which I have posted.

    Given attitudes like this, it is very easy for the layman to get the idea that he or she is not to ask questions about climate science. We're just supposed to shut up and let climate modelers tell us how we should live our lives.

    I am a computer scientist, not a climate scientist. It is very easy to get the impression that climate scientists don't want anyone questioning their assumptions, beliefs, code, data, nor science.

    Thank you,
    Chris Shaker
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    Moderator Response:

    [DB] Let us start off on the right foot, then?

    Welcome to Skeptical Science! There is an immense amount of reference material discussed here and it can be a bit difficult at first to find an answer to your questions. That's why we recommend that Newcomers, Start Here and then learn The Big Picture.

    I also recommend watching this video on why CO2 is the biggest climate control knob in Earth's history.

    Further general questions can usually be be answered by first using the Search function in the upper left of every Skeptical Science page to see if there is already a post on it (odds are, there is). If you still have questions, use the Search function located in the upper left of every page here at Skeptical Science and post your question on the most pertinent thread.

    Remember to frame your questions in compliance with the Comments Policy and lastly, to use the Preview function below the comment box to ensure that any html tags you're using work properly.

    "Is there some way to subscribe to updates to only the questions that I have posted questions or links on?"

    My suggestion, if you cannot come back here often, is to bookmark your comment by right-clicking on the time stamp (sorry, not a Mac user) and selecting the "bookmark this link" option. I would also put those bookmarked links into their own folder, such as "SkS Saved Bookmarked Questions" or somesuch.

    "We're just supposed to shut up and let climate modelers tell us how we should live our lives."

    At SkS we try to focus on the science and leave tone and suppositions (and projecting of suppositions) out of the dialogue.

    "It is very easy to get the impression that climate scientists don't want anyone questioning their assumptions, beliefs, code, data, nor science."

    If climate scientists didn't want anyone questioning their work, code (all freely available, mind you), etc, then why would they donate their time to write up blog posts for this website and then spend many hours interacting here and answering questions? Furthermore, every single climate scientist I have ever reached out to and contacted for help has literally bent over backwards in their efforts to be of service. Without exception.

    To get the most out of your Skeptical Science experience, follow the advice given above and by Sphaerica and Albatross below, and thank you for plying the SkS airwaves.

  6. Hi Chris @55,

    "It is very easy to get the impression that climate scientists don't want anyone questioning their assumptions, beliefs, code, data, nor science."

    " it is very easy for the layman to get the idea that he or she is not to ask questions about climate science. We're just supposed to shut up and let climate modelers tell us how we should live our lives."

    I'm sorry that is your impression. It is, however, the incorrect impression. I'm also pretty sure that no matter how open and transparent we scientists are it will not be enough in your opinion. If you want access to the code and data, here is a good place to start, and while you are make queries, perhaps you could ask Spencer and Christy to release their latest version of their code for the satellite data. For the record, no climate modeller that I am aware of is telling us to "shut up" or telling us "how we should live our lives". That is an inaccurate and unfair characterization, it is also off topic. And climate science does not involve a "belief" system, but a synergy of chemistry, physics, mathematics etc., in addition to data. Such rhetoric that you are engaging in is not helpful, not constructive and not conducive to encouraging people to assist you.

    Regardless, you seem to be ignoring the fact that people have been politely answering your questions here, but that you seem to be ignoring their answers. Your question @51 seems to pertain to the carbon cycle and weathering of rocks, and Dikran @52 explained as much, s/he even provided a couple of helpful links. I do not understand how then you managed to arrive at your assertions quoted above.

    All the best,
    Albatross
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  7. 55, cjshaker,

    At the top of the page you will find a link labeled "Comments". If you click it it will take you to a search page of all of the comments posted, in reverse chronological order. Most people will include your name or handle in a direct reply to you. Hence, it is easy enough to use a browser search on that page to find any comments directed at you.

    If you take a while between visits, you may also have to scan the 2nd or 3rd page of comments, but it really only takes a moment. I do it all the time. I also tend to keep track (in my head) of the threads where I have left comments and expect answers.

    I also have to second Albatross' analysis of your most recent comment. It's another drive by shooting, popping off unsubstantiated allegations and showing a gross misunderstanding of how things work ("supposed to shut up and let climate modellers..." -- where does that come from?).
    I am a computer scientist, not a climate scientist.
    This suggests to me that you have a lot to learn about climate, then, and would be far better served reading and learning than randomly posting. It seems that for you communication is a one way street.
    It is very easy to get the impression that climate scientists don't want anyone questioning their assumptions, beliefs, code, data, nor science.
    That's utterly absurd. If you read the comment threads here, instead of simply posting and running, you'll find a lot of healthy, engaged and high level debate. No one is remotely close to demonstrating the behavior you describe (although you'll find that people who do actively work to derail discussions and troll threads tend to complain a lot when they get properly moderated).

    You sound like someone who made up their mind before you got here.
    I've had one person call me a 'link bomber' before...
    Perhaps you should stop to think about this. Look through the comments both here and on other sites, and see how many people have posting behaviors that mirror your own, and how their comments are received.

    This site exists first and foremost to provide information to laymen about climate science. That John Cook provides comment sections for intelligent discussion is his choice, not a requirement imposed on him by society or "Internet law." It is also moderated to keep that discussion intelligent and focused, which is also his choice.

    This site is neither an open forum for anything anyone wants to say, nor run by climate scientists to promote an agenda. It is about science, but a certain decorum, engagement and behavior is expected from everyone to make any interactions both informative and worthwhile.
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  8. 'It is very easy to get the impression that climate scientists don't want anyone questioning their assumptions, beliefs, code, data, nor science'

    And its very easy to get the impression that you dont want discuss the answers to the questions you make. Why not make ONE post of question that interest you, then discuss the answers properly, (only one part of the blog to look at), and then move onto the next question. What possible point could the be to posted questions that you dont listen to answer for?
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  9. cjshaker wrote: "It appears that Geologic processes bind the CO2 into rock, if I understand the previous answer correctly."

    Yes, that is correct, on geological timescales.

    As to your comments on climatologists. Well I am also a computer scientist, and I have worked with climatologists and have always found them to be perfectly happy to discuss the shortcomings of climate models. Gavin Schmidt @ RealClimate is a leading climate modeller and there are several RealClimate posts where he points out areas where there are problems with the models. However, if you go to any scientist with the "attitude" you display in that post, it shouldn't be surprising if you get a hostile response.


    Is there some way to subscribe to updates to only the questions that I have posted questions or links on? I do not have time to wade through emails of all updates to all questions on this website, but I am interested in updates to the questions in which I have posted."

    How about doing what I do, which is to remember the articles on which I am participating in the discussions and visit them occasionally to see if anyone has posted anything interesting? To save time, you could bookmark them on your browser.

    However, if you don't have time to keep track of the discussion, that does suggest that my point regarding posting lots of questions at the same time might have some value.
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  10. 55, cjshaker,
    59, Dikran,

    For the record, I am also a "computer scientist" (well, my degree is in computer science, but my profession is really more that of an engineer -- software developer, not research scientist -- as much as I wish that was the direction I'd taken my life).
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  11. I am also well versed in computer science and physical modelling; physics, math & computer science in college. I eventually got fed up with being laid off though and went back to school.

    Now, I am a professional gardener at a large botanic garden.
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