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Hot off the projector #3: Atmospheric CO2 to 800 kyr ago

Filed under: — david @ 12 December 2007

Just a few minutes ago Chappellaz et al presented the deepest dregs of greenhouse gas concentration data from the EPICA ice core in Antarctica, extending the data back to 800,000 years ago. In Al Gore’s movie you saw what was at that time the longest record of atmospheric greenhouse gas concentrations, back to 650 kyr, and their astonishing correlation with Antarctic temperature. This iconic superstar record has probably consumed as many eyeball-hours as any in climate science, alongside other classics such as the Jones et al. global temperature trends, the Moana Loa recent CO2 record, and the hockey stick. The Antarctic CO2 record has spawned countless internet rants about the CO2 lag behind temperature, and the circle of cause and effect between CO2 and climate. And the new data say …

The first point to write home about is that the correlation between Antarctic temperatures and CO2 continues unabated. One could imagine a world in which CO2 had no impact on climate, although if you buy that Dick Lindzen has a bridge he wants to sell you. In such a world, it could be that the correlation between CO2 and temperature since 650 kyr was just a coincidence, Mother Nature playing a cruel joke, and maybe in that case a little more data would cause the spurious correlation to start to unravel. That didn’t happen. CO2 continues to be high in warm times and low in cold times. There were no gasps of astonishment from the audience at the continued striking correlation. Ho-hum, of course it still works.

That being said, there are some interesting subtleties in the latest data. The CO2 concentrations were generally lower than average between 800 and 650 kyr. The lowest CO2 value ever measured in an ice core is now 172 ppm, from 667 kyr ago. If you average over the glacials and interglacials, there appears to be a very long-term cycle in atmospheric CO2, low from 600-800, peaking around 400 kyr ago in stage 11, the 50 kyr-long “super interglacial” when the Earth’s orbit was nearly circular as it is now, and then subsiding a bit since then. Perhaps this variability is driven by variations in rock weathering, the longest-term geological carbon cycle. Interestingly, there is no corresponding million-year cycle in Antarctic temperatures, when they are averaged in a similar way. CO2 is a dominant controller of global climate, but it is not the only game in town.

There were also millennial timescale wiggles in atmospheric CO2 and methane concentration during the descent from the stage 19 interglacial to the stage 18 glacial time. There are strikingly similar to the Dansgaard-Oeschger wiggles from 30-70 kyr ago in isotope stage 3. The duration is similar, about a thousand years, and the trajectories of the events are the same, with sharp warming and associated rise in greenhouse gases, then slower recovery. As in stage 3, they occur through the slow cooling transition between extreme climates, not in the full-blown glacial or interglacial states. These abrupt climate changes appear to be business-as-usual for the global climate system.

What would be really cool is if there were enough ice to continue probing back into the deeper past. Around 800 kyr ago, the climate cycles on Earth switched from being dominated by 40 kyr cycles, to the stronger 100 kyr cycles of the more recent times. The time period from 800 – 1000 kyr ago is called the mid-Pleistocene transition, and since the rhythms of the Earth’s orbit didn’t change, it must have something to do with the way that climate on Earth operates, maybe something about the carbon cycle. What would be really cool is if we could get CO2 and methane records back to say 2 million years ago, through this crucial transition time. Alas, 800 kyr is as far back as the useful ice in this location goes. The bottom tens of meters of the ice, like the bottoms of all ice cores, are too scrambled to interpret.


123 Responses to “Hot off the projector #3: Atmospheric CO2 to 800 kyr ago”

  1. 1
    DonaldK says:

    As a non-expert (to put it as flatteringly as possible) I’ve never been able to understand how these ice core data relate to estimates of CO2 levels v. global temperatures from deep-time periods such as the Carboniferous or Ordovician. My skeptic friends always point these ancient periods out to me in an attempt to dispel the idea that CO2 effects temperature. I know they are wrong, but can’t come up with an argument against it. Any ideas? Why were Co2 levels so high in the past with relatively low global temperatures? Is the Quaternary earth a different ball-game?

    [Response: It gets more difficult to reconstruct atmospheric CO2 levels further back in time than the ice cores go, but in general, as best it can be done using “proxy” determinations from isotopic measurements or what have you, periods of warmth coincide with higher CO2 in the past. There’s no argument against the CO2 impact on climate from ancient times. Actually, if CO2 were not a greenhouse gas, it would be impossible to explain past climate states. David]

  2. 2
    Hank Roberts says:

    > What would be really cool is if we could get CO2
    > and methane records back to say 2 million years ago

    I imagine this calls for finding sediments — much harder to drill ocean bed sediment than icecaps, I know there’s a lot less done and very little correlation yet across sites; Gavin remarked on that earlier after his trip to China as a current issue.

    If the ice cores and sediment cores can be correlated as far back in time as the ice is available, then the older sediment will have that as a basis for confidence.

    [Response: Yeah, we got sediments a plenty going back millions of years, but they don’t have bubbles of air in them. No decent isolated water samples from long ago either, more’s the pity. David]

  3. 3
    Milan says:

    I have heard that pollen samples can extend back a lot further than ice. Is that accurate? Are they equally useful, when it comes to working out what the paleoclimate was like at a particular time?

    Richard Alley mentions them briefly in chapter five of “The Two Mile Time Machine.”

    [Response: In principle they could go back further, but they don’t tell CO2 concentrations. David]

  4. 4
    ICE says:

    Yesss , Christmas time! RC keeps on giving !
    Thanks a lot for the reporting !

    so one more question: do we know precisely the mecanisms by which carbon is released (from where ?) to the atmosphere during deglaciation times ? i mean, do ocean warming, circulation changes, soil de-freezing (while vegetation probably develops, so acts as a sink), explain it all ?

    [Response: Not in my opinion. You can clobber an ocean carbon cycle model with everything you can think of, and it won’t bring the atmospheric CO2 concentration down to 200 ppm, let alone 172. Temperature can do some of it, and a high-carbon deep water mass is beginning to emerge in the reconstructions, but no one has a mechanistic model which can explain the CO2 cycles. David]

  5. 5
    Frank R says:

    I am just curious whether there is anything new to report on the lag between CO2 and temperature from this new dataset?

    [Response: No, that part of the record is finished. David]

  6. 6
    Mitchell says:

    “There’s no argument against the CO2 impact on climate from ancient times.” But correlation alone cannot be an argument, can it? How do you explain the lag between temp and CO2? Isn’t this an argument against causation?

  7. 7
    KenH says:

    I’m a non expert in climate science, but I was wondering about the correlation between temperature change and solar irradiation (Milankovitch cycles). Is there a phase lag or delay in the temperature change relative to the change in solar radiation. I guess what I’m driving at is – does CO2 concentration have a damping effect or delaying effect on the on the temperature change relative to the change in solar radiation.

    [Response: No. It has an amplifying effect. – gavin]

  8. 8
    Chris says:

    Re #1 Donald

    Dana Royer has recently compiled the proxy CO2 and temperature data covering the entire Phanerozoic period:

    see “CO2-forced climate threasholds during the Phanerozoic” (2006) Geochim. Cosmochim. Acta 70, 5665-5675

    In general, the temperature proxies rise and fall pretty well in line with the CO2 estimates throughout this period (back over 500 million years). Obviously in the very deep past a stronger greenhouse effect was required to maintain reasonable temperatures in the light of a weaker solar output. So for example right up to “even” 100 million years ago (MYA) atmospheric CO2 levels had to be above 1000 ppm for the Earth not to be in a cool period, and CO2 levels below 500 is considered to be the threshold for full glaciations.

    Interestingly even Jan Veizer, who not very long ago was publishing a paper with Shaviv asserting that the cosmic ray flux variation was the dominant driver of the Earth’s climate in the deep past, has just published a paper indicating that it is actually CO2 levels that couple with the earth’s climate back through the Palaeozoic.

    R.E. Carne, J. M. Eiler, J. Veizer, K. Azmy, U. Brand & C. R. Weidman (2007) “Coupling of surface temperatures and atmospheric CO2 concentrations during the Palaeozoic era” Nature 449, 198-202.

    I’m not sure there are any significant periods where cold temperature proxies are associated with high CO2 proxies. For example, Veizer et al correct a temperature (around 450 MYA) associated with a high CO2 that was previously considered to be cold (about 3 oC cooler than today). Their reassessment indicates that that period was actually 7 oC warmer than now!

  9. 9

    Note, too, that there are no ice cores from the Cambrian or Ordovician. They’re all from the Quaternary.

    Temperature doesn’t just depend on CO2, but on other greenhouse gases, the albedo of the surface, the arrangement of the continents, etc. If all else is kept equal, more CO2 means higher temperatures, but early ages were not like today, so there is no reason to suppose a simple one-to-one link to hold.

  10. 10
    AD says:

    I thought this was a science blog. This is no place for such a personal attack on Richard Lindzen. If I were him I would demand a public apology, but he is probably too much of a gentleman to do that.
    Shame.

    [Response: I apologize. And I’ll sell you the bridge myself. David]

  11. 11
    Lynn Vincentnathan says:

    RE “The Antarctic CO2 record has spawned countless internet rants about the CO2 lag behind temperature…”

    I really don’t know why skeptics are so gleeful over warming causing CO2 to increase. It was only when I learned about that I became much much more concerned about GW.

    It’s one thing that CO2 causes warming in a somewhat linear fashion (I know it’s sort of a log function). But it’s really a lot more serious that warming could cause an increase in CO2 levels, causing increasing warming, causing increasing CO2 levels, and so on to ? oblivion ?

    Is this thanatos (Freud’s death wish) at its worst on their part? Do they really think they have an argument. It just strengthens our need to be concerned about and address GW ASAP.

  12. 12

    D’oh. I didn’t notice that session when picking out my lists of things to see. (It’s too bad AGU doesn’t have a mechanism for highlighting sessions/presentations that are likely to be of broad interest.)

    Did they happen to say anything about when the data is likely to reach the archives? Presumably there will have to also be a paper publication first. I’ve been waiting to update my ice core carbon figure for well over a year after being told that the full record was coming “soon”. Not to mention that there was already at least one press release about “800 kyr” carbon dioxide back in mid-2006. It is nice to know that things are finally coming to completion.

    Incidentally, most of the people I’ve talked to about it seem to think that one can probably get ~1.5 Myr of usable ice out of Antarctica with a truly optimal site, but that’s probably about the limit of what’s achiveable.

    [Response: The paper’s submitted to Nature, ought to be out within a few months. You weren’t the only one who missed the talk, it was at 0800 and not very crowded. David]

  13. 13
    Mark R says:

    Do any scientists speculate that the apparent lag between CO2 and temperature in the ice core record is a false reading, and that there might be some unrecognized factor that makes the lag show up in ice cores?

    [Response: The lag is very uncertain, but there are good physical reasons to expect it to be there, so it isn’t very troubling. The basic issue is that deep ocean carbon takes centuries to adjust to new surface conditions (winds, ocean currents, biological activity). – gavin]

  14. 14
  15. 15
    Bruno says:

    Re: #8

    Chris,

    Thank you for bringing up Came et al, 2007. When the paper was published I thought that RC would be all over it… and I even e-mailed it as a suggested topic… but alas, nothing ever came of it.

    I was taken aback by that paper as well. Does this mean that Veizer has reversed his stance on GCR’s, though? I have not encountered any statements from him that would affirm that… although one would think that this would be a serious blow to his original correlation.

    Nonetheless, it appears that Veizer is still claiming that CO2 was anomalously high during the Late Ordovician icehouse. I have a few problems with his assertion. First, it seems to be based on reconstructions by Yapp, Royer and Berner, which are rather broad in scope (especially in the case of Yapp, 1992). Saltzman and Young’s (2005) detailed isotope stratigraphy of the Mid-Late Ordovician reveals several brief but major d13C excursions that seem to correspond with glacial maxima. These excursions may correspond with rapid drops in pCO2, but no one (to my knowledge) has done any detailed pCO2 stratigraphy from that time period.

    Secondly, Ordovician paleosols were developed prior to the evolution of vascular land plants. Thus, it becomes more difficult to apply Cerling’s model. Furthermore, if Saltzman is right about the icehouse being relatively brief (on the order of 10^4-10^6 years) then one could easily miss those paleosol horizons… especially if one also considers that (as a general rule) paleosols are more likely to develop during wetter, interglacial periods.

  16. 16
    Bruno says:

    So… this must be an update of the Jouzel et al (2007) paper that appeared in Science earlier this year. Did they mention what the CO2 age uncertainty is for the period 650k-800k? Does it become significantly greater with depth?

  17. 17
    Jim Eager says:

    Re Mitchell @ 6: “But correlation alone cannot be an argument, can it? How do you explain the lag between temp and CO2? Isn’t this an argument against causation?”

    No, it’s not. Rising CO2 is not the initial cause of interglacials, but rather a feedback to the initial warming caused by increasing solar insolation, as are changes in albedo as the ice retreats. But then this increase in CO2 leads to yet more warming through the greenhouse mechanism, until equilibrium is gradually reached. [Note: equilibrium is a dynamic state, not a static one.] Since we’re nearing the end of the current interglacial, we’ve been at this natural equilibrium for a long time now. The CO2 we are releasing into the atmosphere by burning fossil carbon fuels is clearly not a feedback to an already underway warming, it is a new forcing.

    BTW, there have been times in the distant past where an increase in greenhouse gases was a forcing rather then a feedback. They were not times that you would have wanted to witness first hand. Look up the Permian-Triassic transition (251M BP), Triassic-Jurassic transition (200 M BP), Paleocene-Eocene Thermal Maximum (55M BP), for example.

  18. 18
    Chris Colose says:

    Wow, everything happening very fast. I do have one question. As I understand, the resolution in antarctic cores is very bad after a (relatively) small amount of time, which is why Greenland cores are used more for the last 100k or so, but is this just for CO2 or temp as well? Or can you just see the “general picture” without details?

    [Response: Time resolution is better in Greenland, but the CO2 in the bubbles there is contaminated by CaCO3 dust + acid aerosols, so CO2 only comes from Antarctica. Methane is good in Greenland, and is used to correlate the two regions since it is essentially well mixed in the atmosphere. David]

  19. 19
    Bob Clipperton (UK) says:

    Several points:-
    1. I thought I had posted on this topic yesterday evening (UK) but there’s no sign of it here now. Am I ‘persona non grata’? (or perhaps I just thought I’d hit the post button but hadn’t
    2. David’s response to post #4 is puzzling me. I read ICE’s post as an enquiry as to what specific mechanisms are considered to be initiators of DE-glaciation but David’s response talks about unknown factors reducing CO2 levels in the atmosphere (the opposite to what I had assumed in de-glaciation). Could you clarify please David (& I don’t mind at all being told I’ve misunderstood.)
    3. The substance of my post that I referred to earlier was that I had assumed, from private advice from Gavin some time ago, that de-glaciations (& indeed glaciations) coincided with the different aspects of the Milankovitch cycles. I recently wondered if increased heat from tidal friction losses could be a deep sea warming mechanism here. (The closer the sun gets, the bigger the gravitational effects leading to bigger to tides & therefore bigger frictional heat losses and therefore warming the oceans differently from direct radiation and finally releasing more CO2).
    I raised several questions on this line of thought. For example, haven’t ‘tidal’ heating effects been speculated for warming of some of the gas giants’ moons? Have any specialists looked at this issue? Is there any conceivable geological evidence that could link increasing tide magnitudes to the start of de-glaciations?

  20. 20
    Ayelén says:

    Just on question:
    Which is the time resolution (in Greenland and in Antartica )?

  21. 21
    Nick Gotts says:

    RE #17: “Re Mitchell @ 6: “But correlation alone cannot be an argument, can it? How do you explain the lag between temp and CO2? Isn’t this an argument against causation?”

    No, it’s not. Rising CO2 is not the initial cause of interglacials, but rather a feedback to the initial warming caused by increasing solar insolation, as are changes in albedo as the ice retreats. But then this increase in CO2 leads to yet more warming through the greenhouse mechanism, until equilibrium is gradually reached.”

    I suspect that arguments involving feedback loops are intrinsically difficult to understand. Although a quick trawl with Google Scholar failed to find any research specifically on this, the frequency with which the point Mitchell makes is raised, and also the more sophisticated misunderstanding that a positive feedback loop must lead to a runaway acceleration, suggest this. So do comments of colleagues of mine who have involved local communities in the design of models of social-ecological systems. More generally, I suspect that contrarian pseudoscience (AGW denialism, HIV-AIDS denialism, smoking-cancer denialism, “Intelligent Design”) makes systematic, though not necessarily conscious use of this and similar “cognitive illusions”. Other examples in the climate area may include the “Golden mean” fallacy – that where there is disagreement between two groups, the truth is most likely to lie between their positions – the “curvilinear impetus” fallacy (young children, and even many adults, think that if you start a ball rolling through a curved tube, it will continue to curve after leaving the tube – McCloskey, Caramazza and Green (1980), Science 210:1139-41), which may be behind the belief that increased solar forcing before 1950 could still be accelerating temperature change, and belief in the “law of small numbers” (Ch. 2 in Tversky and Kahneman’s “Judgement under uncertainty: Heuristics and Biases”), which leads psychologists, and presumably others, to overestimate the importance of results from small samples.

  22. 22
    Nick Gotts says:

    p.s. I forgot one: the common belief that enough invalid arguments can outweigh a valid one!

  23. 23
    Ray Ladbury says:

    Nick Gotts, another trend I’ve seen in denialists is a seeming inability to understand or appreciate inductive science–the fact that if observations A, B, C… all strongly and independently favor hypothesis 1 over hypothesis 2, while observation X could be interpreted weakly favoring hypothesis 2, this does not invalidate hypothesis 2. Indeed, philosophy of science is even kind of weak on its treatment of this situation, since there is no generally accepted treatment of the absolute strength of evidence for a hypothesis, but rather one has to look at its strength relative to other hypotheses. All of this leads scientists to be rather too circumspect in the way they phrase their conclusions.
    Ultimately, though, scientific is the only way I know of that humans can overcome their biases and their poor abilities at assessing risk–both on the high side (e.g. the terrorism threat in post 9/11 USA) and the low side (e.g. climate change).

  24. 24
    Nick Gotts says:

    RE #22 (Ray Ladbury) “another trend I’ve seen in denialists is a seeming inability to understand or appreciate inductive science–the fact that if observations A, B, C… all strongly and independently favor hypothesis 1 over hypothesis 2, while observation X could be interpreted weakly favoring hypothesis 2, this does not invalidate hypothesis 2.”

    I think you meant “hypothesis 1″ at the end?

    I believe there’s some evidence that demonstrating and explaining such fallacies can help (I think this was with regard to courtroom situations, but I can’t remember exactly the fallacy involved). On a related point, it may be possible to use logical and textual analysis to identify when opposition to a widely accepted scientific theory or finding (such opposition is of course not necessarily wrong) has become pseudoscience, by looking at the frequency with which such fallacies, and other features such as constantly shifting grounds of objection, accusations of massively concerted fraud, etc., occur.

  25. 25
    Jim Eager says:

    Re Nick Gotts @20: “I suspect that arguments involving feedback loops are intrinsically difficult to understand.”

    You may well be right, Nick, especially by individuals who tend to see things in a simple one-or-the-other, mutually exclusive way, despite the fact that reality is seldom that simple.

  26. 26
    JH says:

    Did the sampling give us any information on water vapor? I thought water vapor was a much more significant greenhouse gas than CO2?

  27. 27
    Matt says:

    So, why didn’t the ice core diggers get the Nobel instead of Gore?

  28. 28
    paul m says:

    Will the melting of the Ice Sheets have an effect on the orbit of the earth around the sun?

    For that matter does a rising/falling sea levels have an effect on the earth’s orbit (5ft-50ft-200ft-tides)?

    Is the suns radiation still increasing over what it was in the past and do the climate models take this into account?

  29. 29
    jkga says:

    re # 25: These are from air bubbles found in ice samples; I’m not a climate scientist, but I do know that the concentration of water vapor in an air bubble in contact with an ice surface will depend on the *current* temperature of the air bubble and the ice, and not on the conditions when the bubble was formed. (look up “vapor pressure” on Wikipedia.)

    In any case, the maximum possible water content of air drops very quickly as temperature drops, so I don’t think water vapor is an important greenhouse gas *at the poles*.

    Elsewhere on this site you’ll find explanations as to why water vapor is considered a feedback and not a forcing in atmopspheric models.

  30. 30
    Chuck Booth says:

    j# 26 Matt: “So, why didn’t the ice core diggers get the Nobel instead of Gore?”

    Perhaps because the “ice core diggers” didn’t go out on the lecture circuit, produce a documentary, and publish a book for a lay audience, in order to raise public awareness of AGW around the world? I’m just guessing, of course.

  31. 31
    Gary Rondeau says:

    In the ice core data, the onset of warming periods is always quite rapid – presumably because various positive feedback effects enhance the warming. I have wondered, what puts the lid on the temperature increase? Just as suddenly as the increase in temp and CO2 start, it stops increasing – so some mechanism must come into play with its own set of feedbacks that limit the heating at some point. Can anyone tell me what the current theory on this is?

  32. 32
    Bruce Tabor says:

    Re 26 Matt,
    Gore and the IPCC got a Nobel Peace Prize, not a Nobel Science Prize.

  33. 33
    Bruce Tabor says:

    Re 30 Gary Rondeau,
    I will hazard a guess that the warming ultimately stimulates mechanisms that remove CO2 from the atmosphere – increases in biological activity and rainfall are two that come to mind.

  34. 34

    Gary Rondeau posts:

    [[In the ice core data, the onset of warming periods is always quite rapid – presumably because various positive feedback effects enhance the warming. I have wondered, what puts the lid on the temperature increase? Just as suddenly as the increase in temp and CO2 start, it stops increasing – so some mechanism must come into play with its own set of feedbacks that limit the heating at some point. Can anyone tell me what the current theory on this is?]]

    The temperature increase is a converging series; it comes to a maximum. After that, variations in solar illumination with latitude (the Milankovic cycles) eventually start a cooling trend, and the reactions work in the opposite direction, again coming to a certain point and then stopping.

  35. 35
    Ray Ladbury says:

    Gary #30, Most past warming epochs were initiated by mechanisms other than ghg–mainly changes in insolation due to small variations in Earth’s orbit, rotation, tilt, etc. Earth gets more sunlight, so it warms up. As it warms up, ghgs start to be released. There’s no runaway, because the changes (in both insolation and ghg) were rather modest. Remove the increased insolation and Earth has to cool. As it cools, CO2 goes back into the oceans, permafrost, etc. Note that the warming events last thousands of years, so things really aren’t that sudden.

  36. 36
    William Astley says:

    I have two issues:

    1) Why are CO2 atmospheric levels changing over long term periods (million years) and short term periods? (Short term, before anthropogenic CO2 disturbance.)

    Evidence of past rapid changes (less than 100 years) in CO2 levels seems to provide support for an external forcing function such as solar which can abruptly change planetary temperature. CO2 levels then follow the change in planetary temperature. There is no mechanism presented here that can change CO2 levels in a short term period (less than 100 years) semi cyclically, besides a change in planetary temperature.

    Additional support for a non CO2 climatic driving function is evidence throughout the paleoclimatic record of so called cool periods which occur during periods of high CO2.

    2) The second issue is how low current CO2 levels are based on past levels and the limit of C3 plants. The minimum CO2 level, 175 ppm, reached during glacial maximum is the lowest in the history of the planet. At this level there is evidence of CO2 starvation in leafy planets (C3).

    http://www.ecophys.biology.utah.edu/Labfolks/Ehleringer/Publications/229.pdf

    See figure 1, note current CO2 levels (280 ppm prior to anthropogenic change) are the lowest in 300 million years.

    http://irevues.inist.fr/bitstream/2042/4353/4/CG20

  37. 37
    Ray Ladbury says:

    William #34, CO2 is affected by many factors–temperature, volcanism, ecology,… I don’t know what you mean by “semi-cyclically”. A system is either periodic or not. The physics of periodic systems is quite different from that of “quasi-periodic” systems.
    Not sure what point you are trying to make with your second issue. Are you perhaps waxing nostalgic for the Jurassic?

  38. 38

    William Astley writes:

    [[Additional support for a non CO2 climatic driving function is evidence throughout the paleoclimatic record of so called cool periods which occur during periods of high CO2.]]

    You’re dealing with a different time scale. When Earth undergoes a snowball Earth event, CO2 has to build up very high to cause the ice to retreat again. So for a while you have cold temperatures and high CO2. The work of Walker, Kasting, Hays, Lasaga and others on the silicate-carbonate cycle is of interest here. Basically, Earth has a “thermostat” which keeps temperatures in the habitable range most, though not all, of the time.

  39. 39
    Gary Rondeau says:

    Ray #33, I’m not claiming that the warming periods are “runaway” events, just that the general waveform looks a lot like a relaxation oscillator to this electrical engineer. The “synchronization” may come from solar/orbital forcing, but the dynamic of the temperature is not anything like those quasi sinusoidal functions. The main nonlinear effect that I can see is just T^4 radiation finally beating out the GHGs that evolve during the heating epoch, but that doesn’t seem like enough to account for the dramatic change in slope of the temperature and CO2 histories at the end of warming episodes. I’m still looking for a good explanation.

  40. 40
    Ray Ladbury says:

    Gary, there are many nonlinear effects and many feedbacks. First, the orbital variations that affect insolation are periodic. You have damping from the thermal reservoir of the oceans (quite complicated with multiple layers exchanging mass and heat on different timescales). You have feedbacks–water evaporation, GHG emission from oceans and melting permafrost, changes in albedo as ice melts and vegetation changes, and so on. Remember that we have much better resolution on the temperature than on the CO2, and that even then, the resolution is, at best, decades. Also, the driver in most of these cycles is insolation–reduce the driver and the system responds rapidly, just as would your oscillator if you reduced voltage.

  41. 41
    William Astley says:

    In reply to Ray Lamburt’s #35

    William #34, CO2 is affected by many factors–temperature, volcanism, ecology,… I don’t know what you mean by “semi-cyclically”. A system is either periodic or not. The physics of periodic systems is quite different from that of “quasi-periodic” systems.

    Ray, check the attached paper, figure 1. What is causing the abrupt changes in CO2?

    Do the drops in atmospheric CO2 resemble a saw tooth? Is saw tooth like semi-periodic? What is causing that pattern?

    Glacial/interglacial variations in atmospheric carbon dioxide, by Sigman & Boyle

    http://geoweb.princeton.edu/people/faculty/sigman/paperpdfs/Sigman00Nature.pdf

    In reply to Barton Paul Levenson’s #36

    “The work of Walker, Kasting, Hays, Lasaga and others on the silicate-carbonate cycle is of interest here. Basically, Earth has a “thermostat” which keeps temperatures in the habitable range most, though not all, of the time.”

    Barton, The CO2 control device seems to be broken.

    What is missing in this discussion is how low CO2 levels are now and during glacial maximum. During the glacial period roughly a 1/3 of the tropical forest is converted to grass land. Because of CO2 starvation C3 plants (leafy plants as opposed to C4 plants, grasses.) die and are placed by grasses. There is a 10 times increase in dust deposited in the Greenland ice sheet layers during the glacial period which indicates an increase in desertification.

    It is difficult even to explain why CO2 reached such low levels in the glacial maximum. The attached article explains the problem. Sigman & Boyle posit a massive increase in atmospheric dust enables iron deficient regions of the ocean to remove additional CO2 from the atmosphere.

    Excerpt from the above paper:

    “With currently forested or vegetated regions covered under kilometres of ice, an increase in the extent of deserts, apparent conversion of tropical forest to grassland, and exposure of organic-rich sediments on continental shelves, it seems likely that the continental reservoir of organic carbon decreased (My comment: 500 Pg of land based carbon moved to the oceans, in addition to the draw of atmospheric carbon. See paper for details.) during the last ice age, contributing CO2 to the ocean/atmosphere system”

  42. 42
    Hank Roberts says:

    > What is missing in this discussion is how low CO2 levels are now

    How low do you think the CO2 level is now compared to

    >during glacial maximum

    would you give us the numbers you think are correct and your source?

  43. 43
    David B. Benson says:

    William Astley (39) — To put ‘abrupt’ in perspective, consider the Petit et al. paper on the NOAA Paleoclimatology web pages regarding the Vostok ice core data. Indeed, look at just the 3390 years leading to the CO2 maximum of the Eem, the glacial termination just previous to the one we live during. In this 3390 years, atmospheric CO2 rose from 240.4 ppmv to 287.1 ppmv during the 150 years or so of maximum temperature (via proxy) in the ice core record.

    This delta of 46.7 ppmv is about the same as that which has occurred since around 1973, a period only one-hundredth as long as in the ‘abrupt’ paleorecord.

  44. 44
  45. 45
    Hank Roberts says:

    Oops. The link in #42 expires.
    For iron fertilization, go to any recent published work, then
    – click the ‘related’ and ‘cited by’ links or the like
    to track the research forward to contemporary studies. Short answer, it’s s quite uncertain whether there was a lot of iron fertilization during the dry/dusty/windy periods in the glacial cycles, so reason to be skeptical whether it will work as geoengineering.
    Try here, for example, as a starting point for such searching:
    http://www.sciencemag.org/cgi/content/short/294/5541/309

  46. 46
    William Astley says:

    In reply to Hank Robert’s #40

    “What is missing in this discussion is how low CO2 levels are now”
    How low do you think the CO2 level is now compared to >”during glacial maximum”

    & David Benson’s #41
    “This delta of 46.7 ppmv is about the same as that which has occurred since around 1973, a period only one-hundredth as long as in the ‘abrupt’ paleorecord.”

    We have different climatic change concerns.

    I am concerned about global cooling and climatic stability. See for example the follow paper figure 2 d (dust in the ice). The ice house period cycles appear to be increasing in severity.

    http://www.esf.org/fileadmin/be_user/research_areas/LESC/Documents/nature02599.pdf

    Or see figure 1 in the attached which provides a proxy record of planetary temperature over the last 3 million years. It appears that as the planet has become colder the climate has become unstable (larger cycles?).

    http://www.agu.org/pubs/sample_articles/cr/2002PA000791/2002PA000791.pdf

    My concern is that there is proxy evidence that would indicate planetary temperature can be low when CO2 levels are high. I am concerned increasing CO2 levels will not warm the planet which I believe is the opposite of your concern.

    175 ppm is very low from the standpoint of C3 plants. 340 ppm is not high from the standpoint of past CO2 levels, say over the last 300 MMyrs, typically significantly above 1000 ppm. For example CO2 ranged from 2000 ppm to 3000 ppm, 50 MM to 60 MM yr, from this paper. (figure 3).

    http://www.nature.com/nature/journal/v406/n6797/abs/406695a0.html

  47. 47

    William Astley (#39) wrote:

    Barton, The CO2 control device seems to be broken.

    It’s actually just a little slow on the uptake…

  48. 48
    Ray Ladbury says:

    William, not sure what your point is. The ice and temperature peaks are sharper than the CO2 peaks. Look at the abscissa on that graph–each of those points is at least decades. Yes, understanding CO2 uptake during cold periods is interesting, but not really relevent to the current epoch. The real changes in CO2 took place hundreds of millions of years ago when the carbon became sequestered in the petroleum deposits and coal beds–and now we are releasing it into the atmosphere again. So in many ways, the paleoclimate of the past 400000 years is less relevant than the paleoclimate of hundreds of millions of years ago. The thing is, the sun was much weaker then, so climatically, we’re in a different regime entirely.

  49. 49
    Hank Roberts says:

    Worth a read if you can get to a library, or hunt down the information elsewhere; this article’s behind a paywall:

    Is the biofuel dream over?
    New Scientist (subscription), UK – Dec 14, 2007 – Filter
    Can biofuels help save our planet from a climate catastrophe? Farmers and fuel companies certainly seem to think so, but fresh doubts have arisen …

    Someone’s finally done the math on this. Meanwhile the agriculture lobby has gone hog-wild in the US Congress and gotten everything they asked for, calling it ‘energy’ instead of ‘agriculture’ subsidy.

  50. 50
    CobblyWorlds says:

    William Astley – shape of the Glacials/Inter-Glacials.

    In addition to the other points above:

    In “Climate Change and Trace Gases”, pages 1932-1934, Hansen et al put forth arguments for the rapidity of melt vs, the slowness of the freeze process. It takes a long time to build multi year ice that can on average withstand “weather” so as to carry the glacial front forward on a millenial basis. But one hot spring/summer can knock it back rapidly. However note that on pages 1929-1930 they explain how GHGs have to be invoked to explain the gross magnitude of the observed temperature trend. PDF of Hansen et al available here: http://pubs.giss.nasa.gov/abstracts/2007/Hansen_etal_2.html

    Mangled into a simple form…

    The initial forcing is just a small variance of forcing at high lattitudes – Milankovitch. Alone that can’t explain the resultant temerature excursion.

    That initial forcing is first amplified by ice sheet extent, which increase the amount of incoming solar reflected back out to space.

    Then once the oceans have warmed/cooled and atmospheric CO2 has increased/decreased, that implied change in radiative forcing amplifies the temperature change further.

    However because ice sheets melt fast, and form slow. And ice sheet extent change (implying albedo change) is the first main driver, there’s an imposed time asymetry to the resultant output wave form – i.e. it’s a saw-tooth wave.

    Well that’s what this amateur reader understands (numerous non-critical-to-first-order caveats and details excluded).


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