What did temperatures do as the last glacial minimum ended 120K years ago?

A commenter on the most recent edition of het’s AWOGWN asks an interesting set of questions:

How would temperature data have been seen during the last 10,000 years prior to the peak of each of the previous Milankovich cycles? What caused the temperature to reverse course in those cycles and why would we not expect it to occur again this time?

First, here are the quick answers to those three questions, then some discussion.  1. It is not currently possible to resolve the temperature record that long ago to anything close to what we have today.  2. The cause of the temperature reversal is not well understood but probably due to a feedback of falling temperatures, growing ice sheets and falling CO2 levels initially triggered by Earth’s orbital variations (Milankovich cycles). 3. We do not expect this to occur again at this time because the orbital cycle is different and not due to exert a cooling effect for several 10’s of thousands of years and because CO2 levels are simply through the roof compared to anything in the past two or more million years.

So to expand a bit about the temperature records during past inter-glacials: it is true that temperature proxies that far in the past can not currently resolve to time scales finer than centuries.  The current warming period has barely cleared one century.  From a strictly data-centric, context-free, statistical viewpoint, a warming like ours, were it to reverse as quickly as it has come, could come and go in between the dots on our graphs of ice core analyses.  However, many possibilities can be eliminated as implausible or very improbable through our understanding of the factors that control annualized and globalized average surface temperature.  These data are, after all, emphatically NOT just abstract, context-free bits on a hard drive, they represent real physical properties with known behaviors and constraints.

One of the most compelling contextual factors comes from our understanding of the way temperature correlates with CO2 levels.


(image credit: http://en.wikipedia.org/wiki/File:Co2-temperature-plot.svg)

The magnitude and rapidity of the CO2 spike today is stunning and there is no natural mechanism, known or even speculative, that can reduce that level of atmospheric CO2 on anything less than millennial timescales.  Such a pulse could not be invisible in the ice core records.  So were we to postulate a warming as rapid as today’s it would imply a very drastic decoupling of the CO2 – temperature record and therefore some other mechanism of climate forcing.  Now there are of course other climate forcings in nature, but the only truly dramatic ones cause cooling, things like violent volcanic eruptions and asteroid impacts.  Non-greenhouse gas causes of warming are generally very slow: melting ice sheets, continental drift, orbital changes.   It is theoretically possible that the sun brightened rather dramatically and then dimmed equally as dramatically in between our data points, but as well as seeming highly unlikely, that would not be very relevant to understanding what is going on today.  We know what the sun is doing now, even if we don’t know what it was doing on short timescales 120K years ago.

The global climate may well be changing constantly on many timescales but there simply are no plausible mechanisms that fit the data or the current understanding of the earth-atmosphere system that could have produced an as yet undetectable spike in the temperature record analogous to today’s.

As for our understanding of what actually did happen at the end of previous inter-glacial periods, as I said above that understanding is not perfect.  Modeling can explain it roughly as follows: small changes in orbital variations caused the growth of ice sheets in the northern hemisphere.  This increased the albedo (reflectivity) of the earth, a further cooling influence.  Dying forests became permafrost and carbon drawn from the atmosphere became sequestered under the growing ice.  Cooling oceans also began to draw down CO2 levels in the atmosphere.  Falling levels of this greenhouse gas contributed to even more cooling in a self-limiting feedback loop eventually seeing the average global temperature drop around 5 degrees Celsius. The precise timing of all these factors is somewhat murky. It is not a simple story, to be sure, and as such is an opportunity for denialists and contrarians to cherry pick and obfuscate their way to many standard climate “skeptic” talking points.  You can refer to this article or perhaps this one as well.  No one said life was simple!

Today, a process like this would not start for many thousands of years as the Milankovich orbital cycles are not yet aligned in a way to reduce insolation.  Further, if we continue unabated on our current carbon trajectory we may well see the actual end of all permanent ice on the planet, a truly astonishing achievement.  The CO2 we are pumping out today will be here for centuries, even thousands of years, which may be long enough to see the near complete melting of Greenland’s and Western Antarctica’s ice sheets.  I doubt anyone can say with confidence what would happen to the East Antarctica ice sheet.  If it did all melt, who knows when planet Earth would again see permanent land ice?

93 thoughts on “What did temperatures do as the last glacial minimum ended 120K years ago?

  1. The global climate is a very complex interaction of many factors. Even the very simplistic feature we all seem to most focus on, global average temperature, is a complex function of many factors.

    In the case of the long, slow rise from glacial maximum to glacial minimum seen happening roughly every 100-120K yrs in the ice core records, our best understanding is that changes in orbital forcing due to Milankovich cycles began a slow retreat of ice sheets in the northern hemisphere. This set in motion a complex interplay of mostly self limiting feedback effects involving albedo and greenhouse gas forcings and resulted in a total rise in global average temperatures of around 5oC. I *think* the relative weight of these factors is around 40% GHG, 40% albedo and 20% orbital forcing, but that is from memory only.

    No faith involved.


  2. Russell

    Stop with the pretence about trying to be civil and about trying to find answers to your questions – you’re not. And stop with the claims about being a sceptic – you’re not.

    I have already told you what to do if you really want answers to your questions. Stop reading blogs and go away and read some real science. Go to Google Scholar and look for the answers in real science papers written by real experts which have been peer reviewed and published in real science journals.

    All the information is there and has been for decades.


  3. A classic case of “cain’t get no satisfaction” from Russell.

    What do you want, Russell? An alternative paleoclimate? No can do. So we are stuck with what we can *infer* from this one. Which is this:

    ~6W/m^2 forcing and the consequent ~4.5C change in GAT separate and maintain the Holocene from the LGM. So S to 1W/m^2 change in forcing is ~0.75C. I’m sure you can work the rest out for yourself.


  4. coby: “I *think* the relative weight of these factors is around 40% GHG, 40% albedo and 20% orbital forcing”

    this is pure speculation and there is NO scientific proof for this assertion, or can you reference a reliable scientific source?


  5. For people (unlike freddy) actually interested in the answer to his question, I think this is a good starting point:


    One thing apparent in the first figure on that page is that, as I tried to explain to Russel, the different interglacial peaks in the semi regular cycles are not uniform and more detailed analysis suggests that every 400kyr there is a much longer period of warmth. There was one 400kyr ago and the one today is expected to last several 10s of thousands of years.

    Of course, the anthropogenic influences will overwhelm the orbital signal for a long time to come.


  6. If the Holocene is set to be a re-run of MIS11 – and the orbital dynamics do support this – then it kills the old contrarian meme that AGW is “staving off an ice age” stone dead.


  7. I almost feel sorry for Russell; bringing a knife to a gun fight. Cody simplified things though, for his benefit, and made me think when he said that, “I *think* the relative weight of these factors is around 40% GHG, 40% albedo and 20% orbital forcing, but that is from memory only.”

    If we are sitting at a cyclical maximum for temperature, we have the following conditions:

    Maximum CO2 = maximum GHG contribution

    Minimum ice = minimum albedo

    Does orbital forcing overcome 80% of the factors at their most optimum conditions for warming? The only explanation I have is that GHG’s do not contribute as much as theorized or there are other strong factors at play. As stated, it would take a serious leap of faith to get the temperature to turn at face value. Cody may have to keep his mind open to more possibilities, but then he might be in danger of losing HIS religion.


  8. Gordon, the way it works does not require the orbital forcing to overwhelm the entire maximum contributions of the other forcings, it merely tips the balance a bit. This causes those other forcings to begin moving in the same direction. So to describe it as a discreet step simplified process: weak orbital forcings cause the ice sheets to expand a bit, and cause oceans/permafrost to draw small quantities of carbon out of the atmosphere. These small reductions in albedo and GHE further reduce the temperature beyond what the orbital forcing can do on its own. This additional drop in temperature grows the ice more and decreases the CO2 again and so it continues until the ice growth runs into climates that are not cold enough, even if cooled, and the ocean CO2 levels are again in equilibrium with the atmosphere.

    If you want a better explanation than what words alone can provide, (ie better than “the only one you have” hand waving), you have to do some math.

    The link I provided above will provide some of that and get you to all the research papers that have built up this understanding. This one:


    (it’s “coby” BTW)



  9. It is worth recalling that the entire cooling process takes 50K – 90K years depending on the particular cycle you wish to examine. This is because it is much easier to melt an ice sheet than to grow one.


  10. Gordon

    On the issue of whether or not GHG contribute as much as theorised, you might wish to take a look at the following video produced as lecture material by Andrew Dessler, an atmospheric physicist from Texas A&M.

    He makes the point that physics points to a temperature sensitivity of 1.2C from a doubling of CO2, and that this is not a controversial number and it is agreed by everyone who studies the physics of the atmosphere. The issue is feedbacks, and whether or not they are positive or negative, and to what degree.

    In the video he explains that there are some feedbacks which we know for a fact are positive – increased water vapour and decreased albido – which, when factored in, bring the climate sensitivity to around 3C. Then there are others such as clouds and aerosols which we are unable to definitively quantify. Aerosols are almost certainly negative (but to what degree we don’t know), but clouds could be either because they both increase albido and increase infrared absorption. This is where the real debate among is in climate change.

    Orbital forcing obviously exists, but it is over very long timeframes – of the order of thousands of years – and is not a factor in current climate change.


  11. Sorry Coby I’m only about 3 years late on this thread 😦

    Just wondering about Russeli’s question (not sure if you actually explained it or whether it just went over my head?). You explained the mechanism by which past interglacial periods maxed out temperature wise by way of the ever diminishing effect of increased CO2 on temperature and how orbital forcing eventually tipped the pendulum back towards a cooling trend.

    However, I think what Russell was getting at when he talked about “saturation” and “escape velocity” (however improper those terms may be) was why do you think that those same limiting factors that turned around the previous temperature peaks will not restrict our current trajectory (if that is in fact what the concern is)? Is there a danger of reaching a type of event horizon climate wise from which there will be no return like what happened to Venus? (or is the prospect of a naturally limited 5-6 degree temp rise bad enough in of itself?)


  12. Hi Andrew,

    As far as I’m aware, no serious climate scientist believes that a Venus type situation is plausible on Earth. I think this is due to limiting factors such as availability of CO2 (Venus is 95% CO2) and proximity to the sun. Yes, 5-6oC is bad enough on anything less than a multi-thousand year time frame.

    I think the best answer to why the climate will not self-limit in a way/at a level similar to past glacial minimums is this graph from the original post:


    1. Thanks Coby for replying to my late queries. I take your point on the venus comparison.

      So given that there is a temperature lag in response to CO2 increases and a certain degree of thermal inertia, what kind of temperature rise are we likely to get if CO2 concentrations were to somehow stabilize at 400ppm? Is the <2oC target aims of the UN a feasible one or just wishful thinking?

      Also, given the CO2 abundance on Earth, and taking into account other limiting factors, is there a theoretical maximum temperature range that Earth could hypothetically reach?


  13. Stabilizing at 400ppm will not result in warming <2oC, I think that is a scientifically safe assertion. Given the current assessment of climate sensitivity (3oC) to a doubling of CO2 since pre-industrial levels (ie ~560ppm) stopping at 2oC warming would require stabilization in the neighbourhood of 350ppm. We can (theoretically) exceed that level if it is brought down in time and stabilize at 2oC.

    It is often ignored/overlooked that this 3o climate sensitivity only takes into account short term feedbacks (H2O, terrestrial carbon sources and sinks, albedo changes from snow cover and sea ice response). It does not account for long term feedbacks like CO2 out gassing from oceans and melting permafrost (the source of CO2 in glacial-interglacial transitions) or ice sheet melting. Nor does it consider very uncertain things like thawing methane clathrates.

    I don't know much about a "theoretical limit" in modeling, it can go pretty high in the extreme scenarios and very unlikely uncertainty ranges. Geologically, I think there is evidence of possible global climates as much as 8-9oC higher than today.


    1. That’s a sobering thought if the modelling is conservative, especially given that global CO2 emissions are yet to peak.

      Once again, thank you for taking the time to respond to my queries and for sharing your insight on these subjects.


  14. “I think this is due to limiting factors such as availability of CO2 (Venus is 95% CO2) and proximity to the sun.”

    We have plenty of Carbon, but we have plate tectonics and water which Venus doesn’t (and hasn’t for a long time), which ensure that Carbon is locked up in rocks here on Earth and are capable of doing so on Venus.

    For us to get to the same state we’d have to stop plate tectonics at least (the water will eventually boil off as H2O is split into H, O, O, and the H flies off into the interstellar medium.

    If that happens, we’re far more fubar’d than merely AGW.


    1. Haha fubar’d indeed!

      I guess the role that plate tectonics plays in influencing climate is most obvious when it produces large scale volcanic activity which releases previously trapped gases and rapidly affects the atmospheric composition (ie. the flood basalt eruptions of the Siberian Traps at the end of the Permian).


  15. Large scale volcanic activity like the supervolcanoes of half a billion years ago, andrew.

    We have a distinct dearth of those today.

    So our climate change isn’t due to volcanoes.

    And, given you accept that CO2 from volcanoes CAN change the climate if there’s enough of them, the FACT we’re doing 30-60x as much as volcanoes do really just goes to show that AGW is very much acceptable to you.


    1. Yes @Wow I agree totally, I used the super volcano example to highlight the fact that this is the type of natural activity that would cause the type of CO2 increases we’re seeing now. Clearly there’s no equivalent natural process occurring at the moment so the dramatic CO2 spike we’re currently experiencing could only be caused by human activity.


  16. Thing is, deniers won’t agree. They accept the supervolcanoes produce global climate change through their CO2 emissions, but won’t accept that CO2 emissions elsewhere will do so, if that “elsewhere” is “from humans”.

    They don’t see anything wrong in accepting climate changes in the past as evidenced by scientists whilst refusing the evidence that CO2 has a definite effect from human activities.

    It’s like humans produce some magical CO2 that doesn’t do what every other form of CO2 does.

    But it’s probably just they don’t think about it.


  17. Easy answer to wow: volcanoes can influence air temperatures near the surface when they produce dense smoke which sun rays cannot pass. Your obsession with any co2 molecule is irrelevant regarding climate: it cannot be directly observed in an experiment. period.


    1. Yes @Wow, deniers are a strange bunch, they try to use pseudo-scientific arguments to support their positions but usually just end up demonstrating how little they actually know about the science.

      I guess that’s usually because they’re religious and so they already think they have the answer and are just trying to work backwards to justify their belief. No amount of logic or reason will ever convince them because their beliefs are based on emotion, not reason.

      @freddy – The greenhouse effect of CO2 and other GHG’s can be measured and observed, just because the effect isn’t physically visible doesn’t mean anything. Can you see water evaporating?….no but that doesn’t mean it isn’t happening. There isn’t a scientist around who rejects the fact that certain gases in the atmosphere have the ability to trap heat. The only point of contention is from a very tiny percentage who believe that humans aren’t playing a significant role in changing the atmospheric composition, and their reasoning has more to do with their religious beliefs or who’s funding their research than it does with their application of rigorous scientific method.


  18. Andrew, I recommend to you as an obviously ambitious layperson that you study the new handbook of meteorolgy of Möller and Kramm (Alaska) on all the subjects where you show lacking knowledge regarding the atmosphere. Thereafter I promise you that you will have to revise your errors in your copied opinions.


    1. Freddy do you have any links to this handbook? There’s one called ‘Lectures in Meteorology’ by Moelders and Kramm?

      Anyhow, as Wow pointed out, it would be useful to know exactly what errors in my opinion you are referring to? I certainly don’t profess to be any kind of expert on thermodynamics or climate science but I’m interested in the general principles behind the accepted views of the general scientific community.


  19. “@freddy – The greenhouse effect of CO2 and other GHG’s can be measured and observed, just because the effect isn’t physically visible”

    Electricity isn’t physically visible.

    Stick your fingers in an electric socket, freddyboriskaibot.


  20. “handbook of meteorolgy of Möller and Kramm”

    Actually, why?

    There are thousands of texts on meteorology. Tens of thousands.

    But you seem to be missing something, freddykaiboristroll, what it is you think is missing from Andrew’s knowledge.

    As far as you’ve said, there’s NOTHING missing.

    In which case, why should he read anything more than he already has???


  21. “volcanoes can influence air temperatures near the surface when they produce dense smoke which sun rays cannot pass”

    So what?

    It’s already included in the GCMs that indicate the threat of AGW is real and present.


  22. And what handbook are you blithering on about? There is no such handbook.

    Is this yet more indication that there’s nothing new for Andrew to learn,despite your insinuations?


  23. wow wrong: “@freddy – The greenhouse effect of CO2 and other GHG’s can be measured and observed, just because the effect isn’t physically visible”

    You mix up infrared absorption and greenhouse effect. A greenhouse is warmed by blocking convection of warm air. In the atmosphere there is no blocking of convection. You are welcome.


  24. Andrew, here is the link to the handbook coauthored by Prof. Kramm, an eminent scientist in atmospheric physics and micrometeorolgy:

    Prof. Kramm has an estimated 200000 times the knowledge and experience with the atmosphere compared to a Mr. Wow.

    You, as an interested layman should try to learn from first-class scientists in the field of scientific meteorology like Prof. Kramm, instead of listening to other laymen like Mr. Wow.


  25. “You mix up infrared absorption and greenhouse effect. ”

    Since the GHG effect is because of IR absorption, how could that be even possibly true, freddiekaiboristroll?

    “Andrew, here is the link to the handbook coauthored by Prof. Kramm”

    Since you claimed a different authorship, you fail to admit your mistake.

    “an eminent scientist in atmospheric physics and micrometeorolgy:”

    Nah, he’s not eminent. He’s retired. And “believes” the moronic and ridiculous G&T paper, a clear case of wishes blinding him to reality. And meteorology has almost nothing to do with climatology, he’s running well outside his expertise, the expertise of other eminent scientists in similar status who insist AGW is real and present.

    Moreover, you have still failed to explain what it is Andrew has wrong. All you’ve done is try to fluff up his sales.



  26. “A greenhouse is warmed by blocking convection of warm air. In the atmosphere there is no blocking of convection.”

    The atmosphere isn’t a greenhouse. You should stop trying to think that there’s a literal roof to the heavens.

    You’re welcome.


  27. @Andrew, as already said and despite Mr. Wow’s respectless “remarks” I recommend to you to study the handbook coauthored by Prof. Kramm, who is infinitely superior to a Mr. Wow in all matters of atmospheric science. You can only learn from such an expert and not from naked laymen devoid of profound knowledge and experience in a scientific discipline because they did not study it and have not worked as scientists.

    Thank you and best regards


  28. But Andrew has neither got anything wrong, nor have you identified where you “think” he has something wrong.

    All you have is that “something is wrong” and “read this book which I won’t tell you the contents that disprove your errant assumption defined above”.

    IOW fuck all.


  29. There’s only so much room for trees. Planting trees means planting something of the order of 200 billion tons of new growth tree EACH YEAR, and that would only offset what we output, so unless we stop that, we will run out of space for trees.


  30. wow@85 said: “… And meteorology has almost nothing to do with climatology …”

    You cannot believe this seriously, because it is ridiculously wrong. Even J. Hansen and Gavin would object your assertion furiously.


  31. You can believe that seriously, since climatology is a boundary problem and entirely different from meteorology[].

    There should have been a bit of a hint that there are two entirely different academic qualifications for the two enterprises.



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