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Climate change from 40 million years ago shows climate sensitivity to CO2

Posted on 6 November 2010 by John Cook

Around 40 million years ago, sea surface temperatures rose around 5°C in a period called the Middle Eocene Climatic Optimum (MECO). A new study Transient Middle Eocene Atmospheric CO2 and Temperature Variations (Bijl et al 2010) has found atmospheric CO2 was the primary driver of this global warming event. During this period, CO2 levels rose dramatically to 2 to 3 times previous levels. This study gives us further insight into how climate responds to changing CO2 levels and provides evidence for strong climate sensitivity.

The Eocene is a period spanning 56 to 34 million years ago. Over this time, global temperatures showed a long, gradual cooling as the planet moved from a warmer climate to a cooler one (the end of Eocene was marked by Antarctic glaciation). To determine temperature and CO2 changes during the middle Eocene, sedimentary ocean cores were taken from the east coast of Tasmania. The cores contain fossilised microorganisms that provide several independent proxies for temperature. The proxies, or "paleothermometers" as the authors describe them, indicate a warming of the sea surface of around 5°C.

Carbon isotopes in alkenones (produced by algae) serve as a proxy for CO2 levels. Before the MECO warming, CO2 levels were between 600 to 1600 parts per million (note that at these higher CO2 levels, there were no ice sheets in Antarctica). During MECO warming, atmospheric CO2 increased to between 6,400 to 15,000 ppm. The trends in sea surface temperature follow CO2 trends remarkably well.


Figure 1: Sea surface temperatures from two different proxies (red and purple). Atmospheric CO2 levels from algae (light grey band) and phosphate estimates (dark grey band). Yellow shaded area indicates the MECO warming interval.

The advantage of this type of study is it gives us the climate response to changing CO2 levels over long time-scales. This means any temperature changes to rising CO2 will include long-term climate feedbacks like changes in ocean chemistry or changes in vegetation (but in this case, not changes in ice sheets or sea ice as there were no ice sheets during the Middle Eocene). During the Middle Eocene Climatic Optimum, temperatures warmed around 5°C with CO2 increasing by a factor of 2 to 3. This yields a climate sensitivity range of around 2° to 5°C per doubling of atmospheric CO2 (but note that ice feedbacks aren't included). This is broadly consistent with the IPCC climate sensitivity range of 2°C to 4.5°C which is based on a consideration of many estimates of climate sensitivity using instrumental, satellite and proxy data.

One intriguing question remains. Vast amounts of carbon were injected into the atmosphere during the Middle Eocene - where did it all come from? Volcanic outgassing is a possible contributor although it's unlikely it provided all the CO2. It'll be interesting to see what future studies will discover about this distant period.

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Comments 51 to 69 out of 69:

  1. Norman, why do you assume that a thicker ancient Martian atmosphere had the same amount of CO2 as today's Martian atmosphere? Instead, the current CO2 is what's left over after much of the original CO2 degraded in various ways, and crucially different from the case of the Earth, was not replaced by volcanic activity (due to lack of plate tectonics recycling crust). The ability of Mars to have had liquid water depends on several factors, most of which differed from now. The coolness of the Sun in ancient times would have been balanced by other factors.
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  2. Norman, one of the causes for the drastic reduction in CO2 of the Martian atmosphere probably was the stripping of the atmosphere by the solar wind, in the absence of protective Martian magnetic fields.
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  3. Norman: A model can imply a generalization of patterns and perform fairly well, it cannot determine a specific case. I am sure astronomers will see various exceptions to predictions and rules. They may indeed. Still, the fact remains that anyone who chooses to bet on exceptions rather than scientific "predictions and rules" is likely to lose. The smart money tends to be against it, just as it's against the idea that modern climatology will be overthrown, and its adherents revealed as frauds and fools, by a bunch of carping amateurs who can't be bothered to do basic homework on the topics they claim to understand better than the experts.
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  4. Phila #56 "...modern climatology will be overthrown, and its adherents revealed as frauds and fools," This kind of thing doesnt happen. At worst they will say something like, "around the turn of the century, the prevalent theory was..., but since then it has been clearly demonstrated that...". So you have nothing to fear in that sense, but what one does see is a lot of resistance to inquiry, or what might be considered offensive questioning.
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  5. Norman #52: "Mars currently has more CO2 in its atmopshere than Earth does" Yes, but as you note, Mars is further from the Sun. Ergo, it would need alot more CO2 in its atmosphere to have liquid water. Citations of 'fun facts for kids' sites aside, there is more to the temperature on Mars (and any other planet) than distance from the Sun. That is a major factor, but not the only determinant. Again, every examination of the theory that Mars once had liquid water I have seen includes the pre-requisite that Mars once had a thicker atmosphere. You cite Wikipedia for the next bit... take a look at what they have to say about "Atmosphere of Mars". "Instead, astrophysicists come to understand how stars evolve by observing numerous stars at the various points in their life, and by simulating stellar structure with computer models." You seem to have ignored everything before the word "and". The part about "observing numerous stars at various points in their life"? That's exactly what I was referring to. We have observed that stars later in their life cycle are hotter. No model required. Yes, there are ALSO models of the nuclear fusion processes in stars which indicate that they should get hotter over time... but that's just our understanding of nuclear physics confirming what we have observed directly. Your cavalier dismissal of all things modeled doesn't make the observed reality of stellar warming suddenly go away. Rather, it suggests that our understanding of the physics involved in stars may be wrong... but we somehow got the right answer (warming as they age) anyway.
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  6. #58 CBDunkerson, Not dimissing the model. But do you realize how many assumptions you have to make to support your claim? You do understand that the logic to arrive at a conclusion can be valid but the conclusion is still wrong because the initial assumption is not correct. Even if Mars had more CO2 than it has currently (which is an experimental evidence to support it) Carbon Dioxide does not have an unlimited ability to absorb IR and redirect it. It absorbs a percentage of the IR spectrum (mostly at 15 micron range). CO2 at Earth's concentration is already far past the linear part of the logrithmic curve. In the past the Earth is cited as having a CO2 level of around 6000 PPM. Can you calculate how faint our Sun would have to be to keep the temp in the range that is cited? Then take that faint Sun radiation and direct it at Mars to see if Mars could have sustained liquid water at this time. One Historic episode where the majority of scientists were wrong was when locals took them meteorites (somewhere in the 1800's). The scientists used perfect logic and there understanding of Gravity. The locals claimed the rocks came from the sky...the scientists refuted this claim with the reasoning that they could not have come from the sky because they could not get up there (gravity and all), they reasoned the rocks were struck by lightning and that is what the locals saw and why the rocks were warm. Even if the logic was perfectly valid, the assumption supporting it was incorrect.
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    Moderator Response: Regarding "Carbon Dioxide does not have an unlimited ability...," see the Argument "CO2 effect is saturated." If you want to discuss that topic further, please do so over there. (Everyone else please respond to Norman on that thread, not here.)
  7. Norman, Mars does have 70x more CO2 in the atmosphere than Earth does. This is because that atmosphere is 95.3% CO2, but the atmospheric pressure is 0.007 times that of the Earth's. However, CO2 is not the only greenhouse gas. Earth's atmosphere ranges from 0-7% water vapor, while Mars only has 0.03%. The Martian atmosphere also contains no methane. Conditions on Mars: Atmosphere (content, density, sky appearance) NASA - Mars NASA - Earth's Atmosphere Water vapor is the most powerful greenhouse gas (currently argument #25)
    Water vapour is the most dominant greenhouse gas. Water vapour is also the dominant positive feedback in our climate system and amplifies any warming caused by changes in atmospheric CO2. This positive feedback is why climate is so sensitive to CO2 warming.
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  8. RSVP: So you have nothing to fear in that sense, but what one does see is a lot of resistance to inquiry, or what might be considered offensive questioning. Assuming for the sake of argument that one really does see "resistance to inquiry," this may have something to do with the fact that the "skeptical" side relies so heavily on endlessly recycled misconceptions and conspiracy theories (as five minutes on virtually any SkS thread will demonstrate). Perhaps that explains some of the "resistance," along with the fact that extraordinary claims -- like the claim that our understanding of the threat AGW poses is significantly in error -- require extraordinary evidence that "skeptics" never seem to get around to providing. That said, I'd argue that the entire idea of this "resistance to inquiry" is basically false. Apart from SkS, consider the EPA Endangerment Findings, which patiently address nearly every "skeptical" argument in existence, from the plausible to the laughable. "Resistance" to the ignorance and misinformation on display in threads like this one is exactly what we should expect and want to see from scientists. The fact that all these demands for more (and more, and more) "inquiry" tend to come from people who display childlike credulity toward repeatedly debunked anti-AGW arguments makes it even harder to take them seriously. Real inquiry into this subject is happening where it really matters: among experts in the relevant fields.
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  9. Norman, there is graph titled "Atmospheric CO2 concentrations required to compensate for reduced solar luminosity in the past,..." in a slide presentation by Daniel Kirk-Davidoff titled "Paleoclimatology: An Introduction." It is Slide 4, Figure 2. You might find the rest of the slides instructive, too.
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  10. Norman wrote: "Even if Mars had more CO2 than it has currently (which is an experimental evidence to support it)" Well... there is the evidence of possible past surface water. :] Look, you said that the only way that Mars could have had liquid water in the past was if the Sun had been hotter then. I have shown that this is incorrect... there are other things which impact the temperature of planets. DID Mars have a thicker atmosphere and liquid water in the past? That is the prevailing theory, but not something I'm advancing as established fact. I was correcting your 'Sun only' assumption... not stating any conclusion of my own. "Carbon Dioxide does not have an unlimited ability to absorb IR and redirect it. It absorbs a percentage of the IR spectrum (mostly at 15 micron range)." See the presentations the moderator and Tom Dayton linked to. Or consider the planet Venus. Further from the Sun, but more than 100 C hotter than Mercury. Pretty much tosses out the 'solar only' and 'limited CO2 warming' claims in one neat package. "In the past the Earth is cited as having a CO2 level of around 6000 PPM." Which would be about 'five doublings' from the pre-industrial revolution CO2 level. So 5 C warming from the increased CO2 and maybe 10 C to 30 C from feedbacks if we were to assume that climate sensitivity then was similar to now (which we shouldn't). Earliest evidence of liquid water on Earth was back when the Sun's output was about 70% of current. Lining those and various other factors (e.g. orbital differences) up gets more than a little involved, but the graph Tom cited seems like a good rough guide. Also worth mentioning that there is some indication that the Earth MAY have frozen over, or nearly so, in the past. Lookup 'Snowball Earth' for details.
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  11. TIS - the paper is about a transient event at 40my. The plate reconstruction in the paper infers drake strait had not opened and I am not aware (but could be uninformed) of evidence for persistent ice sheet in the antarctica at that time. I dont see really what you points have to do with this paper. However, you claim that development of ice sheets in antarctica would reduce CO2 and cool the ocean (presumably you mean ocean cooling reduces CO2??). I wonder what you think the mechanism for this is? Ice sheets definitely increase albedo but if you think changes to GHGs are an insignificant element in determining climate, then I am interested to you how the ice sheet development cools the global oceans. Can you make the arithmetic add up? Warming oceans can cause higher CO2 levels (amplifying warming - that is what "sensitivity" is about) but you do know that the elevated CO2 levels at present are from fossil fuel and not from out the ocean?
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  12. #50: "It is fully accurate to say that cooling oceans CAUSE lower CO2 levels. Much like warming oceans CAUSE higher CO2 levels." Not so fast. If you expect warming oceans to release CO2, then why do we see increased acidity in today's warming oceans? The oceans are absorbing carbon dioxide (CO2) from the atmosphere and this is causing chemical changes by making them more acidic (that is, decreasing the pH of the oceans). ... Calculations based on measurements of the surface oceans and our knowledge of ocean chemistry indicate that this uptake of CO2 has led to a reduction of the pH of surface seawater of 0.1 units, equivalent to a 30% increase in the concentration of hydrogen ions. -- Raven et al 2005 Similarly, warming temperature and increasing ocean acidity during the Eocene --> oceans absorbing CO2. During the most prominent and best-studied hyperthermal, the PETM, the global temperature increased by more than 5C in less than 10000 years. At about the same time, more than 2000 Gt C as CO2 -- comparable in magnitude to that which would occur over the coming centuries -- entered the atmosphere and ocean. -- Zachos 2008 CO2 entering just the atmosphere could be ocean outgassing. But entering both ocean and atmosphere hardly indicates that the ocean is giving up the gas. A controlling factor is the 'carbonate compensation depth,' which can vary considerably with both ocean temperature and acidity. Tripati et al 2005: ... report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from 42 million years ago, with a permanent deepening 34 million years ago. ... suggest that the greenhouse–icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago.
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  13. #62 Tom Dayton Thanks for linking the slide show. Some of the slides remained blank. Here is a graph for you that shows little relationship between CO2 and temp over the eons. Not much of a match between CO2 and Temp. Faint Young Sun and Paradox. During the faint Sun, the CO2 levels were low.
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    Moderator Response: Regarding your first link, a simple correlation between only CO2 and temperature over the entire history of the Earth of course will not be straightforward, because CO2 was not the only forcing of climate during that entire period, and also acted as a feedback. But recently CO2 has been the dominant forcing; see "There’s no correlation between CO2 and temperature," and for background see "CO2 is not the only driver of climate," where you should be sure to click the Intermediate tab. Regarding your second two links, I don't understand your point.
  14. Also not to forget this one. Sun may have not been dim in the past.
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    Moderator Response: I don't understand your point. The article you linked to says that the notion that the Sun was not dim in the past "remains highly speculative." The article focuses instead on describing several mechanisms that could have made the Sun dim.
  15. On the 'Sun may have not been dim in the past' link... in addition to the moderator's point about the article actually exploring mechanisms for the OPPOSITE argument (that the Sun WAS dim) it should be noted that the article was written in 1987. The whole 'the sun may not have been much dimmer in the past' idea which the article describes as "highly speculative" back then would now, with the benefit of 23 years of additional research, fall into the category of 'implausible fiction'.
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  16. CBDunkerson #63 "Look, you said that the only way that Mars could have had liquid water in the past was if the Sun had been hotter then. I have shown that this is incorrect" I agree (for a change ;) ), please check the following for more plausible explanations... "Hale-Bopp's orbital position was calculated as 7.2 astronomical units (AU) from the Sun, placing it between Jupiter and Saturn and by far the greatest distance from Earth at which a comet had been discovered by amateurs.[10][11] Most comets at this distance are extremely faint, and show no discernible activity, but Hale-Bopp already had an observable coma.[5]" Now, for any of this to have significance in the discussion, you have to assume that the coma is caused by melted water, (which is not necessarily the case as there are other more volitile components), however, comets generally get noticed before they pass within Earth's orbit, and they do contain a lot water. Aside from what has been brought up in all these earlier comparisons iwith Mars, the rotation and tilt of planets might be easier to assume having altered over time and having affect on climate, than the Sun's brightness. On the link about Hale Bopp, it says, "Comet Hale-Bopp's activity and outgassing were not spread uniformly over its nucleus, but instead came from several specific jets. Observations of the material streaming away from these jets[42] allowed astronomers to measure the rotation period of the comet, which was found to be about 11 hours 46 minutes.[43]" Then you data on the rotation and tilt of Mars itself... "For all of its differences, here’s one situation where Mars is very similar to Earth. Mars’ rotation is 24 hours, 39 minutes, and 35 seconds. In other words, Mars only rotates 39 minutes more slowly than Earth." "Astronomers know that the current tilt of Mars’ axis is just a fluke. Unlike Earth, the planet‘s tilt has changed dramatically over long periods of time. In fact, astronomers think that the wobble in the tilt might help explain why vast underground reservoirs of water ice have been found at mid-latitudes, and not just around the planet’s poles. It’s possible that in the distant past, Mars was tilted at a much more extreme angle, and the ice caps were able to grow across the planet. When the tilt was less extreme, the ice remained, and was covered by a layer of dust." If I have to make a point (based on all of the above), it would be that even if the mean temperature of the Earth were supposely 33 degrees colder without CO2 in the atmposphere, there would still likely be portions of the Earth with temperatures high enough to melt water.
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  17. #69 RVSP Using this calcualtor tool: Calculates temp in absence of GHG. Is it possible Mars could have had water with a faint Sun? In the past, if the Sun was 70% as bright as today, Earth would receive 958 watts/m^2. Mars receives 2.5 less radiation than Earth so it would have only received 383 watts/m^2. The lowest the calcualtor goes is 800 watts/m^2 you have to interpolate the rest. Find a higher number on the solar scale and drop it by 417 watts/m^2. Watch how that lowers temp. Then look at the Earth's temp at 800 watts/m^2. Take that temp number and subract from it what you get with the difference of 417 to see what the temp would be at 383 watts/m^2. I was getting -100 F for a global temp (-59 F + -41 F). If you add a nice Greenhouse effect of 70F you are still -30 F. It would be like having a flowing river in Antartica in the Winter.
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  18. Norman, a greenhouse effect greater than 70 F is entirely possible (after all, Earth's is currently about 60 F). Also, the 70% factor on sunlight was about 4.4 billion years ago while the timeframe Mars is believed to have had water is in the 4.1 to 3.5 billion years ago range... when the Sun was closer to 75% of current brightness. We know Earth had liquid water in this timeframe. There is strong evidence Mars did too. Yet sunlight alone clearly couldn't have caused that. Sunlight and more pronounced greenhouse effects (hardly surprising in the early volcanic history of the newly formed planets) could have.
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  19. # 49 .... If you remove all the CO2, the climate will cool as 1/5th of IR absorption by the atmosphere is due to CO2 (that is well measured). That's not just by the absorption spectrum is it? Any account that co2 is 390ppm vs 30000 ppm water? On the topic of paleo-carbon control knobs, the enthusiast scientist Dr Alley has given the fantastic explanation why co2 lagging doesn't in fact disprove causality. The credit card, interest playing the carbon role, with support by debt as temperature. - axiom interest increases debt - observation debt increases lead to interest raised. Thus observation of a lagging event of a causal factor. (that debt is due to banks boosting the economy in a low wage environment is beside the point). Dr Alley has a problem with causality. I wonder if he studied quantum mechanics, that subject causes serious damage to rational minds. That interest rate had to have travelled forward in time like a positron.
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