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The significance of past climate change

Posted on 21 April 2010 by John Cook

A common skeptic argument is that climate has changed naturally in the past therefore humans aren't causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. When I try to explain why to people, I usually get blank, confused stares. I gave a presentation to a roomful of engineers this week and after explaining the significance of past climate change, complete with slides of climate sensitivity PDFs and examples of positive feedbacks, the result was a long, silent pause. I asked if anyone understood what I'd just talked about. A few asked some follow-up questions which made it clear they didn't. So I'm reworking my whole explanation of past climate change in an attempt to make it as clear and simple as possible. Comments, particularly on anything confusing or unclear, are welcome!

In the past, climate has changed, sometimes very dramatically. This has gone on long before SUVs and coal fired power plants. If climate can change on its own, couldn't current global warming be natural as well? To answer this, first you have to ask why climate has changed in the past. It doesn't happen by magic. Climate changes when it’s forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes.

This can happen in a number of ways. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, all the particles suspended in the atmosphere reflect sunlight and the planet cools. These effects are referred to as external forcings because by changing the planet's energy balance, they force climate to change.

Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, we can work out past temperature change, the level of solar activity plus the amount of greenhouse gases and volcanic dust in the atmosphere. From this, we can determine how temperature has changed due to past energy imbalances. What we have found, looking at many different periods in Earth's history, is that when the Earth gains heat, positive feedbacks amplify the warming. This is why we've experienced such dramatic changes in temperature in the past. Our climate is highly sensitive to changes in heat.

What does that mean for today? Rising CO2 levels are an external forcing. They're causing an energy imbalance and the planet is building up heat. From Earth's history, we know that positive feedbacks will amplify the CO2 warming. So past climate change doesn't tell us that humans can't influence climate. On the contrary, the past tells us that climate is highly sensitive to the CO2 warming we're now causing.

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Comments 1 to 50 out of 87:

  1. Few comments on some issues I think should be explained if you want this to be as simple as possible:

    - The role of greenhouse gases in the past climate (or where they come to the atmosphere) might need an explanation.

    - You should explain what positive feedback means and the factors which are positive feedbacks.

    - The part about CO2 being an external forcing might call for additional explanation - I don't think it's readily clear for someone not familiar with the issue.
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  2. I suspect that the argument has to proceed by easy steps. One step is to assert that it is not logical to exclude a new factor, without further examination. Before 1890, there were plenty of reasons for accidental deaths, but being involved in a motor car accident was not one of them - for obvious reasons. But we do not use that as an argument to say that car accidents should be ignored. In the case of CO2, it was not previously present in large quantities (unless you're going back before human history), but it now is and its projected increase in the time scale we're interested in will dominate other factors.
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  3. You don't really believe this do you?
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    Response: It's not a question of belief - this is the state of the science as explained in the peer-reviewed literature. I go into more detail with peer-reviewed references at Climate's changed before - this was an attempt at a simpler, easier-to-understand explanation.
  4. VnceOZ

    Which bit of 'this' are you referring to? And why do you use a word like 'believe'?
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  5. John,

    In the past . . .

    What happened as Surface Temps rose, due to CO2 on occasion, with regards to the rates of convection and evaporation?

    Did they increase, decrease, or stay the same?

    If convection and evaporation rates increased as surface temps increased, did that cool the surface, heat the surface, or did the surface temps remain the same?

    Thanks
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    Response: See, this is the thing. In my presentation to the engineers, I did up this gorgeous little schematic of positive feedbacks, showing increased evaporation, more water vapor causing an increased greenhouse effect, more clouds, etc. Very colourful, I was very proud of it :-) But I think I went into so much detail describing the positive feedback processes and the derivation of climate sensitivity, I think everyone had forgotten the point of the explanation by the time I got to the end.

    The basic point was to say that there are a number of feedbacks - both positive and negative. Finding the net feedback by adding up all the individual feedbacks is a complex job. But you can cut through all that in one fell swoop by looking at past change. By just comparing temperature change to changes in the energy balance, you determine the net feedback without having to know all the individual feedbacks.

    So I skipped all those details in this blog post in the effort of a simpler argument. But going through the explanation too fast leaves people wanting more details. Where to draw the line?
  6. Every time I hear the "Climate changed naturally in the past, so humans can't be responsible this time around" I always respond by saying "so does this mean that 'because many forest fires start naturally, humans are *never* responsible for forest fires?'" The comments are equally nonsensical, yet it never ceases to amaze me how often the Contrarians try & push this illogical position!
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  7. I think its important to mention the speed of climate change today vs the past.
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  8. I agree with pretty much all of what you say above except for one major point-the issue of the time scales involved.

    The past geological record indicates that changes on a global scale are invariably very slow. We're talking tens of thousands, to hundreds of thousands and even millions of years in the vast majority of cases. For something like acidification of global oceans by volcanoes, and indeed the majority of mass extinction events (except in rare cases of eg bolide impacts), that is the sort of time frame the geological record indicates is invariably required.

    To take a few examples, the output of greenhouse and other gases by Siberian Traps volcanism at the end of the Permian occurred on a major scale over several hundred thousand to several million years, and that is how long it took to cause major climate changes such as (possibly) acidifying the oceans, the collapse of coral reef ecosystems, and mass extinction. It was not a ‘rapid process’ when compared to the scale of human lifetimes. The break-up of the Gondwana supercontinent is likely implicated in eg the Mid-End Triassic mass extinction-continents do not break up 'rapidly'. This occurred over millions of years - with possible stress-related tipping points etc, as rift-related volcanism increased over very long time periods. Many other examples from the geological record indicate much the same thing, (eg oceans don't ‘acidify’ within short human timescales when similar amounts of c02 have been added to the atmosphere in the sort of time scales involved as is currently the case).

    This is also the major reason people such as the gradualist Charles Darwin were so skeptical of the presence of 'mass extinction' events in the geological record in the first place, and also I suspect why the person on the street is with current climate change and the extreme predictions around it as well. It's like worrying about continental drift changing the climate within this century.

    This is one of the major skeptical arguments, that major global changes such as those predicted by the IPCC to occur within the next century by human emissions of greenhouse gases will NOT occur, not so much because the concepts and theory is wrong, but because the scale of time involved is far too short, and that the geological record provides very good support for this contention. Academics and other pro AGW advocates get the concepts largely right, but get the time scales involved largely wrong.
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  9. I would say that in my experience the key point to get across to sceptics is that during past climate changes there were no humans around to experience the effects. Consequently from today's human viewpoint past changes do not seem such a big deal. But the truth is they were huge. Although climate change of three, four degrees or more over tens of thousands of years would allow most plant and animal species to move, adapt or evolve to cope with that change; the more serious and/or sudden climate changes would cause huge extinction events. The problem, in explaining this, of course, is that such events seem no big deal to us today, looking back over millennia. So what, if large mammals -- which we're aware of only through scant fossil evidence -- once died, to be replaced by other equally unfamiliar species?

    On the other hand the current rapid Climate Change -- when humans are now one of our planet's larger animal species -- is a highly risky for both us and most of the other living species; though as an extinction event it will probably be no worse than many in the past. Arguably it's made worse by the unprecedented fact that humans are responsible for creating the problem this time.

    It would be interesting to conjecture (is it possible to know for sure?) the extinctions and changes to the nature of life on the planet as a result of past climate changes, and then extrapolate how the current climate change being instigated by humans could alter the nature of life on the planet today.

    Of course one of the problems in doing this is that global heating is only one of many serious issues now being caused by human influences on life's existence on our planet. Pollution; deforestation; industrial agriculture; fresh water diversion; resource depletion; over-population -- to name just some of the most obvious -- are all key components in the environmental timebomb we're creating.
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  10. It may seem a bit off-beat, but I personally think paleoclimate more often than not is about red-herrings.

    While a number of false arguments try to use paleoclimate to deny AGW, I must admit I have never been convinced that the paleoclimate record is terribly useful to the debate. Fundamentally, there is nothing it gives us that is not more clearly communicated by contemporary data.

    For example, the "Hockey Stick" shows that the contemporary rate of warming diverges enough from the last few thousand years to raise a smoking gun type of question, but the graph itself is not an explanation -only a way of flagging an anomaly.

    Even if the hockey stick was proven wrong, it wouldn't matter - because arguments about medieval warming, past CO2 levels, higher ancient temperatures etc. all miss the point:

    In the contemporary world, we can identify the rate of global warming, the mechanisms of warming and the predicted (and confirmed) effects.

    Until there is a better explanation for the colossal body of evidence of the last 100 years, the opponents of the AGW thesis will try and drag our attention into the distant past, where the ground under any given argument is weaker.
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  11. Thingadonta-how can you make such claims when global warming of +0.6 degrees has *already* occurred over the past 60 years, with nearly +0.5 degrees of that warming occurring in just the past 30 years?!?! The fact that virtually *every* natural climate change event in the past (with the exception of those caused by extremely massive natural disasters) has occurred at a rate of between 1/2 to 1/10th of the rate of recent change is simply further proof that nature is not the cause of the most recent climate change event. What humans are doing, by burning the accumulated & compressed CO2 storage of tens of millions of years of tree & plant life, is to effectively compress millions of years of geological activity into the space of just 250 years. To suggest that geo-engineering on this scale could *not* be the cause of rapid climate change simply shows how *weak* the contrarian argument actually is!
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  12. re #6 and #9:

    I get that argument sometimes and my response is that it wouldn't matter if there was either a more rapid or larger warming at some point in the distant past - there weren't billions of people living near the sea then. "The Planet" is not endangered - it is a small percentage of the planet's flora and fauna, plus a big percentage of its human beings. Of course, it's like asking for anesthetic at the dentist and being told you shouldn't have it because the dentist's grandfather had to suffer more than you!
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  13. I think part of the problem is that you are really conflating two different issues... which if separated might be phrased as;

    1: 'Climate has changed in the past so how do we know humans are causing it now?'
    2: 'How do we know that the climate change we are causing is going to be significant?'

    I think you can really handle the human causation question using CO2 alone (past correlations of CO2 and temperature, satellite and ground measurements confirming energy imbalance in CO2 absorption spectra, various proofs of increased CO2 levels being human caused, et cetera)... possibly following that up with explanation of why it ISN'T the Sun, cosmic rays, volcanoes, martian death rays, or whatever. All of which can be handled by showing lack of correlation - some match past changes, but none match the current.

    Once you have established CO2 as a major 'control knob' in determining temperatures then you can get into feedbacks amplifying that effect and what the historical record tells us about those.

    Basically, handle one issue at a time. Don't start talking about positive feedbacks before they've bought into CO2 increasing temperatures at all.
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  14. How about this:

    Rather than list all the possible primary drivers (forcings), start by describing a couple of periods when the climate changed in the past from different forcings. For example, Milankovitch and volcanoes as the initial forcing.

    For each of the two periods in turn, describe in simple terms what the earth was like before the change (eg ice free Antarctica) and what it was like afterwards (eg lots of ice everywhere). Describe the forcing that led to the change (and define the terms - engineers most probably use slightly different jargon from climate scientists).

    Then move to today and show the evidence that the climate is changing (temperature, ice, oceans, sea levels etc). And explain that the forcings that caused the prior changes you discussed are absent, and the only primary driver or initial forcing operating today is CO2 / greenhouse gases.

    Then show how greenhouse gases also acted as a positive feedback, amplifying the changes you discussed in the two first examples (warming or cooling).

    You can deal with the 'but couldn't it be due to ...' in the question time / discussion - keep your detailed slides on hand for when specific questions are asked.

    If there are particular points you want to elaborate on (eg role of water vapour/clouds which some often ask about), you can probably do this during the discussion by saying, 'someone often asks me xyz' and then whip out your extra slides :).

    (As a broad (over?) generalisation, engineers are often clever, but their thought processes tend to be linear rather than lateral. Simple works best. One step after another, with a bit of repetition and reiteration of key points along the way. No offense meant to engineers, it's just an observation from working with groups of engineers in a variety of contexts.)

    The following publication is rather good, and will no doubt be more useful than my off the top of the head idea: CRED guide
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  15. I concur with #13. I think the first principle is explain what is causing warming now, then climate history comes in to a supporting role.
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  16. An excellent presentation of past climate change is available online: Richard Alley, "The Biggest Control Knob: Carbon Dioxide in Earth's Climate History" at the Fall AGU meeting, see http://www.agu.org/meetings/fm09/lectures/videos.php To be sure this is for a scientist audience but the main points can be adapted for others... admittedly not easy. And of course, bear in mind that people have a hard time grasping knowledge that conflicts with what they want to believe!
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  17. The thing with this line of argument that your tackling is that it fails basic logic: Human activity of course didn't cause past climate changes, but that’s no evidence that it doesn’t now.

    Try that line of argument in a court of law against a arsonist, by saying that forest fires have always happened naturally; it won’t fly.

    Indeed, GHG had an important role to play in many past climate changes (even though their concentration changed without human involvement). Looking at the past actually strenghtens the evidence for a climatic effect of GHG.
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  18. I think you have the facts well explained, but there are a lot of them. Is an analogy helpful? Eg:

    Climate change history is like precedent in law - you have a good chance that a previous finding or two might establish where things stand 'when deciding subsequent cases with similar issues or facts'.

    Yes; the climate has always changed, man-made climate change is just a new precedent. Looking at 'precedents' or past forcings helps us with what might come next.
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  19. I tend to find the non-scientific mind has a poor conception of order of magnitude and probability. For example they are quite happy to imply that climate change happening now is natural by referring to climate change during an epoch perhaps hundreds of thousands or millions of years ago.



    All indications are that climate hasn't changed significantly for the best part of 10,000 years, so for it to suddenly change in the last 150 years just at the same time as mans influence on the biosphere has become significant, strongly implies this is the most likely cause. This is without any knowledge of greenhouse gases or other causes, it is a pure statistical explanation. The other more direct experimental evidence such as radiation exchange simply increases this probability further.
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  20. It is a temperature reconstruction from a Greenland ice core (close to summit, at 72.6 N, 38.5 W, altitude 3200 m) for the last fifty thousand years.



    As you can clearly see, temperature is much more variable when it is cold. In other words: climate sensitivity diminishes with increasing temperatures.
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  21. Berenyi Peter, I suspect you say stuff that you know is wrong or illogic (but you say it anyway).

    (1) The Dansgaard-Oeschger events in your graphics are measurements of rapid temperature changes in Greenland cores. They are barely recognizable (and asynchronous) in Antarctic cores. Thus they are not representative of global warming/cooling events.

    (2) In fact these events are recognised likely to arise from major rapid shut down and restarting of the major ocean circulations (thermohaline circulation) that strongly participiates in bringing heat (or "thermal energy" if we're stil being pedantic about that!) from the equator to the high Northern latitudes.

    In other words they are indicative of major redistribution of Earth heat as opposed to global scale warming or cooling events.

    (3) Thus they have nothing to do with "climate sensitivity" which is the equilibrium response of the Earth's global temperature to changes in radative forcing.

    (4) The fact that D/O events (and similar large scale jumps/drops in high Northern latitude temperatures) aren't apparent in the Holocene part of the record you posted, is that these events only occur under conditions that major ice sheets occupy the high N. latitudes (that's part of the likely mechanism of the D/O events). Since we're in an interglacial period when these major ice sheets have melted away, we can't have D/O events.

    It's got nothing to do with temperature "being much more variable when it's cold" or "climate sensitivity diminishing with increasing temperature".

    One should really address these issue in terms of what we know!
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  22. I assume this is intended to replace the text on the “Climate’s changed before” page. It seems like a good explanation to me – but then I understood the explanation on the original page (at least I think I did). I also think it’s worth including all the climate sensitivity info that was in the old version. I recommend that you keep your more detailed explanation and link to it from the new version.

    Also, is it really possible to derive solar activity from ice cores? I’m sure there are other proxies (carbon-14 comes to mind), but ice cores?
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    Response: The more common method of working out past solar activity is using radiocarbon data from tree-rings. But Beryllium isotopes from ice cores are also a proxy for solar activity, going further back than tree-rings.
  23. chris #21

    I would add in your response to Péter that this interglacial is remarkably stable when compared to the previous ones. So his assertion "temperature diminishes with temperature" has little supporting evidence.

    Besides, some events like PETM support a self-reinforced warming hypothesis.
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  24. nautilus_mr #10

    I agree with you. Paleoclimatology is just one of the many lines of evidence to climate sensitivity. And if you pick one specific period -like the Medieval Warm Period- it's just one tiny fraction of this.

    So even if there had been a strong MWM... not much would change in the science.
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  25. My first thought was - do positive feedbacks kick in as strongly when the energy balance shifts towards cooling? My initial guess is "not so strongly", or, "negative feedbacks come into play more", judging by the slow cooling period compared to warming in the late quaternary ice ages. I don't know if covering this would dilute the message, but I think it would help give a more balanced assessment - skeptics scorn commentary that only talks about warming, for example.

    And I don't know if this goes beyond your intent, but it might be worth mentioning the rate of change now compared to past warming events.

    What you already have, though, is pretty clear I think. I'm not a scientist or an engineer, but it makes sense to me. (I am a fairly well-read layman, though)
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  26. Seems like the question "why hasn't the climate changed over the millennia ought to garner some interest as well. It's been stable enough to grow stuff for a long time. Don't need esoteric statistics to see that.
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  27. "See, this is the thing. In my presentation to the engineers, I did up this gorgeous little schematic of positive feedbacks, showing increased evaporation, more water vapor causing an increased greenhouse effect, more clouds, etc. Very colourful, I was very proud of it :-)"

    Do you have a link to this schematic? I am an engineer and would be pleased review it as well. I find your answer lacking since it has been dumbed down. It is, with all due respect, meaningless in 'simplified' form.

    What I am especially interested in is the detail of the positive feedbacks and whether you have satisfied the negative feedbacks sufficiently.

    Regards
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    Response: Here's the slide I showed at the talk. Note - its purpose was just to introduce the concept of positive and negative feedbacks - that warming initiates a series of climate responses. It was not about accounting for every individual feedback (sea ice feedback is noticeably absent) - the general gist was to communicate that there are a myriad of different feedbacks which makes it difficult to work out the net feedback. But the way to cut through all that and calculate the net feedback without having to worry about the individual components was to look at past climate change.


  28. If you don't mind my asking, do you know what kind of engineers you were presenting to? In general, different engineering disciplines have different levels of understanding of advanced math, statistics, physics, chemistry, and so on. So what works for a room full of electrical engineers won't necessarily work for a room full of software engineers, mechanical engineers, nuclear engineers, or chemical engineers.

    Unfortunately, that means you'll need a few different presentations instead of just one, and you should ask the group that's sponsoring your presentation what the audience is likely to be before (and maybe ask the audience too and tailor your presentation accordingly).
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    Response: They were mechanical engineers but let me be clear that I'm not commenting on their ability to understand past climate change but on my ability to explain it. The blank stares I receive are fairly universal. A university group once borrowed my info to write a short debunking flyer and I noticed they went with a completely different answer to the 'climate's changed before' argument - presumably they either didn't like or didn't understand my explanation (or thought they could do a better job). Hence I'm going back to the drawing board.
  29. john, as usual, a great post that is thought provoking and a joy to read.

    what was the reason for the MWP? there seems to be a new paper on this topic that points to this happening in the Indo-Pacific so it appears this event wasn't localized to Greenland. were we in a solar optimum at that point? i've heard the MWP debunked as a localized event but there are other papers such as this that show it happened other places on the Earth at the same time.

    I've also seen graphs of vostok ice cores showing we are due for another glacial period and i'm surprised i've never seen comments related to the fact that maybe we humans are counteracting that normal temperature decline wiht our GHG production. the up shot being, even if you subsribe to the AGW theory, it might not be a bad thing for the next 1000 years or so since plus 1 to 2 degrees C is easier to adapt to than negative 8 to 10 degrees C.

    forgive me if this has been already discussed ad nauseaum.
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  30. I think one important aspect missed in your discussion is the distinction between internal and forced variability. I think engineers, in particular do get that. You make it sound like all climate change is "forced" by some external mechanism. But it is not. There is internal natural variability that causes ups and downs and those are much more difficult to understand. The challenge in climate science is to separate those two and examine whether the "forced" variability is much greater the expected internal variability over the time or spatial scales one is interested in. Daansgard-Oeschger might stand as an example for internal variability that is indeed larger than than recent variability, but the mechanisms involved are tied to glaciated periods and the spatial scale is not global.

    Another aspect, I have found difficult for engineers and many other scientists to grasp is: How a system where all feedbacks sum up to a net "positive" does not constitute a run-away system. This is not and easy thing to explain and typically requires too much time in a short talk. But may constitute a stumbling block for engineers in particular.
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  31. Hi John,
    You may already be familiar with this presentation in June 2008 to the Tällberg Forum by David Wasdell: Planet Earth - We Have a Problem
    In case not, you might want to review it - he does an excellent job on feedbacks, and you may find some ideas therein.
    I find this an excellent presentation, not only for it's content, but also for the passion with which it is delivered.
    Thanks for all your excellent work!
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  32. garythompson,
    you should always refer to the original scientific papers. The link you provide on the MWP has been "adapted" to hide the rise and erroneously draw to the conclusion that "Medieval Warm Period was about 0.4°C warmer than the Current Warm Period.". Nothing similar can be found in the original fig. 2b. On the contrary, it explicitly shows that the average 1997-2007 SST is higher than any other period in the last 2000+ years.
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  33. i'm surprised i've never seen comments related to the fact that maybe we humans are counteracting that normal temperature decline wiht our GHG production

    I have, and not just from the skeptical optimists. ;-) The next ice age isn't due for tens of thousands of years. The earth has cooled by about half a degree C since the end of the climb out of the last glaciation 10k years ago. We've countered that and more within a century. Our immediate concern is the next hundred years or so.

    Here is the issue I suggested earlier regarding rate of change. We can adapt to long-term slow changes much more comfortably than relatively fast changes in the near-term (relatively speaking). Perhaps in 10 000 years, if we haven't warred ourselves to extinction or succumbed to a devastating plague, we may be knowledgeable enough to fashion some kind of thermostat for the planet that doesn't interfere adversely with long-term, possibly necessary climate changes. We're not yet wise enough to deploy any form of geo-engineering.

    i've heard the MWP debunked as a localized event

    The language is too strong. We think the Earth was generally warm, but datasets all over the world show 'medieval' warmth at different times, as much as 500 years apart. And most of the data we have is from the Northern Hemisphere. The MWP may or may not have been a global event, but it would seem there is some evidence for that. Whether or not the warmth for some sequent decades in the past was comparable to the last few decades is the qualified assessment most discussed (probably not).

    Here's a map of data sets often deployed by skeptics. Check the warm dates for each of the time series. Ironically, skeptics don't realize that they're buttressing the 'not global' argument when they reference this - they don't investigate much further than the message.

    http://pages.science-skeptical.de/MWP/MedievalWarmPeriod.html

    The map, by the way, documents a small number of paleo data sets (47). There are now hundreds. No doubt these have been selected to buttress the message. Ironic then...
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  34. Gary Thompson, it has, by GallopingCamel. The next glacial would settle over the next 20000 years or so, which is way more than enough for humans to devise solutions or exterminate themselves.

    See this thread or this paper
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  35. Most engineers would understand the concept of 'signal conditioning'. If you put a signal of a certain amplitude into the box, what amplitude do you get out of the box? Likewise, if you put external forcings of certain amplitudes into the earth-box, what temperature amplitude do you get out of the box? If the amplitude out of the box is larger than you'd expect just on the basis of the input forcings, then the box is a amplifier: it contains internal reinforcings that are more positive than negative. What they are, exactly, is unimportant and maybe even confusing to discuss initially. After the point has been driven home, that earth is an amplifier, you can go into the details about what those mechanisms are that climatologists have discovered: albedo, methane, water vapor, etc, that make it so.
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  36. thanks Philippe (#34). i enjoyed reading the comments on the skepticalscience thread from gallopingcamel and i did print out the paper you referenced and i'll take a look at that for a description as to why we aren't due for another glacial period now.

    #32 Riccardo, that figure on CO2 science appears to match the figure 2b in the paper. the graph in 2b does show temperatures during the MWP that are above modern temperatures. here is the graph from the Nature website. what rise are you talking about and why would you call the graph on the CO2 Science website a lie? i didn't read the article on the Nature website but instead just went to the graph that you referenced.

    thanks all (barry at #33 too) for the quick feedback - just another reason why this is the place to come when I have questions.
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  37. garythompson,
    in the original fig.2b the line labeled "1997-2007 mean annual SST" is higher than any other line for the whole period shown. So, when co2science says that following the paper it was 0.4 degree warmer, it is blatantly false.
    You may be right that this is not a lie, they possibly can't read a graph.
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  38. garythompson - Mann et al 2009 for more on the latest data and analysis of MCA, including causes. All data and programs available in supplementary materials so you can reproduce their results.

    thingadonta- rates. The "Heinrich" events as we moved out of last ice age look to have dramatic temperature swings in the scale of decades. However, we have no reason to believe that such swings are feasible in an interglacial from natural causes.
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  39. Being engineers I’m sure at least one asked to see a graph of atmospheric CO2 concentration verses temperature for our planet. Since such a graph does not exist how were you able to explain that to your audience?

    You could point out that over a very short period of time mankind has removed and burned an enormous quantity of fossil fuels that had been comfortably buried for, roughly, the past 500 million years of earth history. I think if you can graphically show an estimate of how much fossil fuel by weight has been burned since the industrial revolution and compare that with the rise in atmospheric CO2 concentration over the same period, it might convince some. Past climate change is used to help us understand what changes we will likely see for the future. When climate has changed in the past scientists have struggled to understand the cause. This time the cause is obvious.

    One more question. I did not know that ice cores could reveal solar activity. What in the frozen ice gives us that information?
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  40. "One more question. I did not know that ice cores could reveal solar activity. What in the frozen ice gives us that information? "

    C14 and Be10 production are proxies for solar activity. I believe Be10 is used in ice core.
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  41. "On the contrary, the past tells us that climate is highly sensitive to the CO2 warming we're now causing."

    I would suggest not using words like "highly". That word has little scientific value.
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  42. scaddenp,
    Thanks for the help on that!
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  43. Keenon350: I would be very wary of David Wasdell. He is an odd example of someone who uses denialist logic and tactics on the opposite side - to claim that climate scientists are all underestimating the scale of the problem and we are all going to die tomorrow.

    He has repeated classic denialist rubbish in the past while putting the opposite spin on it - like that climate models don't include the role of water vapour (utter nonsense). He also frequently says misleading things about his CV to suggest that he is a climate scientist when in fact he is a psychotherapist.

    After he accused the IPCC of political corruption in the New Scientist (or rather Fred Pearce did on his behalf, quoting him) a letter "From the co-ordinating lead authors of Working Group 1 of the IPCC Fourth Assessment Report" signed by 20 major climate organisations was posted in protest, pointing out major falsehoods in Wasdell's claims.

    It can be viewed here: http://www.newscientist.com/article/mg19325960.900-climate-with-care.html
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  44. Few comments.

    It seems fairly straightforward to me but I supposed I've been following the science for a year or so. As you say it depends on audience and what you can assume about them.

    One thing that jumped out is that you switch between energy/heat/temperature alot. You could think about just talking about one of them. I think the clearest is talking about energy building up in the system. I don't know whether by sticking to one term it makes the ideas flow better. With one line to at the start or end to say that energy build up = rising temperatures.
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  45. RE#36 garythompson, #37 Riccardo

    Two things:

    From eyeballing the adapted "CO2 science" graph (with my ruler on the monitor I get 0.27 deg warmer in the MWP than today based on matching the red peak to the blue peak so I don't know how they got that.

    But when I quickly read over the journal paper as far as I can tell it was incorrect method anyway to interpret figure 2b or at least to compare it to modern times as they author's state a few things:

    ...reconstruction suggests that at least during the Medieval Warm Period, and possibly the preceding 1,000 years, Indonesian SSTs were
    similar to modern SSTs....Contrary to the Indonesia SST reconstruction, however, the Northern Hemisphere temperature reconstruction does not estimate temperatures as warm as modern at any time during the past two millennia.


    The author's then continues to say: We note that the high-amplitude variations resulting fromthese hypothesized changes in G. ruber seasonality also preclude accurate estimates of the rates of SST change in the past and a meaningful comparison to the rate of SST increase during the past
    decade.


    Reading this, makes me instantly skeptical of the claim at "CO2 Science" that the Medieval Warm Period was warmer than the Current Warm Period. I infer that they intentional (not just that they can't read graphs) with their lack of transparency over how they interpret the science.
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  46. CO2 science has something of history in misrepresentation of science papers by selective quoting. I think you can get a list of other examples by putting "CO2science misrepresent" into google and follow links sounding like annoyed authors.
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  47. Everything in the "dumbed down" talk sounds good and true, but it seems to miss the point. The skeptics, for example, could immediately make it all SOUND meaningless by pointint out that in the past (was it Late Triassic? I forget), we had MUCH higher CO2 levels, but rather small climate change due to it.

    This would APPEAR to overrule the presentation's observations that 1) climate changes took place in the past due to forcings and 2) CO2 is a forcing.

    What is the right approach to this? My understanding (I would certainly welcome Cook's corrections) is that the degree to which the climate is sensitive to forcings has itself also been highly variable. Specifically, that with the Late Triassic configuration of continents and corresponding ocean currents, even a rather large CO2 forcing had a surprisingly small effect; but with modern ocean currents, it is well-expected to have a MUCH larger effect.

    So if that understanding is correct, then it certainly should be included in the presentation for engineers.
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    Response: Funnily enough, this was one of the questions I was asked at the presentation - "are you saying climate sensitivity was lower in the past?"

    On the contrary, climate sensitivity has been surprisingly consistent in the past, even going back millions of years. So if someone asked about CO2 being much higher in the past (noone did bring that up at the talk), I would say that the sun has been getting steadily brighter over Earth's history. Millions of years ago when CO2 was much higher, solar output was lower. The combined effect of sun and CO2 show good correlation with climate. For more details, see CO2 was higher in the past.

    In fact, it's because the sun was cooler in the past that CO2 was so much higher. CO2 acts as a natural thermostat for our climate, regulated by rock weather which is the process of removing CO2 from the atmosphere by chemical reactions. When it's cooler, rock weathering activity slows so there's less removal of CO2 from the air. This means CO2 builds up, warms up the planet. Then as it gets warmer, rock weathering activity increases which removes CO2 out of the atmosphere.

    This process is a natural way of keeping our climate within a certain temperature range. It means that if the sun was cooler, temperatures get cooler so rock weathering slows down, increasing CO2 levels. This is a fascinating process but of course, I didn't go into that much detail at the talk - I just mentioned that climate sensitivity has been consistent in the past.
  48. Oh, and about understanding thermodynamics: I had a VERY long argument in Topix with somebody who clearly had an engineering background, yet believed that EVERY climate scientist claiming AGW was relying on violations of the First Law of Thermodynamics (occasionally he claimed 2nd too) in their understanding of the energy balance. He even claimed to find this flaw in Trenberth's famous diagram.

    But when I turned to the diagram (and Trenberth's original paper, whose title I forget), what I saw was something very different: I could not find evidence of any such violation (of course), but I DID find evidence of some very disappointing mistakes in presentation that made it difficult to verify that the First Law is satisfied by his numbers.

    So what is the implication of all this for this article? I would say two things 1) do NOT assume a deep understanding of Thermodynamics, not even from engineers 2) make sure that quantities, variables, scales etc. are CLEARLY marked in the diagrams, and that simple questions like which way how much heat/energy flux flows are easily visible on the diagram. Diagrams like the one in post#27 in this thread are damn near useless.
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  49. John, this mechanical engineer had no problem understanding either your previous explanation, or the new simplified one.

    As you've mentioned, though, perhaps the issue is that you got bogged down in the details. A different order of presentation may be required, where you present a simplified overview, then explore a few areas in more depth, perhaps having some of those detailed explanations in reserve to answer questions.

    Also, from personal experience - if you were presenting to a bunch of students, then the 'blank looks' & shortage of questions aren't that uncommon while people are digesting the information just presented.

    Explaining AGW in terms of radiative heat transfer, though, is something any fourth-year engineering students should understand (heat transfer was 3rd year subject when I studied at UQ, but that was nearly 20 years ago...)
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  50. Petroleum reservoir engineers construct numerical models to forecast the production from an oil or gas field. They calibrate their models against recorded production rates in a process they refer to as a "history match". See, for example, this abstract which would, I guess, look familiar in many methodological aspects to climate modellers. So, if you were presenting to a bunch of petroleum engineers, you wouldn't have much problem explaining why palaeoclimate was important in calibrating forecast models.

    I think a major problem with people's misinterpreting the significance of palaeoclimate is the naturalistic fallacy: what's natural is good for us, or the Nietzschean corollary (recently popularized by the Koch brothers in the Smithsonian exhibit)that what doesn't kill us makes us stronger. What the general public doesn't usually understand is the extremely rapid rate of current changes compared to past changes, as well as the fact that the most extreme palaeoclimates would not permit the survival of our current civilization.

    There's also the logical fallacy that bverheggen noted (about past and present causes) that seems so elementary (to me) I'm amazed that people keep making it. Nevertheless, I've lost count of the number of times that I've used the forest fires/arson metaphor.
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