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Debate over the Early Anthropogenic Hypothesis

Filed under: — gavin @ 5 December 2005

There have been a few mentions of the ‘early anthropocene’ hypothesis recently (cf. the EPICA CO2 results, and Strange Bedfellows). We therefore welcome Bill Ruddiman to RealClimate to present his viewpoint and hopefully stimulate further discussion – gavin.

[Addendum: For a non-technical backgrounder on the ‘early anthropocene’ hypothesis and its significance in the context of anthropogenic climate change, see Bill Ruddiman’s article “How Did Humans First Alter Global Climate?” from the March 2005 issue of “Scientific American” (first two paragraphs available for free; full article must be purchased). -mike]

Guest posting from Bill Ruddiman, University of Virginia

The hypothesis (Ruddiman, 2003) that early agriculture caused large enough emissions of greenhouse gases millennia ago to offset a natural climatic cooling remains controversial. The centerpiece of the hypothesis was a comparison of the increases of CO2 and CH4 values in Vostok ice during the current (Holocene) interglaciation versus the (natural) drops during similar portions of the three previous interglaciations.

Recent attempts to assess the hypothesis have centered on an earlier interglaciation (‘stage 11’, around 400,000 years ago) when low eccentricity created orbital trends similar to those in recent millennia. Two recent papers that presented long-term CO2 and CH4 trends from EPICA ice (Siegenthaler et al. 2005; Spahni et al., 2005) concluded that the ‘early anthropogenic’ hypothesis is invalid, a conclusion previously reached based on deuterium trends in EPICA ice (EPICA Community Members, 2004). All three papers used the same (seemingly reasonable) approach: they aligned the deglacial warming that initiated the stage 11 interglaciation with the one that initiated the Holocene interglaciation and counted forward in estimated ice-core years to compare the durations. They found that 16,000 more years must elapse before the current Holocene interglaciation reaches the length of the fully natural stage 11 interglaciation (Fig. 1a). The obvious implication is that present-day warmth is accounted for by natural causes, and an anthropogenic overprint is not needed.

It certainly makes sense to turn to stage 11 to find the closest natural analog to climate-system behavior during recent millennia, but I have previously pointed out (Ruddiman 2005) that the EPICA strategy produces an odd alignment of insolation trends for the two interglaciations (Fig. 1b). It positions the present-day northern summer insolation minimum against a stage 11 insolation maximum (409,000 years ago). This inverted alignment raises an obvious question: Does it really make sense to use a past insolation maximum as the ‘closest analog’ to the present-day insolation minimum?

A positive answer to this question would imply that the actual ‘details’ of orbital insolation changes through time can be ignored in testing my hypothesis. Yet the reasoning I used was specifically tied to two well-known theories that invoke orbital changes as central to the operation of the climate system: Milutin Milankovitch’s theory that northern summer insolation plays a prominent role in driving ice volume, and John Kutzbach’s theory that northern summer insolation forces north-tropical monsoons. The inverted insolation alignment arrived at by the EPICA approach (Fig. 1b) ignores the climate-system physics embedded in these theories, as well as the central role they played in my hypothesis. As such, EPICA’s inverted insolation alignment is not a valid test of my hypothesis.

A valid test requires finding the closest stage 11 analog to recent millennia. Years ago, Andre Berger and colleagues identified this interval — the decrease in northern summer insolation to a minimum value not long after 400,000 years ago (Fig. 1d). During this interval in both EPICA and Vostok ice, methane and carbon dioxide concentrations were dropping toward the values predicted by the early anthropogenic hypothesis. Also, deuterium concentrations (Antarctic temperature) were plunging toward typical glacial values. These results from the closest stage 11 insolation analog to the present day show that peak-interglacial warmth had ended (Fig. 1c), as the hypothesis proposed.

All time scales obviously have some amount of error as far back in time as interglacial stage 11, but the major difference between the two views shown in Figure 1 is actually not about time scales. The heart of the debate is about how to align the two interglaciations. Does it make more sense to count forward in elapsed time from terminations (and thereby align the current insolation minimum against a stage 11 maximum) or to align the present-day insolation minimum with its closest analog in stage 11? The first approach refutes the early anthropogenic hypothesis; the second confirms it.


EPICA Community Members (2004). Eight glacial cycles from an Antarctic ice core. Nature 429, 623-628.
Ruddiman, W.F. (2003). The anthropogenic greenhouse era began thousands of years ago. Climatic Change 61, 261-293.
Ruddiman, W.F. (2005). Cold climate during the closest stage 11 analog to recent millennia. Quaternary Science Reviews 24, 1111-1121.

45 Responses to “Debate over the Early Anthropogenic Hypothesis”

  1. 1

    Prior to reading this guest posting I had just become aware of the following paper. It is in French, and I would be interested to know if an english translation could be made available. I have posted the English abstract below and it seems to contradict the arguments above. Personally I prefer the idea that man has acted as a negative feedback on natural cooling, but I suspect Berger is correct in assigninging water vapour a key role in climate change.

    Cheers, Alastair.

    Andre Berger, “Les causes astronomiques des grandes variations du climat au Quaternaire” Comptes Rendus Palevol, In Press, Corrected Proof, Available online 28 November 2005 doi:10.1016/j.crpv.2005.09.009

    Astronomical origin of the large climatic variations during the Quaternary. The astronomical theory of palaeoclimates aims at explaining the recurrence of glacial-interglacial cycles during the Quaternary. It is based upon the long-term variations of the latitudinal and seasonal distributions of the energy that the Earth receives from the Sun. The response of the climate system to this forcing has been simulated by means of the Louvain-la-Neuve (Belgium) model of intermediate complexity. This model reproduces the main climatic features of the last 3 Myr: the entrance into glaciation 2.75 Myr ago; the obliquity cycles from Late Pliocene to Early Pleistocene; the emergence of the 100-kyr cycle 900 kyr ago, the glacial-interglacial cycles of the last 600 kyr, and the climatic variations of the last 200 kyr. In addition to the role played by insolation and greenhouse gases, the model stresses the importance of the feedbacks related to the water vapour, the planetary albedo, the vegetation, the inlandsis and the isostatic rise. Moreover, the analysis of the Quaternary climate has shown that Marine Isotopic Stage 11, 400 000 years ago, was the best analogue of our Holocene. Simulations and sensitivity analyses show that our interglacial will last much longer than most of the others, even without taking into account the impact of human activities on the climate.

  2. 2

    Dear Bill Ruddiman
    I think that’s an interesting and fair comment, certainly the debate is not over with the new EPICA publications. I have two remarks/questions.
    1) Methane has a very strong precessional component in the past. What happens when you aline the nearest 20°N (strongly precessionally influenced) June insolation equivalents from the Holocene and from stage 11?
    2) When you say ending of the Interglacial you refer to the Vostok or EPICA isotope record. However Antarctic temperatures might be affected by the obliquity dominated southern high latitude insolation. What happens when you repeat the exercice relative to the 65°S insolation?
    Last, I dont know if you ever commented on the following problem. Your original numbers of anthropogenic CO2 emissions necessary to explain the “Holocene anomaly” underestimated largely the oceanic CO2 sink, a fact Fortunat Joos mentioned some time ago. Your numbers would also imply a huge 13C isotope signal in atmospheric CO2 which is unobserved even when considering all the uncertainties of this particular isotopic measurement. Have you somehow reconciled the necessary CO2 input over the last 10.000 years with our knowledge of ocean carbon chemistry?
    Let me just finish saying even though I am not convinced by your hypothesis I really like to discuss it with my colleagues and I already ordered your book on the early anthropogenic impact.
    All the best Georg

  3. 3
    Jeffrey Davis says:

    When there’s a technical comment such as this, I wish there would always be a paragraph or two which explained the science and its import for dummies like me. What is the issue here? Is this a curiosity or is it a Big Idea. Are there implications for this beyond academia?

    [Response: Your point is well taken. Please see the “addendum” we have added to the beginning of the post. – mike]

  4. 4

    # 3
    Dear Jeffrey
    yes, the entire issue is important and the links above give you some more informations. Basically if Ruddiman is right (i.e. mankind is modifying greenhouse gas concentrations in the atmosphere since about 8000 years significantly and is changing the “natural” temperature evolution, that is a cooling, over the last couple of thousand years) then we could use this period with its hypothezised human CO2 (40ppm) and CH4 input and its theoretical warming effect (1°C) as an analog for future climate changes.

  5. 5
    Gavin says:

    I think there are two additional points that could be made. Firstly the alignment of the extremely well dated insolation curves and the necessarily imprecisely dated ice cores will always be a problem, absent some independently dated tie point. I therefore think that Bill’s point is most fundamentally a criticism of the EPICA timescale than a complete test of the idea. Secondly, I don’t think that the length of the Stage 11 is that controversial any more. i.e. it clearly lasted longer than the more recent interglacials. If that is correct, looking at the amount of GHG variability over that period gives us a good null hypothesis for how much GHG variability might be expected in the Holocene without any anthropogenic influence. Additionally, if the length is on the order of 30,000 years, it implies that there must have been a downturn in NH summer insolation (which occurs every 20,000 years or so) in which a glaciation was not triggered. One test of the ‘Early Anthropocene’ hypothesis is then whether the that non-ice-age-triggering insolation change was closer to the Holocene changes than the ice-age-triggering insolation changes in the subsequent glaciations were.

  6. 6
  7. 7
    Douglas Chuhran says:

    What do you guys make of this paper: When I, a diehard skeptic of man-made global warming having any impact on climate, and a climatologic layman, read it I can only surmise that earth’s climate has forever undergone dramatic and often rapid changes in climate. That it has occurred so often with such magnitude to cause glacial-interglacial-glacial cycles, and within those, rapid decades or shorter temperature/weather/climate changes WITHOUT any discernable input from man or industrialization can only mean our impact now must be negligible. If back then with no man, how and why now with man? Take man out of the equation and we’ll still have glacial-interglacial-glacial cycles of wildly varying natures. So why can’t that logic, as elementary as it is, put all this greenhouse reduction discussion to bed forever? We couldn’t change our climate if we tried….BTW, in that paper, I just had to eliminate the references to “perturbing the system” so I could quit laughing long enough to finish reading it….

    [Response: Your logic is flawed. If waves can rock a row-boat all on their own, does it mean that if I stand up and rock it voilently from side to side I won’t capsize? Obviously not (as I know from bitter experience), so therefore the existence of natural climate variability does not preclude human influences as well. The more interesting question is whether our ideas of how climate changes match up with both the natural changes that occured in the past, and the changes that are occuring now. It turns out that we can’t explain what’s going on now without invoking human influences – predominantly greenhouse gases and aerosols. – gavin]

  8. 8

    Douglas Chuhran’s comment #7 is typical of the illogical response of a climate sceptic. He conceeds that the climate is highly unstable, yet he denies that man can have any effect. Surely it must be obvious that if the climate can flick from an interglacial state, such as that of today, into a glacial state without warning, then if we increase the controlling greenhouse gas by 30%, which we have done, then we are playing with fire!

    The point of this thread is that the global climate has been stable for the last 10,000 years. If climate is so unstable, in the way Douglas Chuhran describes, then why has the Holocene remained so stable? Bill Ruddiman’s answer is that it is due to anthropogenic global warming! Does that not explain why Bill’s thesis is an interesting question?

  9. 9
    Blair Dowden says:

    Re #7: The paper shows that rapid climate transitions “WITHOUT any discernable input from man” have occured frequently in the past. That has no bearing on whether or not human actions can also influence climate. Ruddiman’s hypothesis is that we have been affecting climate for the past 8,000 years, and may have delayed the conditions that lead to a rapid climate transition. The much larger human impact due to industrialization will have larger effects. The real consequence of natural climate instability is our actions could trigger unexpectedly large effects. This is not cause for comfort. Remember how your paper ends: ‘Climate is an ill-tempered beast, and we are poking it with sticks’.

  10. 10
    Pascal says:

    The Ruddiman hypothesis is based on the fact that:

    1-Earth should be in a cooling phase for astronomical reasons
    2-antropogenic GHG avoided this cooling several ky ago.

    For the first point I’m very surprised to note that there is such an uncertainty (or ignorance?) concerning orbital Earth parameters.

    very surprising indeed.

  11. 11
    Tom Fiddaman says:

    Re 10

    The orbital parameters themselves aren’t uncertain; you can get the resulting insolation from NCDC.

  12. 12
    Douglas Chuhran says:

    It must be that agreement about the stability of the Holocene period being so stable is lacking, assuming authors Adams, Maslin and Thomas know anything about what they write. They write,”Less intensive, but significant rapid climate changes also occurred during the present (Holocene) interglacial, with cold and dry phases occurring on a 1500-year cycle, and with climate transitions on a decade-to-century timescale. In the past few centuries, smaller transitions (such as the ending of the Little Ice Age at about 1650 AD) probably occurred over only a few decades at most. All the evidence indicates that most long-term climate change occurs in sudden jumps rather than incremental changes.”

    As far as my logical being flawed, I disagree. We all know naturally formed waves can rock a row boat to the point it can capsize. So if you happen to be standing up and rocking it when the waves hit and you still capsize, should mankind spend a couple quad-billion dollars, ruin economies and lives, waste peoples’ tax dollars that could be spent on more results-oriented programs and policies, make laws that no man shall stand up in a row boat, and on fancy contraptions that prevent any man from doing so? Should who strap every person on earth who ever might sit in a row boat down so they can’t get up (or out)? And when all that money is spent and misallocated so no one can ever stand up in a row boat, and many people drown because they can’t stand up or get out after those nature-formed waves capsize their boats will you still demand that it’s just those damn, ignorant, illogical, selfish, hoi polloi, who just don’t get how damn smart you are to demand we all just sit down,listen to you and do what you say? The climate “waves” come as they will. They have before, they will again. If man didn’t cause any single one of equal or greater magnitude than you see now, it is not possible for you to attribute what’s happening now to man. Not possible!

    [Response: Possible. By examining the physics of previous climate changes – due to Milankovitch forcing, freshwater pulses, volcanoes, solar, ocean variability, etc. – and then seeing whether those processes can account for present changes, they have been eliminated as important factors now. So unless you have a brand new mechanism of climate change, the ones that remain (GHGs) are probably responsible for the completely consistent climate changes seen over the last few decades. And at the risk of over-extending the analogy, all I am suggesting is that it might be wiser to sit down and stop rocking the boat. -gavin]

  13. 13
    Pascal says:

    Thank’s Tom for your link

    Orbital parameters aren’t uncertain.
    I’m sure of that.
    So, in accordance with these parameters and as Ruddiman says (it’s what I understand) might we have a cooling several thousand years ago, maybe towards an ice-age?
    In RC article we can read:

    “Recent attempts to assess the hypothesis have centered on an earlier interglaciation (‘stage 11’, around 400,000 years ago) when low eccentricity created orbital trends similar to those in recent millennia. ”


    “Does it really make sense to use a past insolation maximum as the ‘closest analog’ to the present-day insolation minimum?”

    So are we to-day in a insolation minimum?

    [Response: Yes. This refers to the fact that the nearest approach to the sun now occurs in northern hemisphere wintertime (early January), and so summer insolation in the north is at a minimum, compared to say, 10,000 years ago when the closest approach was in July. The summer insolation in the North is thought to be important for allowing snow/ice to persist during the summer (and therefore grow an ice sheet) – so when it’s low, there is more likelihood of a glaciation. – gavin]

  14. 14
    Douglas Chuhran says:

    Gavin- that you had to use the word “probably” to describe the process-of-elimination role you ascribe to GHGs tells the whole tale. Causation has not been established. Even Ruddiman, [ad hom deleted] can’t establish causation between man, GHGs and climate change. It’s all about effect not affect. He states that each of that last four deglaciations have been marked by a rapid CO2 rise to a maximum timed just ahead of ice volume minimum. What’s happening now? The same damned things. C02 rapidly rising and ice volumes decreasing. So please explain what man did during those past four deglaciations to cause CO2 to rise and ice volumes to decrease. Please explain to me how 10-20 million people on earth during Ruddiman’s anthropogenic time period caused anything that’s ever happened climatologically. Look beyond the existence of man on this earth and tell me you don’t see climate changes that exceeded what’s ever happened while man has inhabited the earth.

    Please tell me what would you and your ilk would be saying if we lived around 11,000 years ago and experienced what Ruddiman shows in his Figure 1 where CH4 rose about the same 250 ppb we’re so worried about now. I don’t know what ‘YD’ means, maybe “yucky data.”

    This is the last I’ll post here because I must just be too dense to “get it.” But you guys are going to have to come up with a whole lot better causal data before I’ll stop shouting from the rooftops that this “global warming-stop burning fossil fuels because it’s ruining the planet” tripe is just that. You don’t know what you’re talking about just as much as I don’t know what you’re talking about. If you’re successful, the next thing I’ll have to fight is your planned forced relocation and dispersal of humanity because our concentrated locations are too weighty distorting tectonic plate movements which threatens our very existence.

    [Response: Sigh… absolute proof only exists in mathematics, not the real world. YD = Younger Dryas, a cold period during the last deglaciation. While CH4 increased by about 250 ppb at the end of the deglaciation (as the globe warmed by roughly 5 deg C) – anthropogenic increases are around 1100 ppb – 4 times as large. You are correct about one thing – you don’t know what you are talking about. In such circumstances, I would counsel you to stop jumping to conclusions about what you think the ‘agenda’ is, and simply look at how science works and why scientists have come to the conclusions they have. Start with the IPCC summary for policy makers. -gavin]

  15. 15
    Andrew Dodds says:

    Douglas –

    Funny you should mention that (distorting tectonic movements):,3605,1655977,00.html

    But speaking more seriously – if the climate has changed rapidly and to a large degree in the past, as a result of fairly minor natural pertubations, then why is it so unreasonable that it can change as a result of man made pertubations of a greater size? If the historical climate was extremely stable – that is, it never changed over millions of years, THEN you could conclude that man made changes were unlikely. For instance, if this were the mid cretaceous period with no icecaps, warm seas and temperate poles, the possibility of man changing the climate would be remote indeed. However, we have the misfortune to be living at a time when the climate is highly sensitive to small pertubations, and we can tell this from the dramatic shifts seen in the ice core data.

    Ruddiman’s hypothesis – and it is no more than that at present – is that large scale land clearence by burning combined with the development of livestock caused small changes in Carbon dioxide and methane levels, just about enough to stop a very slow cooling.

  16. 16
    Douglas Chuhran says:

    I can’t help myself…I’ve tried sitting on my fingers but can’t. Please forgive me. You have that exactly backwards. If earth’s climate had been stable for millions of years and then low and behold man appears and begins to exert his influence, greatly perturbing the system,and climate changes for the first time ever I would say you have a pretty good case for a causal relationship. But it’s the exact opposite. Earth has had climate uninhabitably too hot and too cool long before man. Variations inbetween therefore cannot automatically be attributable to man. I think you’re hoping against hope it can.

    What if the measurements you rely on to back-fill this desire to link man to climate change, accurate as they might be, are just part of a cycle or cycles you can’t see/measure? Ruddiman’s hypothesis, or one very much like it has to be correct for there to be any chance of linking man to climate change.

    You can’t equivocate with words like “probably”, “likely”, “possibly”, etc. and establish cause (man) and effect (climate change) and then insist on such a dramatic change in human behavior. When you boil it all down the assertion is “we can’t afford to do nothing and then find out we were right.” I say we can’t afford to do what you insist and find out you were wrong. Especially after all the attempts you’ve made to make this causal link. As far as me admitting I don’t know what I’m talking about, you might be well-served to view me as the skeptic you should try to convince. Granted, you’re much closer to your goals with so many more who feel rather than think.

    I will read anything you think should convince me otherwise. But what I’ve read so far is totally lacking. Pretend I’m Denzel Washington’s character in the movie “Philadelphia” and explain it to me like I’m two….

  17. 17
    Don Baccus says:

    Re: #16: “If earth’s climate had been stable for millions of years and then low and behold man appears and begins to exert his influence, greatly perturbing the system,and climate changes for the first time ever I would say you have a pretty good case for a causal relationship. But it’s the exact opposite. Earth has had climate uninhabitably too hot and too cool long before man.’

    This is like saying “since people died of heart disease and cancer before tobacco was first smoked, smoking can’t cause heart disease or cancer”.

    Sorry. Doesn’t follow.

  18. 18
    Stephen Berg says:

    Re: #12, “If man didn’t cause any single one of equal or greater magnitude than you see now, it is not possible for you to attribute what’s happening now to man. Not possible!”

    Possible, for another reason than Gavin’s, as well.

    How many humans existed at any one period back thousands of years ago? Maybe a few thousand? A million at most?

    Certainly not the 6.3 billion that exist today. Therefore, the impact the early humans had on the climate system would barely register on a graph which displays the impact of humans today.

    This is evident in the graphs on page 286 of:

    Also, Douglas, you employ one of the primary logical fallacies in your argument. The one that says if A causes B and B causes C, then A causes C. However, you use the opposite, that if A did not cause B and B did not cause C, then A cannot cause C. It is completely irrational.

  19. 19
    Steve Bloom says:

    Re #17 (DB): And of course this argument about smoking was actually used by the tobacco industry some years back. Even today, they continue to use a variation on it with regard to second-hand smoke.

    Re #16 (DC): “What if the measurements you rely on to back-fill this desire to link man to climate change, accurate as they might be, are just part of a cycle or cycles you can’t see/measure? Ruddiman’s hypothesis, or one very much like it has to be correct for there to be any chance of linking man to climate change.”

    As to the first statement, this question got asked a long time ago. Scientists then went out and took measurements to establish what all the possible causes are. There is still some debate as to the exact role of the various factors, in particular the precise effect on climate of GHGs versus solar versus aerosols, but this is essentially an argument over the details. Just out of curiosity, how can a cycle that affects climate be something that cannot be seen or measured, to say nothing of missed by thousands of scientists. (BTW, if it’s completely unkown, it’s a little odd to describe it as a cycle.) You seem uninterested in actually reading the TAR, but I would suggest you read the portion relating to the basic physics of GHG-induced warming and whatever references are appropriate to get a basic understanding of how increases in GHGs will warm the atmosphere. If you can find a flaw in the physics, I suspect there may be Nobel prize in it for you.

    As to the second one, I doubt that Ruddiman thinks this is true.

  20. 20
    Hank Roberts says:

    For Dr. Ruddiman — Do you have footnotes/references on the web somewhere to supplement your Sci. American article, that supplement those available in your 2003 _Climate_Change_V61 article? (That is, what’s new since 2003?)

    And a long question about whether you see the human change as a slow steady one or one that’s episodic for reasons that may go beyond plague events. I’ll outline the bits that made me wonder:

    I’ve just finished “Thin Ice” (on Lonnie and Ellen Thompson’s work with mostly high elevation tropical ice cores records).
    I’m wondering if the “dust events” that show up figure in your thesis.

    Seems to me that failures in agriculture leading to erosion of topsoil may produce more methane — that there’s more carbon tied up in the living part of the soil than in the plants in pre-agricultural soils, and each year’s plowing removes some from the top few inches when it’s disturbed, with a net loss. One such comparison here:

    One reference to the dust events — discussed more by anthropologists than climate experts, til recently (headline writer confused “BC” and “AD” but article is decent enough) — is here:

    (It begins: “Some 4,000 years ago, a number of mighty Bronze Age cultures crumbled. Were they done in by political strife and societal unrest? Or by a change in the climate?”)

    I’ve seen suggestions — that ought to be testable by checking the contents of the dust layers in ice cores, though I haven’t happened on such results — that the climate impacts may have been due to dust from meteors that came in swarms but were small enough not to leave impact craters, for example here:

    I’d imagine this all can be correlated with your time series of plague events — though I doubt plague leaves a layer of anything identifiable in any sediments (rat DNA? I think I’m joking …).

    But basically I’m asking what other events tie in with your suggestion and whether you think it might be failures or interruptions of human land clearing and agriculture, rather than the steady development of it, that could be associated with the methane. Whether you see a steady growth or episodic changes associated with the ice core data.

    I know we’re still coming across new comets;
    “an unexpected burst of meteors on October 5, 2005 has occurred, which betrayed the presence of a thusfar unknown, potentially Earth-threatening, comet.” —

    A grab-bag pick from Google on the same lines (probably outdated, not sure if it has a publication date) is here:
    “Passing through a Giant Molecular Cloud – Snowball Glaciations produced by interstellar dust.” (Alexander A. Pavlov, Owen B. Toon, Anatoli K. Pavlov, John Bally, David Pollard) Nature submitted, 2004.

    At any rate, I’m curious whether you find the notion of dust from a meteor storm as a climate shock believable and whether it fits your hypothesis as a cause of, for example, plague and failure of agriculture — or whether you see no need for anything besides a slow steady change starting with agriculture 8000 years ago.

    Pardon the ramble here, I’m just an interested reader, picking at odd bits and wondering what fits with what.

  21. 21
    Joel Shore says:

    In response to #16 (“Pretend I’m Denzel Washington’s character in the movie “Philadelphia” and explain it to me like I’m two….”):

    (1) Humans are radically altering the composition of the atmosphere by their emissions of CO2 and other greenhouse gases. CO2 levels are at their highest in over 650,000 years and the increase in CO2 can be definitively attributed primarily to our fossil fuel emissions.

    (2) These gases have effects understandable from the most basic physics of causing additional absorption of energy that the earth would otherwise radiate back into space, thus warming the earth’s climate.

    (3) In order for significant warming not to occur, one would have to posit a mechanism by which the complicated “feedbacks” in the climate system miraculously counteract this warming. However, such a mechanism cannot stabilize the atmosphere too much because this then makes it very difficult to explain the known rapid oscillations in climate that have occurred naturally in the past in response to rather modest “forcings”. Furthermore, no such plausible mechanisms have thus far been proposed. In fact, there are various mechanisms that lead to “positive feedbacks” that magnify the effects of the greenhouse gases.

    That is the story in a nutshell. Of course, this is really just one line of evidence in a much larger body of evidence. For example, I have not discussed the whole field of “detection and attribution” of the current climate change nor how the magnitude of the earth’s response to past forcings can be estimated from glacial-interglacial cycles.

    Look, the reason that we are having this argument has nothing to do with science and everything to do with the fact that you just don’t like the political or economic implications of the science. If you didn’t like the political implications of quantum field theory, you would be challenging it too. However, if you are like most people, you are willing to accept scientific authority to a large extent as long as it doesn’t conflict with strongly-held religious or political beliefs or economic self-interest. If it does, you adopt a new higher standard of having to have the scientists convince you that they are right even without any real attempt to obtain the background to understand their evidence and arguments.

  22. 22
    Andrew Dodds says:

    Re: 16. You have completely misconstrued the argument here – the point is that changing a climate that was demonstrated to be stable would be hard; changing a climate that was demonstrated to be unstable (or highly sensitive to small changes in inputs) is easy. We are just unlucky to be living at a time when climate change is easy to achieve.

    However, others have already noticed that. What I have real issue with is your continuing assertion that I advocate drastic changes in human behaviour; this is simply untrue.

  23. 23
    Jeffrey Davis says:

    It sounds like “Hooray for GHGs!” or at least “Hooray for GHGs! (oops)”

  24. 24
    Dennis Sweitzer says:

    All good things in moderation… If it is good, beneficial & tasty to bake cookies at 375F, is it even better to bake them at 575F?

    So a big (provisional) hooray for modest GHGs.

    Of course, debating the past effect of anthropogenic emissions is academic; by long experience, we know that a beneficial climate is associated with pre-industrial GHGs (whatever their source); The $trillion question now is what is the effect of increasing emissions well beyond pre-industrial levels. One argument by skeptics seems to boil down to “if a little bit is good, a lot is better or benign”.

    If it ain’t broke, don’t break it!

  25. 25
    Doug Percival says:

    Dennis Sweitzer commented: “The $trillion question now is what is the effect of increasing emissions well beyond pre-industrial levels.”

    From Radio New Zealand and BBC reports today: “The scientist who first raised the alarm about global warming says the planet will be irrevocably damaged within a decade if greenhouse gases continue to be produced at current levels … Dr James Hansen told a meeting of American Geophysical Union in San Francisco that just 1C more of warming would take the Earth into climate patterns it has not experienced for more than 500,000 years.”

    As to “greenhouse gases continuing to be produced at current levels”, the BBC reported a month ago that a mainstream projection of greenhouse gas emissions, the annual World Energy Outlook report from the International Energy Agency (IEA), projects that global greenhouse gas emissions will rise by 52% by 2030.

    Draw your own conclusions about what the future holds.

  26. 26

    Ruddiman reads and reasons spot on, I just don’t understand why Epica C.M. chose a disparate insolation comparison? Is there more logic than it seems?

  27. 27
    Jeffrey Davis says:

    Re: 25

    Getting change is a tough sell. 1 degree C? We know 1 degree C. It’s the difference between 8AM and 9AM. We have no way of knowing what 1 extra degree C, applied consistently, to the planet will do, and we do know what a recession looks and feels like. And since the brunt of the change will be felt at the extremes? That makes it harder.

    Dr Hansen may be right. One of the consequences may be drought and famine on a Biblical scale. But at present, we pay farmers NOT to grow stuff. So, possible future indefinable consequences over the difference between 8AM and 9AM? With lots of us still freezing every winter anyway? That’s not going to be easy.

  28. 28
    Chris Reed says:

    #1 Alastair, I’d understood that Water Vapour is an amplifying factor in climate change (possibly of all sorts?). But that it’s atmospheric residence time is too short for it to maintain an effect. i.e a trend in WV alone could be stopped by events such as Mt Pinatubo (generally plinean eruptions), thus preventing it from sustaining long term changes without an underlying forcing change.

    #2 Georg, You imply that a signature in Carbon isotopes should be apparent. However the non-industrial anthropogenic effects would surely be down to a combination of land use change and burning of recently fixed carbon in forest clearance. As pointed out in RC on CO2 increase attribution the isotope difference presently seen is the result of the isotope signature of fossil fuels. Would the early changes not represent a shift in the carbon budgets of bio/atmosphere that would not necessarily be seen in isotope ratios?

    Personally I second Wayne Davidson (#26). Without the EPICA team’s comments it looks (to a lay-reader like me) like their phasing of insolation that they’ve chosen is a construction that does not fit with what I’ve read about the role of Milankovitch cycles in glacial/interglacials. Even if it does otherwise suit a refutation of Ruddiman’s hypothesis. I am really interested to read their response to his arguments here as I’m sure this is a misinterpretation on my part.

    Thanks to RC and to Bill Ruddiman for an interesting post.

  29. 29

    BTW for those who do not know I think it is fair to call Berger the world expert on Milankovitch cycles, and he is mentioned in Bill Ruddiman’s viewpoint.

    Re 28 I have translated Berger’s paper with the help of AltaVista’s Babel Fish. He only mentions water vapour to state that it is a greenhouse gas included in his EMIC type LLN model, so discussion of it is not really relevant here. For my views on the effects of water vapour see my next post to the “Busy Week for Water Vapor” thread.

    Berger argues that although in the 1970’s it was believed that interglacials only lasted 10,000 years, and so we then thought that another glacial was imminent, we now know that 400,000 years ago, an interglacial lasted much longer, and he believes that this may be true for this interglacial too. He argues that the effects of eccentricity amplify the effects of precession, and since we are entering a period of low eccentricity then we will not be pushed back into a glacial by the precession cycle alone.

    He also writes “Only the scenarios where the future CO2 concentration is lower than 220 ppmv [ 21 ] lead to an early entry in glaciation”
    [21] M.-F. Loutre and A. Berger, Future climatic changes: are we entering an exceptionally long interglacials?,, Clim. Change 46 (2000) (1-2), pp. 61-90.

    However, he appears to be prescribing the levels of CO2 based on those measured in the Vostock core for 400,000 years ago. If CO2 is primarily controlled by temperature, then it may be that prehistoric man’s intervention in keeping the CO2 levels high by burning the forests prevented the re-entry into glacial conditions. Moreover, the temperature that controls CO2 levels is likely to be the sea surface temperature in the warmest, largest ocean – the South Pacific. Milankovitch and Berger calculated the solar flux at 65 N. For CO2 levels driven by SSTs then we may need to use the flux at 20 S.

  30. 30
    Mike Atkinson says:

    I have read that Australia had far more vegitation in previous interglacials. Is this not another possible cause of the raised CO2 in preindustrial times?

  31. 31

    #29, “Moreover, the temperature that controls CO2 levels is likely to be the sea surface temperature in the warmest, largest ocean – the South Pacific. Milankovitch and Berger calculated the solar flux at 65 N. For CO2 levels driven by SSTs then we may need to use the flux at 20 S. ”

    The same sea with El-Nino cycles, quite compelling argument. Moreover present day eccentricity resonates well with AGW causationâs, given that obliquity has been tilting away from the sun for quite some time and that summer solstice is at the furthermost distance away from the sun for the year. Despite this, we still are experiencing the warmest year in history for the Northern Hemisphere.

  32. 32
    JimR says:

    #29 Alastair- Berger wrote: “Only the scenarios where the future CO2 concentration is lower than 220 ppmv [ 21 ] lead to an early entry in glaciation”
    [21] M.-F. Loutre and A. Berger, Future climatic changes: are we entering an exceptionally long interglacials?,, Clim. Change 46 (2000) (1-2), pp. 61-90

    Note the figure 220 ppmv is lower than any during the past several interglacial periods. This would indicate that natural conditions have been above this threshold and there is no need to credit early man for preventing a the re-entry into glacial conditions. Berger is saying that with current orbital conditions the only scenario for entry into glacial conditions would be unnaturally low (below 220 ppmv) CO2 levels… quite the opposite of saying that unnaturally high (manmade) CO2 levels have prevented the onset of glacial conditions.

  33. 33

    #32 “This would indicate that natural conditions have been above this threshold and there is no need to credit early man for preventing a the re-entry into glacial conditions”

    Don’t think so, current orbital conditions need to be matched with equal insolation comparisons. Doing so would mean that the current interglacial should have been over by now.

  34. 34
    JimR says:

    #33 Berger and Loutre do analyze current orbital conditions and compare insolation and came to the conclusion that with the insolation of the current Holocene CO2 below 220 ppmv would be required for glacial conditions.

    Eccentricity (the roundness of the Earth’s orbit) is currently very low (round) and in 25,000 years will be almost perfectly round. This roundness lowers the amplitude of the the other orbital influences on insolation and is the primary reason Berger and Loutre say our current Holocene interglacial will naturally last another 40,000 to 50,000 years. Similar conditions existed during the long MIS11 interglacial.

    See Berger and Loutre: An exceptionally long interglacial ahead?

  35. 35

    Re #34 Doesn’t the idea that no eccentricity will have no effect imply that the natural condition is interglacial. However, most of the last 400,000 years were spent in glacial times. What I am saying is that we may need an eccentricity peak to get us out of glacial, but lack of such a peak may not prevent us falling back into one.

    One could argue that as insolation decreases at 65 N, then the Artic sea-ice spreads out into the N. Atlantic and intensifies the THC. This would lead to a larger CO2 sink, which would reduce atmospheric levels of CO2. It is only because man has artifically kept CO2 levels above 220 ppm that has prevented the next ice age.

    Berger and Loutre seem to think only that CO2 drives temperature. They do not seem to be aware that the opposite could also be true, CO2 could also be driven by temperature, or at least the indirect effects of changes in solar flux.

  36. 36
    Pat Neuman says:

    In comparing trends in ENSO and global CO2 growth (links below), increases in CO2 occur at the same time as big El Nino years. It seems that increases in ocean temperature and CO2 outgassing go hand in hand.

    Multivariate ENSO Index (MEI)

    Global Average Growth Rate based on CO2 measurements

    From 2001 IPCC Chapter 3 Carbon Cycle and Atmospheric CO2:
    “The total amount of carbon in the ocean is about 50 times greater than the amount in the atmosphere, and is exchanged with the atmosphere on a time-scale of several hundred years.

    Dissolution in the oceans provides a large sink for anthropogenic CO2, due in part to its high solubility, but above all because of its dissociation into ions and interactions with sea water constituents”

    “CO2 solubility is temperature dependent, hence air-sea heat transfer contributes to seasonal and regional patterns of air-sea
    CO2 transfer (Watson et al., 1995). Net cooling of surface waters tends to drive CO2 uptake; net warming drives outgassing.”

  37. 37
    JimR says:

    #35 Alastair I believe the point made by Berger and Loutre is that low eccentricity diminishes the effects of the other orbital forcings meaning less change in insolation.

    For example with a very round orbit (low eccentricity) the actual changes in insolation from both obliquity and precession are greatly diminished. That is why Berger and Loutre feel that we are naturally in a long interglacial, we currently have low eccentricity and it is slowly becoming lower (rounder orbit). Similar conditions existed during the long MIS 11 interglacial.

    I would disagree that Berger and Loutre think that only CO2 drives temperature. Their paper seems to indicate that it is changes in insolation that are the drivers of glacial/interglacial conditions with some feedback from CO2. That is why they say that only for CO2 below 220 ppmv would we expect the onset of glacial conditions. Since CO2 @ 220 ppmv is not natural for an interglacial they are saying that we would quite naturally be in for a long interglacial.

  38. 38

    #37 Eccentricity may becoming lower, but obliquity is slowly tilting away from the sun for almost 2000 years. May be Berger is inferring to a time when obliquity and eccentricity are synergistic, tilting closer and being lower (giving a high insolation boost). Dr Ruddiman’s point is still is very strong, identical insolation cycles should be compared, as with the right hand graph above.

  39. 39
    JimR says:

    #38 – Obliquity tilts one pole towards the Sun at the same time the other tilts away. Decreasing obliquity reduces the insolation difference between summer and winter. Similar to low eccentricity, lower obliquity reduces the amplitude of precession driven changes in insolation, but does not cause an actual decrease in insolation.

    As for alignment of the graphs I would question if panel D in the graph above is the best alignment. Note the much lower insolation at that time, so low that it ended a long interglacial period. A much better analog would be the (NH summer) insolation minimum prior to that one, 420,000 years ago. That insolation minimum was very similar to our current insolation low and did not end the MIS 11 interglacial. As shown by the EPICA team MIS 11 did not even show reduced temperatures associated with that very similar insolation minimum.

    With Andre Berger co-authoring ‘An exceptionally long interglacial ahead?’ Science, 2001 I would question if he would find the insolation minimum that ended the MIS 11 interglacial an analog to our current insolation minimum. That paper was based on the lower amplitude of insolation changes currently due to eccentricity and the fact that there will not be such a low NH summer insolation value for the next 50,000 years.

  40. 40
    Bill Ruddiman says:

    Thanks to many for their comments on my post and to Gavin for handling some of them. Replies to a few others:

    Comment #5: My main point was that the EPICA method produces a specific alignment of the insolation trends, and the one shown is clearly not the closest possible analog to recent millennia. The Ruddiman 2005 paper listed at the bottom of the post covered all this in more detail, including:

    comment #2: the fact that methane values fell to or below the 450-ppb level my hypothesis predicted soon after 400,000 years ago (and within the interval of the ‘real’ closest insolation analog);

    #2, 28: the fact that good arguments based on carbon and carbon-isotope budgest led to a revision of part of the hypothesis. I conceded that humans can only explain perhaps 1/3 of the 40-ppm CO2 anomaly by direct emissions (specifically, the rise from the natural peak of ~268 ppm 10,000 years ago to the pre-industrial value of ~282 ppm. Still, the same 40-ppm difference remains between the pre-industrial value of 282 ppm and the natural peaks of 240-250 ppm reached in previous interglacials, and it still needs explaining. I still think that humans are the explanation. I now think that our methane and (smaller) CO2 emissions kept climate warm enough to produce feedbacks in the climate system that stopped the large CO2 drops that had occurred in previous interglaciations at similar times.

    #29: the fact that Berger’s model produced an extremely long interglaciation if he only used insolation forcing, but a much shorter one if he added CO2 (from Vostok) to the forcing;

    Other replies: (#18) There may have only been 10-20 million people thousands of years ago when the greenhouse trends began to slowly depart from natural, but there were 750 million by just before the start of the industrial era. Also, the link between population and GHg emissions need not be linear. For example, the Domesday Book survey of England in 1089 found 90% forest clearance and 1.5 million people. By 1500, the population was up to 5 million, but they were still feeding themselves from about the same amount of cleared land. How? By using manure and crop rotation to shorten or eliminate the fallow period between plantings and produce 3 times as much food from the same area. So — the link between human populations and land cleared (and CO2 emitted by deforestation) breaks down (is not linear).

    #20: I will be writing a summary “Challanges-and-Responses’ paper on all aspects of the hypothesis this winter, but for now interested readers might consult “Plows, Plagues, and Petroleum” (Princeton University Press, 2005) for an informal update.

  41. 41
    JimR says:

    I have a question on the alignment of the insolation as represented in panel B of Dr. Ruddiman’s graph above. The EPICA team in ‘Eight glacial cycles from an Antarctic ice core’ aligned the deglacial warming of MIS 11 with the deglacial warming of the Holocene. Since the insolation signal is driven by precession and has a period of ~22,000 years how do the two graphs end up out of sync by more than 10,000 years?

    I suppose the question is how was panel B created? If the two interglacials started out aligned (as per EPICA) why does panel B align a period 24,000 years after the start of MIS 11 with current day which is ~ 12,000 years after the start of the Holocene?

    Looking at our current cycle we are roughly at the first NH summer minimum insolation after deglaciation. Looking at MIS 11 the analog would be the insolation minimum of ~420,000 years ago which is deep in the warm period of the MIS 11 interglacial.

  42. 42
    dave says:

    Re: Ruddiman’s #40

    “Still, the same 40-ppm difference remains between the pre-industrial value of 282 ppm and the natural peaks of 240-250 ppm reached in previous interglacials, and it still needs explaining. I still think that humans are the explanation. I now think that our methane and (smaller) CO2 emissions kept climate warm enough to produce feedbacks in the climate system that stopped the large CO2 drops that had occurred in previous interglaciations at similar times.”

    If the claims that climate is so sensitive to small (~40 ppmv) CO2 increases over the last 5 thousand years (fig 1b) are true–with the (unspecified and unknowable) feedbacks–so much so as to offset the Milankovitch forcing indicating a minimum solar insolation at 65°N at present (and not much less prior to the Industrial Age), then I am surely frightened now when we’re looking at 380 ppmv (+100 ppmv) at present and rising. But, I don’t believe it–how does the GHG (CO2, CH4) forcing the Ruddiman hypothesis presumes compare with current climate modeling results incorporating anthropogenic GHG forcings going forward? Not much, I think, there would seem to be large discrepancies depending on the timescale and feedbacks. What are these unspecified feedbacks Ruddiman refers to? What are the implications of Ruddiman’s hypothesis, if true, given the much larger GHG levels now existing in the atmosphere?

  43. 43
    Hank Roberts says:

    Thanks for the responses Dr. Ruddiman.

    I came across another reference — secondary source reporting on the 2004 AGU, here:

    — to add to my questions in #20. Quoting Lonnie Thompson, who at least at that point was talking about a sudden climate change a bit over 5,000 years ago that shows in the ice cores. He mentioned to the interviewer — not clear if it’s an idea from his publication or a not — that such a shift may have been triggered by a spike-and-dip in solar output. The writer says there’s evidence for such a solar change at that time; I haven’t found it yet. The Ohio State abstracts and PDF files go only through 2003 right now.

    I’ll watch for your summary Challenges and Responses, and am asking my local library to get your book in promptly.

  44. 44

    UMass microbiologist Lynn Margulis makes good, evidentiary based claims on how early anaerobic bacteria helped create Earth’s 1/5th oxygen atmosphere by expelling oxygen into the troposphere as a waste product. Independent lines of evidence (ie. unique banded iron formations in 2 billion year old rock) lend support to this conjecture.

    Margulis’ hypothesis states that the outgassing of oxygen from archaeo-bacteria as a waste product forced these same bacterial organisms to evolve to use oxygen as a metabolic energy source once it became abundant in the atmosphere. If Margulis’ line of reasoning and evidence is sound, we have empirical support of life fundamentally altering the composition of Earth’s atmosphere as early as 2-3 billion years ago, and essentially, making life possible for most of the phyla which exist on Earth today.

    If we accept that humans ultimately evolved from early bacterial life forms 2-3 billion years ago, and these very simple organisms were capable of radically changing the composition of the Earth’s atmosphere just by living and excreting gaseous waste products, the concept that humans can similarly alter the Earth’s atmosphere becomes a rather predictable and easily testable hypothesis.

    If something as “primitive” as bacteria can greatly affect the Earth’s atmospheric composition, then certainly “advanced” humans can register a similar chemical imprint as well. This seems logical. With the concept of life changing its own chemical environment established, what is left is an empirical question as to the degree to which life (ie. humans) are changing the Earth’s chemical and atmospheric environment. As regards humans, the type and degree to which we are changing the atmospheric environment can be measured within widely accepted norms (calculate output from smokestacks, for example). Gasses emitted from smoke stacks must go “somewhere” just as the gaseous waste products of Earth’s bacteria and volcanoes must go “somewhere.” That somewhere is first the Earth’s atmosphere. Once in the atmosphere, CO2 or any other gas is relatively indistinguishable as to its source. The laws of physics and chemistry mandate that CO2 or CH4 molecules behave in the atmosphere the same regardless of whether they originally came from a smokestack, a volcano or a bacterium.

    If during the past 100 years, volcanic activity on Earth was several orders of magnitude greater than in past centuries or millennia, and the gaseous contribution of this activity to Earth’s atmosphere could be accurately measured, I doubt many people would deny that volcanoes were in fact erupting over much of the Earth. How different it is when the gasses are human-induced, rather than bacterially or volcanically induced? Does human induced CO2 behave differently in the atmosphere than volcanically induced CO2?

    The sheer weight of scientific evidence shows that living organisms on Earth are capable of invoking massive changes on the physical and chemical character of Earth. If not for a thick bed of planktonic skeletons of calcium carbonate, Mount Everest would be much lower than it is today, since its summit is made of metamorphosed limestone. Thus we must dismiss claims that living organisms on Earth are incapable of profoundly changing the character of the very Earth they live on.

    The claim that humans are so minuscule as to be incapable of altering some of Earth’s physical and chemical cycles is not supported by evidence. If smokestacks release as much CO2 as X number of volcanic eruptions, science dictates we analyze this human-induced contribution just as if it were contributed by volcanoes. If one accepts that bacteria can cause measurable changes in Earth’s atmosphere just by gasses released during their metabolic activity, science dictates we analyze this human-induced contribution just as if it were contributed by bacteria.

  45. 45
    Rod Smith says:

    It seems from the comments that there is violent agreement that we are experiencing waves that threaten to capsize our boat. Critics of the “don’t rock the boat” approach seem to forget that we are rapidly depleting our CO2 emitting energy supplies, and it is only a matter of decades before it will be too late to develop alternatives. Since we must do something anyway to develop alternative energy supplies, why not accrue the added benefit of reducing the amount we rock the boat just in case it makes a difference.

    My understanding of complex dynamical systems is that small changes can have large consequences. No doubt the climate system is extremely complex and we may not have all the answers yet–but how smart is it to assume all the waves occur naturally and go about rocking our boat as if it doesn’t matter? For example, we may be witnessing a positive feedback loop of runaway methane gas emissions as the permafrost thaws. Must we add more fuel to the fire through carelessly emitting CO2?

    While the transition period to alternative energy may be financially painful, if we wait until we run out of traditional energy before we develop alternatives that will be even more painful. Mitigation approaches may actually yield economic benefits. For example if the largest consumer of carbon based fuels could wean itself off of foreign oil through development of alternative energy it might be able to avoid the next half-trillion dollar war.

    We need to keep in mind that sudden change has had devestating impacts on human systems throughout history, and that society is facing major changes in coming decades in the areas of climate, energy, and water. Must we look for trouble?