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What we can learn from studying the last millennium (or so)

Filed under: — mike @ 15 May 2010 - (Español)

With all of the emphasis that is often placed on hemispheric or global mean temperature trends during the past millennium, and the context they provide for interpreting modern warming trends, one thing is often lost in the discussion: space matters as much as time. Indeed, it is likely that the regional patterns of past climate changes, rather than simple hemispheric or global mean temperature trends, will best inform our understanding of the dynamical mechanisms involved. Since much of the uncertainty in future projections relates to regional climate change impacts, it makes particular sense to focus on those changes in the past that involve regional changes and the underlying mechanisms behind them.

For instance, melting of the cryosphere (and consequent rises in sea level), subtle shifts in drought and rainfall patterns, and extreme events, are all regional effects that could be important threats to ecosystems and our environment. Such changes are often associated with phenomena like ENSO or the North Atlantic Oscillation. Yet there remain large uncertainties about how such mechanisms will respond to anthropogenic climate change.

There are a number of potential ways forward to improve our understanding. A first step is to look directly at the time-series of specific systems (like the ENSO index or the ocean temperatures in the North Atlantic) and try to extend them as far back as possible using proxy data. This gives more information on what the natural variations in these phenomena look like, and thus a better idea of how big a forced response would need to be before it could be reliably detected. Secondly, we can look to see if there is a relationship between various natural drivers of climate change (volcanic eruptions, solar variability or orbital forcing say) and any characteristics of these phenomena – amplitude, frequency or duration. Do volcanic eruptions appear to affect El Niño for instance?

For phenomena that need annual or decadal resolution data to be resolved, the last millennium or so is an obvious (and only) time period to be looking at for it is only for that period that there is sufficient paleo-data coverage of high enough temporal resolution. Other periods – such as the mid-Holocene 6000 years ago – are also useful, but the results are more long-term in nature (there is also a discussion of the value of different periods for reducing future projection uncertainty in this recent paper).

There are a number of different approaches to looking at reconstructions in recent centuries – some rely on high density regional networks (as seen in this recent paper by Guiot et al concerning European temperature trends for which they mostly used pollen data) and some rely on wider networks of more diverse proxies which aim to capture longer-range correlations to specific phenomena (such as the recent Mann et al (2009) paper).

When this is done, people usually find that while it was relatively cool in global mean temperatures from the 1400s to the 1800s known as the “Little Ice Age” and relatively mild in the 900s to 1300s interval ( sometimes termed the “Medieval Warm Period”). But the spatial reconstructions reveal, however, why such global terms can be quite misleading, and why alternative phrases such as the “Medieval Climate Anomaly” are being increasingly favored by the community. This latter terminology recognizes that while the interval displayed significant climate anomalies, they varied greatly, even in sign, from region to region. Many of the more profound climate anomalies, moreover, involve variables other than temperature, such as drought, rainfall, and atmospheric circulation. Though the medieval period is seen to be modestly warmer globally in comparison with the later centuries of the Little Ice Age (the peak global mean warmth is likely comparable to mid, but not late, 20th century warmth), some key regions appear to have in fact been colder, while other regions appear to have been warmer. Southern Greenland, for example, appears within uncertainties to have been as warm as today. However, much of the tropical Pacific was unusually cold, suggestive of the La Niña phase of the ENSO phenomenon (a similar conclusion was reached by Trouet et al (2009)). Thus even though some locations may have been as warm or warmer than today, the hemispheric mean appears not to have been.

Why does this matter? It matters because there are plenty of factors that can affect the overall mean temperature (solar variability, volcanoes, greenhouse gases, internal variability etc.) and so it is hard, given the uncertainties in the solar or volcanic reconstructions to precisely attribute the paleo changes in the global or hemispheric mean to these factors. But if we can look at more complex fingerprints of the changes, it might be possible to be more quantitative in those attributions since the spatial fingerprints of the different factors are easier to distinguish. Furthermore, if we can clearly tie the regional patterns to the different forcings, we might be able to use that information to inform regional projections under future conditions.

Thus we can basically say that the warmer conditions of the Medieval era were tied to higher solar output and few volcanic eruptions and the cooler conditions of the Little Ice Age resulted from lower solar output and more frequent volcanic eruptions. But these drivers appear to have had an equally important, though more subtle, influence on regional temperature patterns through their impact on climate phenomena such as ENSO and the North Atlantic Oscillation. The modest increase in solar output during Medieval times appears to have favored the tendency for the positive phase of the NAO, associated with a more northerly jet stream over the North Atlantic. This brought relatively greater warmth in winter to the North Atlantic and Eurasia. A tendency toward the opposite negative NAO phase helps to explain the enhanced winter cooling over a large part of Eurasia during the later Little Ice Age period.

There is some model support for these patterns (see also instance Shindell et al, 2001) when the models include interactive ozone photochemistry to produce this dynamical response to solar forcing, but it is not captured in a simulation of the NCAR CSM coupled model which lacks those processes. Neither model simulation reproduces the apparent La Niña pattern seen in the Medieval temperature reconstructions:

Figure 1: Spatial pattern of mean temperature difference between the MCA and LIA periods (defined at the intervals AD 950-1250 CE and 1400-1700 CE respectively) compared with simulations of two different climate models forced with estimated differences in natural (volcanic and solar) radiative forcing between the two periods (Mann et al, 2009).

Other model simulations, however, using a climate model that exhibits a particular tropical Pacific mechanism, do reproduce such a response. In such models, the tropical Pacific counter-intuitively tends to the cold La Niña phase during periods of increased heating, such as provided by the increase in solar output and low volcanism of the Medieval era. If this response holds for the future, something that is still vigorously debated, it could imply a more La Niña-like response in the future. Most of the state-of-the-art climate models, e.g. those used in the IPCC Fourth Assessment, by contrast, suggest the opposite–a more El Niño-like future climate. The credibility of the models with regard to this phenomenon is not high, however, and lots more work is going to be needed (both on paleo-reconstructions and model improvements) before we can be confident in the future projections of changes in ENSO-like dynamics and mean state.


690 Responses to “What we can learn from studying the last millennium (or so)”

  1. 651
    Completely Fed Up says:

    “no one, not even the folks who believe there is a link between solar cycle length and global temperature believe it’s the earth’s atmosphere which has an influence on the sun.”

    Then your argument was threaded the wrong way.

    Your version had the effects of earth temperature on the length of this solar cycle as the driving position. Why else is it that you are concerned that the link between cycle length (as opposed to solar output) and global temperatures is being ignored?

    After all, solar output is an INPUT to the global models. This makes it rather hard for a GCM to ignore solar output…

  2. 652
    manacker says:

    David B. Benson

    I have looked at your analysis of decadal temperature changes versus CO2 concentrations based on the Barton Paul Levenson analysis, which you cited. As you state:

    From well understood physics, we suspect the logarithm of CO2 concentrations (lnCO2) explains most of the variation and indeed from a standard correlation
    http://bartonpaullevenson.com/Correlation.html
    this is confirmed. Now we want to explain the GISTEMP global temperature anomaly product (GTA) from the Arrhenius formula which (approximately) explains the warming due to ln(CO2).

    I cannot argue with the mathematics.

    But let’s take a closer look at this analysis. And let’s keep it simple (Occam), without any hypothetical considerations of warming “hidden in the pipeline”, etc.

    The Arrhenius formula explains atmospheric warming due to ln(CO2).

    Let’s see how well this works on a decadal basis, using the 2xCO2 climate sensitivity of 3.2C (mean value of IPCC model simulations), taking the actual observed linear rate of temperature change over each decade and the logarithm of the change in observed CO2 concentration over the period.

    Years prior to 1958 will be difficult to analyze, since there are no physical observations of atmospheric CO2 concentrations prior to the Mauna Loa record, so let us forget these.

    Let us use the HadCRUT temperature record (rather than GISS), since this is the record preferred by IPCC.

    This record shows:

    1958-1967 ln(C2/C1) = 0.0229 dT(theo) = +0.106C dT(act) = -0.163C
    1968-1977 ln(C2/C1) = 0.0329 dT(theo) = +0.152C dT(act) = -0.014C
    1978-1987 ln(C2/C1) = 0.0392 dT(theo) = +0.181C dT(act) = +0.050C
    1988-1997 ln(C2/C1) = 0.0344 dT(theo) = +0.159C dT(act) = +0.127C
    1998-2009 ln(C2/C1) = 0.0619 dT(theo) = +0.286C dT(act) = +0.040C (12 instead of 10 years)

    The correlation is weak (more of a “random walk” than a real correlation), and does not validate the assumed 3.2C climate sensitivity

    If we look at longer periods (breaking the 1958-2009 period into two 26-year periods) we see:

    1958-1983 ln(C2/C1) = 0.0849 dT(theo) = +0.392C dT(act) = +0.195C
    1984-2009 ln(C2/C1) = 0.1249 dT(theo) = +0.596C dT(act) = +0.442C

    Even here, the correlation is weak and does not support the 3.2C climate sensitivity.
    (The first half suggests a CS of 1.6C and the second half a CS of 2.4C).

    So the result obviously depends very much how one analyzes the data.

    Max

  3. 653

    Furry 645,

    You’re arguing against a straw man. If solar and temperature are BOTH sinusoidally linked, a linear regression will show high correlation. You specified the length of the cycle might be important. I just did the regression. It isn’t important. It isn’t even there. It’s not the sun. Deal with it.

  4. 654

    Ray @ 649:

    Yes, previous cycles =have= had similar lengths, and other transitions have been similarly slow.

    And when that has happened, there has been a period of cooling in some part of the planet. That we’ve yet to make a new record high on HadCRUT since 1998 — to me — confirms the hypothesis. That’s part of why I made that Live Journal post 2 years ago — I’d been talking about SC24 and why I felt we’d not made a new record high for a few years before that post. So, I’m pretty set on this solar cycle length influences climate thing.

    Unless “random chance” somehow causes these things to just line up. You know, like “random chance” is somehow mostly “right” …

    As for SC23, it was more normal than SC22. SC24, as others have mentioned, is not at all like any we’ve seen in quite a while, and the buildup is still on-going, so it may yet make new records. On the other hand, SC24 is starting to ramp up and the question is how “quiet” does the sun have to be to keep a lid on setting records. My personal stake in the ground is that we won’t see one until 2014. Or rather, that we =will= see one in 2014.

  5. 655

    max 652: Years prior to 1958 will be difficult to analyze, since there are no physical observations of atmospheric CO2 concentrations prior to the Mauna Loa record,

    BPL: Where in the world did you get that idea? Have you never heard of ice cores? We have a high quality record of carbon dioxide levels for the past 800,000 years.

  6. 656
    Completely Fed Up says:

    “And when that has happened, there has been a period of cooling in some part of the planet. That we’ve yet to make a new record high on HadCRUT since 1998 — to me — confirms the hypothesis.”

    This is not a hypothesis that is missing from the GCM models, however.

    What these models don’t do is model the magnetosphere of the sun (much like it doesn’t model when a volcano is going to happen), therefore the prediction in 1992 for the temperature in 2002 would not have included the actual solar constant (nor could it). Just like Hansen’s 1980;’s model had *a* “Pinatubo-strength” volcano but didn’t actually have the real Pinatubo eruption in it.

    And I think that because of the fact that solar input is already included in models yet you’re making a big song-and-dance about it is why Ray is skeptical of your point: there doesn’t seem to be one.

    So where are you going with this?

  7. 657

    Furry 654: I’m pretty set on this solar cycle length influences climate thing.

    BPL: We can see. In the face of all the empirical evidence to the contrary, too.

  8. 658
    manacker says:

    BPL (655)

    You probably misread what I wrote. I said that there were no physical observations of atmospheric CO2 concentrations prior to the Mauna Loa record.

    Ice core data are not quite the same as actual physical observations.

    Max

  9. 659
    Ray Ladbury says:

    Max suggests: “And let’s keep it simple (Occam), without any hypothetical considerations of warming “hidden in the pipeline”, etc.”

    Bzzzzzz! Oh, but thank you for playing. Sorry Max, but ignoring warming in the pipeline is unphysical. The upper oceans represent a considerable thermal reservoir that equilibrates on a much longer timescale–e.g. ~30 years. Occam would not have approved. In the search for simplicity, it does not pay to over-simplify.

  10. 660
    manacker says:

    Ray Ladbury

    Sorry. The “hidden in the pipeline” postulation has been invalidated by the physical observations.

    The “heat reservoir” was supposed to have been the upper ocean. But the upper ocean has cooled since Argo measurements replaced the inaccurate expendable XBT buoys in 2003.

    At the same time the atmosphere (surface as well as troposphere) has also cooled slightly since 2001.

    The latent heat from melting ice or net water evaporation is too small to make a difference.

    As a result, the “hidden energy” is nowhere to be found in our climate system, so does not exist.

    Kevin Trenberth thinks it may be going back into outer space, with clouds acting as a natural thermostat to reflect more incoming SW radiation.

    See “The Mystery of Global Warming’s Missing Heat”:
    http://www.npr.org/templates/story/story.php?storyId=88520025

    Another postulation has it disappearing into the lower ocean.

    In either case, it is not “lurking in hiding” somewhere to come out and eventually cause more atmospheric warming, as some scientists have postulated.

    And the “hidden in the pipeline” hypothesis has been falsified.

    Occam would definitely have approved of eliminating the somewhat implausible and rather complicated postulation of “energy hidden out of sight”.

    Max

    [Response: Since Max's understanding of this is so low, please do not indulge him in more pointless explanations. He is not going to get the point and this boring and highly repetitive diversion will continue. Enough already. - gavin]

  11. 661

    BPL @ 657:

    Well, I put that post up so that today I can say “I put that post up years ago and we’ve yet to break the HadCRUT record because I believe in the link between solar cycle length and global temperature”.

    Meanwhile, the “There is no correlation!” crowd scratches its head.

    I have a prediction that held, you have “There is no correlation!” Actually, you have “The correlation between solar cycle length and global temperature diverged in 1975″, which is what the articles I referenced above showed =and= that agrees with what would be seen as man-made CO2 began to dominate the global climate. Except that we’re in a Grand Solar Minimum, so …

  12. 662
    Hank Roberts says:

    Why just keep going ’round with the guy who denies science works?
    Suggestion: look at the original post again and pursue the topic?

    “… there are plenty of factors that can affect the overall mean temperature (solar variability, volcanoes, greenhouse gases, internal variability etc.) and so it is hard, given the uncertainties in the solar or volcanic reconstructions to precisely attribute the paleo changes in the global or hemispheric mean to these factors. But if we can look at more complex fingerprints of the changes, it might be possible to be more quantitative in those attributions since the spatial fingerprints of the different factors are easier to distinguish. ”

    That’s a challenge right there. Retyping basic science to the guy who just repeats his argument from ignorance is helping him derail the topic.

    “… we can look at more complex fingerprints of the changes … the spatial fingerprints of the different factors are easier to distinguish”

    There must be a lot of scientists each looking at different locations and different fingerprints across locations. If we can talk about the topic, the conversation might attract more scientists.

  13. 663
    Jeffrey Davis says:

    “Ice core data are not quite the same as actual physical observations.”

    Think through the words you use. Of course, ice core data is an actual physical observation. What else could it be? If you see an ice cube melt you know what the temperature is just as surely as if a column of mercury in a graded tube says so.

  14. 664
    Completely Fed Up says:

    “I have a prediction that held, you have “There is no correlation!””

    Actually, he has “only 2.5% of the variation is correlated with solar”.

  15. 665
    Doug Bostrom says:

    Jeffrey Davis says: 1 June 2010 at 11:33 AM

    “Ice core data are not quite the same as actual physical observations.”

    Think through the words you use. Of course, ice core data is an actual physical observation.

    No, no! Once the air samples from Mauna Loa etc. are captured they’re no longer observations! The very act of their sequestration changes their composition, somehow. Don’t ask for an explanation of how that happens; the transmogrification part has not quite been figured out yet and is still in the realm of imagination.

  16. 666

    CFU @ 664:

    Go read the referenced posts on Skeptical Science, okay? It’s one of the “Good Guy” blogs.

    Other than “random chance”, do you, personally, have any thoughts on why there’s not been a new HadCRUT record since 1998? Any predictions, projections, hypotheses, ideas, concepts, etc. that you’ve been advocating for, say, 5 or more years?

    There’s a hypothesis — a deeper solar minimum will reverse (or delay increases in) global temperature.

    There’s an experiment — G-d has provided us with a deep solar minimum.

    There’s a result — no new HadCRUT record since 1998.

    Looking at past iterations of this “experiment”, the same result.

    Come up with something besides “Are Not!”, okay?

  17. 667
    Completely Fed Up says:

    “Go read the referenced posts on Skeptical Science, okay? It’s one of the “Good Guy” blogs.”

    That neither changes what BPL said nor what you said he said (which was different).

    “There’s a hypothesis — a deeper solar minimum will reverse (or delay increases in) global temperature.”

    And that hypothesis has no physical basis and (if is part of the GCR scare) has been shown insufficient.

    “There’s an experiment — G-d has provided us with a deep solar minimum.”

    I think you’ll find it’s just the Sun.

    “There’s a result — no new HadCRUT record since 1998.”

    1998 was an extreme El Nino. Are you now purporting that the El Nino is Solar Cycle driven?

    And there’s been a new GISS record, and HadCRUT has a joint top-spot. And indicates that this year is going to be the new top spot.

    “Come up with something besides “Are Not!”, okay?”

    Ah, we’re back on to the “say something the other person didn’t say” schtick.

    2.5% of the variation is accorded by solar changes.

    You’re the one saying “Is not.”. Come up with something.

  18. 668
    Completely Fed Up says:

    Gavin, just don’t let his stuff through. Heck [edit] the whole schmeer out.

    It’s been done before by other moderators who remain anonymous.

  19. 669
    Ray Ladbury says:

    FCH,
    I recommend the following post by Tamino:

    http://tamino.wordpress.com/2009/06/26/breaking-records/

    Be careful when taking records as your metric. Records are inherently in the realm of extreme value statistics, and the tails of the distribution are difficult to pin down.

  20. 670
    Jacob Mack says:

    Ray # 669 that is a good post by Tamino. I printed that one up.

  21. 671

    Ray @ 669:

    Yes, I’m well aware of both the post by Tamino and the perils of using records as a metric. However, since the general direction for temperature is “up”, I feel that records are more than appropriate in this particular case (while still acknowledging that they are prone to their own set of issues), and make a better point.

    That said, I’ve been putting out the same message for quite a while now. It’s not like I decided just last week to hang my hat on solar cycle length. There was even a thread a while back where Lindzen, I think it was, refused to take an even-money wager on which way global temperatures were going. When I said I’d be happy to take the wager, all the sudden people clammed up.

    So, I’m right on the “no new records”, I’ve been at this for a while, and I was willing to put my money where my mouth was. I know this makes me extra annoying, but it’s part of my charm!

  22. 672
    manacker says:

    FCH

    You and I may not agree on everything, but I believe you see that arguing with those who already have their minds made up is an uphill battle.

    To agree with you just a tiny little bit shakes up the whole belief system.

    Max

  23. 673
    John E. Pearson says:

    FurryCatHerder said: “I said I’d be happy to take the wager,”

    WHat wager is it that you will take?

  24. 674

    Max 658: You probably misread what I wrote. I said that there were no physical observations of atmospheric CO2 concentrations prior to the Mauna Loa record.
    Ice core data are not quite the same as actual physical observations.

    BPL: What are they, spiritual observations? Data is data.

  25. 675

    furry 666: do you, personally, have any thoughts on why there’s not been a new HadCRUT record since 1998?

    BPL: Because we haven’t yet had another record-breaking El Nino like we did in 1998?

    The fact that there was a brief peak in 1998 is MEANINGLESS. Will you PLEASE crack an introductory stats book? It’s the trend that matters, not one outlier.

  26. 676

    Gavin,

    I was going to list the references showing the ocean is warming, not cooling, but after your post I won’t.

    Wouldn’t it be easier on all of us, though, to just kick Max’s annoying butt off the echo? Rather than let him post misinformation we’re not allowed to answer?

  27. 677
    Nick Gotts says:

    “I put that post up years ago and we’ve yet to break the HadCRUT record because I believe in the link between solar cycle length and global temperature”. – FCH

    You think your beliefs have affected global temperatures???

  28. 678
    Ray Ladbury says:

    FCH, try the following experiment:

    Flip a coin until it comes up tails. Note the number of times you had to flip it before it came up tails. Now keep flipping until you have a streak that breaks your previous record. Note how many times you had to flip the coin before your record streak started. Repeat many times. This will give you an idea of how things compound as we seek to break ever higher records.

    Now in the case of climate, things are more complicated. Our probability distribution is not stationary. However, to a first approximation, the mean is increasing roughly linearly. The extreme value statistics, though can increase much more quickly than linearly, though. The 1998 El Nino was a really big event–over 2 sigma, as I recall. It’s going to take awhile to break that.

    You can also play around with this in Excel or R. Tamino’s post gives you an idea how to proceed. When looking for patterns, it is very important to assess statistical significance. Our visual cortex is hardwired to see patterns. It’s why we see faces in clouds or why heat rising off of a highway appears as a body of water.

  29. 679
    John E. Pearson says:

    Nick 677 and BPL 675: shhhhhhhh. Please see 671 & 673. I don’t know whether there’s one born every minute but a fair bet (based on an assumption of no underlying trend and a 1988 temperature nearly 3 sigmas above the mean which would suggest that the 1998 HadCrut record was a roughly once in a hundred years event) strikes me as potentially lucrative.

  30. 680
    manacker says:

    BPL

    You ask why ice core data on CO2 are not as valid as actual physical measurements for determining the atmospheric concentration for a given year.

    Are you serious?

    (If so, check all the info out there on the reasons.)

    Max

  31. 681
    manacker says:

    BPL

    BTW, I do not challenge the validity of any ice core based CO2 data for telling us (more or less) what the atmospheric CO2 levels were over a composite number of years in the Antarctic or Arctic.

    I just think that the data show that it is stretching the imagination a bit to believe that this will give you an accurate indication of global CO2 concentration for a given year.

    If you believe otherwise, so be it.

    “Belief” is a wonderful thing, BPL.

    Max

  32. 682
    Patrick 027 says:

    Re 681 CO2 averaged over x years derived from an ice core record is still good information about CO2 changes on longer time periods. Which is good enough for some important purposes.

  33. 683
    Patrick 027 says:

    … and the physics and ecology suggest that there are limits to the likelihood of large excursions that would not be resolved for x-year averaging…

    … and CO2 is to a good first approximation well-mixed over most of the mass of the atmosphere.

  34. 684
    David B. Benson says:

    Max Anacker (652) — You analyzed the last 13 decades wrongly and so clearly failed to understand what was in the link I posted for you. In particular, I made no assumption about the transient reponse (OCTR); it is calculated by best fit.

    Go back and study it more carfully.

    Regarding ice core CO2 data, I checked Law Dome against the Keeling curve for the interval of overlap. These agree.

  35. 685
    Hank Roberts says:

    Quoted from inline above, for those who missed it and will respect it:

    [Response: Since Max's understanding of this is so low, please do not indulge him in more pointless explanations. He is not going to get the point and this boring and highly repetitive diversion will continue. Enough already. - gavin]

  36. 686
    manacker says:

    Patrick 027

    You wrote:

    CO2 averaged over x years derived from an ice core record is still good information about CO2 changes on longer time periods. Which is good enough for some important purposes.

    I agree fully. It just isn’t as good as real measurements at the time for establishing the actual CO2 concentration for a particular year (to use for establishing decadal trend correlations between CO2 and temperature, for example), which was my argument.

    Max

  37. 687

    Let’s just stop answering max altogether. I admit I’ve been a chief offender in this regard. But I think the moderator was right–he’s one of those “bad attention is better than no attention” people. Stop responding and he’ll eventually go away.

  38. 688
    flxible says:

    re Hank’s note @685, as a “change management leader” mr Anaker doesn’t seem to have changed much in the last 3 years, he’s still following the money.

  39. 689
    manacker says:

    [edit of irrelevant manipulations of data to prove meaningless points. Please stop]

  40. 690
    manacker says:

    Hey, Gavin, it’s your blog, so you can censor out anything that is too inconvenient for you. Congratulations!

    Max

    [Response: Repetitive and irrelevant recitations of the same generic 'Max' talking points in every other comment thread is not 'inconvenient' - it is just tedious. Feel free to assume that my boredom with your tactics equates to my helplessness in front of your brilliance, but good luck trying to convince anyone else. - gavin]


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