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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. 601
    Septic Matthew says:

    572, Ray Ladbury: If they want to convince me and other sciences, they have only to come up with an alternative theory that explains the evidence–all of it–equally well or better with equal economy. Until then, they have nothing to offer and are firmly in the anti-science camp.

    I agree with sentence 1 but not sentence 2. I used to think that the solar theorists were mostly cranks, but now I think that their evidence is non-ignorable. As far as I can tell, all of the scientists discredit some of the evidence for reasons that I find insufficient. Say there’s no large body of evidence substantiating a hypothetical causal mechanism underlying the correlation between solar activity and earth temperature swings. Here, the old saying applies: Absence of evidence is not evidence of absence, and there are bits and pieces of evidence. Hence, to me the lack of a complete mechanism is not sufficient to discredit. Some stuff causes cancer without the mechanism being known. Some people seem not to get AIDS from HIV, and no one knows how.

    For warming or cooling or random variation humans require more energy and are running out of oil. The development of new energy supplies is something that about 75% of people agree on, with the biggest disagreement being about federal funding of nuclear power. India, China, and Russia have no worry about federal funding, only we Americans have a worry. I, C and R will show (I bet) the feasibility of using what is now called “nuclear waste” as fuel, so the US won’t lag much, if at all.

  2. 602
    David B. Benson says:

    Max Anacker — Looking forward to yuor comments on “Global warming, decade by decade”. My goal is something sufficiently simple that most can follow without overly simplifying, just keeping it simple.

  3. 603
    Ray Ladbury says:

    Oh, come on. Who do you know wh says the sun plays no role. You’re sequestering a lot of carbon in all that straw! The last solar cycle was long–the second longest in about 150 years. However, it is nothing like the levels of a Dalton or Maunder minimum.

    I would appreciate it if you would take sufficient time to represent my views as they are rather than distorting them so even I don’t recognize them. For the record: Yes, Mr. Sun does play a role. However, even if we were to get a Grand Minimum, it would buy us at most a few decades. The effects of CO2 persist for centuries. I have never contended otherwise and I do not appreciate your distortion of my views.

  4. 604
    Jacob Mack says:

    Interesting here in California the recent rainfall and cooler temperatures were such that only a handful of times in the last 150 years for this time of year matched these conditions. Of course there would be some who might try to jump on that fact and make a false claim that warming is done basd upon an ephemeral regional, but relatively drastic weather change. The climate is certainly changing overall and this example of precipitation changes and cooler temperatures in terms of regional weather is a good example of such predictions made by GCM’s the IPCC report and the literature we have been citing here.

    It is easy to confirm what I am saying: wacthed it on the weather channel, and four major network news programs with their various meteorologists.

  5. 605
    John Pollack says:

    Ray – 575 – I fear that your point 3 about nuclear power and human stupidity is even worse than you make it out.

    That’s because it’s more human cupidity than stupidity. Take something that’s undetectable to normal human senses, but really dangerous, such as radiation. There is a permanent financial incentive to treat it and dispose of it carelessly. It saves money, and the consequences are invisible – for a while.

    Eventually, regulation gets lax, more safety measures are ignored or disposed of, and a disaster results. (Rather like oil extraction and coal mining, to name a couple of more visible but ignorable dangers.)

    In that sense, it clearly resembles the buildup of CO2 itself, and will (partially) substitute a different problem of the same general type.

  6. 606
    John Pollack says:

    Jacob – 598 I believe that the GCM’s are doing a fairly good job with the big picture, but I am unimpressed with attempts to derive regional trends in my part of the world (western U.S. Corn Belt).

    For quite a while, many of the GCMs have been suggesting we get a hotter and drier growing season. However, what I saw in 31 years as an NWS forecaster was wetter with little temperature trend.

    A lot of the wet comes from moisture recycled through evapotranspiration and convection, and much of the rest from moisture transport from the Gulf of Mexico.
    In either case, I don’t trust the GCM’s to get it right when the higher resolution daily forecast models can’t do it reliably! They tend to break down when it comes to evaluating vertical stability, and require human interpretation to produce reasonable results.

    That said, I’m not drawing firm conclusions. It is clear from the last millennium that we are subject to multi-decadal droughts and wet periods. Perhaps we are enjoying one of the latter. Perhaps there is enough positive feedback between moisture and vegetation, heightened by AGW, that the next drought will by a biggie, and dwarf our current wet tendency.

  7. 607

    BPL @ 595:

    What’s the trend for sin(x) for x = 0 to 2 pi radians?

    Oh, right — 0!

    [Response: Just for kicks, fit a linear model to that at say steps of pi/6 and see what slope you get.–Jim]

  8. 608
    Jacob Mack says:

    606 thanks for your response. I will be looking more into GCM predictions and regional variations later next week.

  9. 609
    Completely Fed Up says:

    “What’s the trend for sin(x) for x = 0 to 2 pi radians?”

    Which has relation to this discussion HOW?

    Or are you saying that temperature is a sinusoid? If so, how is this *correlation* *caused*?

  10. 610
    Completely Fed Up says:

    “I am not claiming that this is a “trend”.”

    cf earlier statements:

    “temperature records show a slight linear cooling trend”

    Max, there is no cooling trend in the records since 2001.

  11. 611
    Completely Fed Up says:

    manacker says:
    29 May 2010 at 3:18 PM


    You wrote (584):

    Max #573, there is no cooling trend in the Hadley or GISS dataset since 2001.

    The annual records for 2001 through 2009 show:”

    They show no cooling trend.

    Tell me, Max, you’ve done the maths here.

    Is there any statistically significant cooling trend in the data?

  12. 612

    SM 601: Some people seem not to get AIDS from HIV, and no one knows how.

    BPL: HIV-immune people have a defective gene that normally makes cell walls permeable, but in them (recessive trait requiring two parents with the defective gene), the cell wall blocks HIV entrance. I think this news is a few years old now.

  13. 613
    Completely Fed Up says:

    “My former post to AC referred to my desire to see more green tech implemented here.”

    Jacob, not wasting resources is THE GREENEST tech possible.

    cf Micawber:

    “Annual income twenty pounds, annual expenditure nineteen pounds nineteen and six, result happiness. Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.”

  14. 614
    Ray Ladbury says:

    Septic Matthew,
    The problem with the solar models is that they could at most explain a tiny fraction of the evidence–for instance, they couldn’t explain the stratospheric cooling in conjunction with tropospheric warming. Also, to get such correlation, the solar scientists have to introduce artitrary parameters–e.g. lags–and you know how risky this is with a purely statistical model. Without a mechanism, I just can’t take these guys seriously. It is not sufficient to merely show a correlation between solar cycle and temperature. Lots of things change with solar cycle.

    Finally, none of the work the solar guys have done changes the fact that we have very strong evidence that CO2 sensitivity is around 3 degrees per doubling. The evidence there is overwhelming. Ironically, if the solar guys are right, it might wind up implying a higher CO2 sensitivity rather than a lower one.

    That is my problem with all these guys trying to “disprove” global warming. They are looking only at the temperature record–nothing else. That’s not science. What they need to be doing is trying to understand the climate rather than focusing only on late 20th century warming. Show me the model.

  15. 615
    Ray Ladbury says:

    John Pollack,
    Very true. We need to always remember the way the Mongol hordes breached the Great Wall–by bribing (and eventually killing as unreliable) a guard.

    My comment was more directed at plant safety. The idjits at Chernobyl had to bypass–I think it was–8 failsafes to bring about their own little version of hell on Earth. Every significant nuclear accident has involved astounding stupidity.

    Perhaps what both cupidity and stupidity have in common is the fact that humans suck at gauging risk. That is why there’s a whole science of doing so–if we’d just frigging use it.

  16. 616
    Didactylos says:

    It always amuses me when people get pedantic about when the 21st century started. Generally, the people getting all warmed up about the subject don’t understand why the date is when it is, don’t understand why the “right” choice is the wrong choice for most situations, and don’t understand why it couldn’t matter less in nearly all contexts.

    In short, exactly the same deficiency exhibited by climate deniers. Understand the whole problem, don’t just be contrary for the sake of being contrary.

  17. 617
    Hank Roberts says:

    CFU writes:
    > Tell me, M ….

    Oh, he loves seeing his name and claims repeated, and having people ask him to go on saying his stuff. You’re hooked, sir. Spit out the hook.

  18. 618
    Jacob Mack says:

    BPL # 612: Right the chemokine CCR5 deletion along with other receptor changes (a co-receptor).

    CFU: again agreed, and we produce as little waste as possible.

  19. 619
    John Pollack says:

    Ray – 615, I didn’t know that about the Mongols. Good illustration!

    I agree totally that humans suck at gauging risk (especially long-term).
    Risks with invisible causes (radiation, climate change)are worse, if possible.

    Science certainly helps a lot, but there are a disturbing number of “black
    swan” unanticipated risks that even get by the science.

    In the case of climate change, while the deniers like to yammer about the economic risk that we take expensive conservation measures for no “good” reason, I’m a lot more concerned about very rapid change.

    For example, there is a reasonable tendency for climate scientists to interpret paleo data conservatively, and not call for an extreme change unless absolutely forced to by the data. There are disturbing hints that the climate system is vulnerable to very rapid change.

    For example, the melting of the Greenland Ice Sheet in the last interglacial. From the concluding paragraph of “High rates of sea-level rise during the last interglacial” Rowling et. al. Nature Geoscience 1, 38-42 : “A 1.6 m global sea-level rise per century would correspond to the disappearance of an ice sheet the size of Greenland in four centuries
    (modeling suggests 1000 years or more.)”

    What if a mere 300 ppm CO2, with somewhat different orbital parameters, really is enough to melt the GIS in 400 years? We’re in a heap of trouble!

  20. 620

    Ray @ 603:

    Have you ever read my post on Live Journal? What you wrote is exactly how I explain it. However, my overall point is that by reacting to various and sundry arguments — arguments that are often true on a shorter time scale, such as the length of a solar cycle or two — the way they are reacted to, all you do is feed the denialist engine.

    If you read this post on Skeptical Science — — it states that until 1975, there was a strong relationship between solar cycle length and global temperature. But SC23 wasn’t as weak a cycle as SC24 is shaping up to be, which indicates to me that there is a stronger cooling bias during SC24 than during SC23. That we’ve gone 12 years without a new record high for HadCRUT would seem to confirm this.

    To me, that’s the better message — acknowledging that there has been an influence, but that whatever the influence it is either being swamped, or will end with SC24 in another decade.

  21. 621

    CFU @ 609:

    What’s the graph of y = mx + b + a * sin(cx)?

    Pick various values of m, x, b, a and c. Let a * sin(cx) be the change in temperature associated with solar activity. Let mx be the CO2 related rise in global temperature. Let b be the baseline temperature. Let a be the change in temperature attributable to the length of a solar cycle. Let c be a scale factor for Grand Minima and Maxima.

    Do there exist values such that the temperature rises on multi-decadal scales while still going sideways, or even downward, during deep solar minima? See the Skeptical Science post I referenced earlier for an answer to that.

    Now, if that’s true — that past relationships between solar cycle length and global temperature have existed — wouldn’t it be =better= to acknowledge that, as well as the implications of that, than to make snarky responses?

  22. 622

    John Pollack @ 619:

    Worse scenario — what if 300ppm is enough to cause another global Ice Age in 500 years and the only solution is burning everything made of carbon we can get our hands on?

    One reason to sequester carbon in places where we can get to it is that some day we may =need= to have 450ppm CO2. Sure, let’s get back to 280 or so. But let’s not shoot ourselves in our collective feet the next time Mr. Ice Man cometh.

  23. 623
    David B. Benson says:

    FurryCatHerder (620) — Look at the past 13 decades this way:

  24. 624
    John Pollack says:

    FCH @ 622: Considering that 300ppm is at the high end of what we’ve been seeing during interglacials in the past 800k years or so, I’m not concerned about inviting another glacier in 500 years if we were somehow able to return to that value (which is not by any currently reasonable technology). 250 ppm and you’d have a point, but we can’t get it that low. 450 ppm looks immensely more likely before we get a handle on emissions, if then.

    450 ppm puts us back in the Middle Miocene, when temps were 3-6C higher than present, and sea level 25-40m higher. I refer to Tripati et. al. in Science 326, 1394-7. The full article is still paywalled, but the abstract isn’t:

    Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years
    Aradhna K. Tripati,1,2,* Christopher D. Roberts,2 Robert A. Eagle3

    The carbon dioxide (CO2) content of the atmosphere has varied cyclically between ~180 and ~280 parts per million by volume over the past 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, the partial pressure of CO2 (pCO2) is much less certain, and the relation between pCO2 and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO2 during major climate transitions of the past 20 million years. During the Middle Miocene, when temperatures were ~3° to 6°C warmer and sea level was 25 to 40 meters higher than at present, pCO2 appears to have been similar to modern levels. Decreases in pCO2 were apparently synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago) and Late Pliocene (~3.3 to 2.4 million years ago).

    1 Departments of Earth and Space Sciences and Atmospheric and Oceanic Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA.
    2 Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK.
    3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

  25. 625
    Anonymous Coward says:

    FCH (#622),
    As has been pointed out several times already, there are better GHG gases which could be synthesized for the purpose of avoiding a glaciation such as HFCs.

    Still no explanation from Jacob for the link he asserted between immigration and the lack of investments in renewables and/or emission cuts by the way…

  26. 626
    Jacob Mack says:

    # 625: go to the Due to the high influx of unskilled immigrants and a weakening economy coupled with the high costs of solar panels, there is a deficit growing in Germany. There are also less people percentage wise who are skilled to design, build and maintain alternative energy applications.

    From that site you can do your own reading,as I promised to be brief on this topic so we can continue to have fruitful discussion,so any future responses from you will simply be ignored.

  27. 627
    Jacob Mack says:

    AC this will get you started:

    If you do not have a subscription you can get a 14 day free trial to see the full content. Little hints: solar panels are still very expensive to install and take awhile of use to pay for themselves. Standard power uses are still cheaper and allowing so many unskilled immigrants has reduced the productivity of technological development and application.


  28. 628
    Jacob Mack says:

    Just when I thought I

  29. 629
  30. 630

    David Benson @ 623:

    I have looked at a wide variety of data. That the article I referenced admits that pre-1975 there was a connection between solar cycle length and global temperature tells me that the link is still there, it’s simply been swamped by CO2. And that means that it becomes increasingly difficult for any other cause — besides CO2 — to change global temperatures. So, what might have once caused a wiggle that include “down” no longer does. It takes an even longer cycle, with an even stronger downward bias, to change the upward trend.

    Fortunately for the cause of advancing scientific knowledge, the Sun is providing us with precisely that experiment …

  31. 631

    John Pollack @ 624:

    There are myriad and sundry ways to reach 300ppm in the next 500 years. But that’s beside the point, which is just that we need to reduce CO2 concentrations in a smart way because we might find ourselves needing to tweak CO2 levels upward in the future.

    In my post-fossil-fuels utopia, I’d like to see the “Carbon Pickup Service” collecting refuse from our homes and return bottled or canned liquid fuels back to us the next week. Could make for interesting ads — “Drive your car on junk-mail and kitchen leftovers!”

  32. 632
    Rod B says:

    Ray Ladbury (614), how on earth could the solar guys possibly alter your overwhelming evidence??

  33. 633
    David B. Benson says:

    FurryCatHerder (630) — Consider Tung & Camp (2008?). But do note that uses decadal averages smooths out the minor wiggles due to solar cycles; think in terms of centuries.

  34. 634
    Completely Fed Up says:

    “What’s the graph of y = mx + b + a * sin(cx)?”

    Not the graph you used earlier.

    PS guard against curve fitting: it only fits if past behaviour is a guide to future behaviour.

  35. 635

    David Benson @ 632:

    In the words of what’s his face from Back Draft — “You’re doing it wrong”.

    Your approach is to say “No, the sun isn’t affecting climate (because if you use century scales the wiggles all get worked out).” My approach is to say “Right now we’re in a Grand Solar Minimum. That’s a strong cooling bias. That’s why the temperature isn’t setting new records. After SC24 ends, we’ll see plenty of new records, unless we take aggressive action.”

    Think Judo or Hapkido, not Boxing or Thermonuclear Warheads.

    Long term — the trend is up. Right now — sideways. Explain why it’s sideways and sweep their feet out from underneath them.

  36. 636

    David Benson @ 632 redux:

    Here’s another from Skeptical Science —

    Keep in mind — SC24 isn’t an ordinary cycle. We’re going through one of the longest and weakest transitions between solar cycles in a century or so. 2007 and 2008 were two of the four weakest years on record, and 2010 ain’t all that exciting either.

    This is my basis for asserting that solar cycle length / sunspot cycle is creating a very strong cooling bias that will end either with SC24 or SC25 (I’m betting on SC24 since SC23 was already winding down from SC22).

    The Gore Minimum =will= end. Then we =will= resume Global Warming. But right now, we’re sideways and we’re going to stay that way.

  37. 637
    Ray Ladbury says:

    While there is some evidence of a solar effect on global temperatures, I would not call it tremendously convincing–mainly because there is no known mechanism.

    I am reluctant to invoke a cause–especially one that supports my argument–if I don’t know the mechanism. This is particularly true for a noisy system like climate. That would put us in the same league as the denialists. So, by all means, we must look at this, but
    1)Until we have a mechanism, it ain’t science
    2)it does not in any affect the properties of CO2 as a greenhouse gas.

    As I have said before, a well mixed, long-lived greenhouse gas has left its fingerprints all over the paleo and current climate. The only viable candidate is CO2.

  38. 638

    Ray @ 636:

    I don’t know that something with a cycle of 200 or so years is going to show up as other than noise in a very long term climate picture.

    I suspect that the reason there is no “mechanism” is more political than scientific. Mechanisms have been proposed, and there is certainly evidence that, for example, the Dalton and Maunder Minima were cooler than the surrounding time periods. Dueling papers have been published, and I’m sure that more papers will be added to the controversy.

    There are a lot of areas of science where the exact reason or mechanism isn’t know, but the evidence is fairly solid. Like, what causes gravity? We don’t know what =actually= causes gravity, but ignoring gravity because you don’t know the precise mechanism isn’t much of a way to do science. So I’m not convinced that failing to nail down the =exact= cause of the relationship between solar cycle length and global temperature means it gets to be ignored.

  39. 639

    Cat 621,

    Which part of “solar variation only accounts for 2.5% of temperature variation” did you not understand?

  40. 640

    Cat 622,

    The chimera of a nearby ice age is not supported by the facts. Milankovic cycles are celestial mechanics, a very precisely known science. The next stade isn’t for 20,000 years, and it’s so shallow we’ve probably already prevented it. The next deep stade is 50,000 years from now–not 500. No need to save CO2.

  41. 641

    Here are regressions A) of Hadley Centre dT on years since solar minimum, B) years since solar maximum, and C) both. These regressions use the Wolf annual sunspot number to determined peaks and troughs.

    A) dT = -0.170 + 0.00235 SinceMin

    N = 149 (1860-2008)
    R^2 = 0.000847
    Adjusted R^2 = -0.00595
    t on intercept = -4.30 (p < 0.0000305)
    t on coefficient = 0.353 (p < 0.724)

    B) dT = -0.149 – 0.00207 SinceMax

    N = 149
    R^2 = 0.000633
    Adjusted R^2 = -0.00617
    t on intercept = -3.93 (p < 0.000129)
    t on coefficient = -0.305 (p < 0.761)

    C) dT = -0.161 + 0.00190 SinceMin – 0.00146 SinceMax

    N = 149
    R^2 = 0.00113
    Adjusted R^2 = -0.0126
    t on intercept = -2.71 (p < 0.00760)
    t on SinceMin = 0.269 (p < 0.788)
    t on SinceMax = -0.203 (p < 0.839)

    Conclusion: There is not even the smallest hint here of any cyclical effect of Sunspot number on dT. After adjustment for number of variables, no variance is accounted for at all.

    Note: Similar results are obtained with TSI.

  42. 642
    Completely Fed says:

    “So I’m not convinced that failing to nail down the =exact= cause of the relationship between solar cycle length and global temperature means it gets to be ignored.”

    Why do you think that the earth’s temperature is causing a longer solar cycle length?

  43. 643
    Ray Ladbury says:

    FCH, there is a huge difference between attribution of, say, planetary motion and the putative solar mechanisms. The latter system is quite noisy, and there are many factors that change with the solar cycle. CO2 stands out because it is such a persistent effect. The Milankovic cycles stand out due to their regularity.

    However, the Grand Minimum/Maximum “cycle” is not really periodic so much as oscillatory. Thus, without a mechanism that predicts where to look for a bump in the frequency spectrum, it is very easy to fool yourself with noise. The difference between science and simple empiricism is that the former is empiricism guided by theory. Moreover, none of the mechanisms I’ve heard are particularly compelling.

    Going 12 years without a new record is not all that unusual–Tamino has posted on this. Going 15 years would be odd, but it probably wouldn’t entirely invalidate current models. And finally, the past 12 months have been the warmest on record.

  44. 644

    CFU @642:

    Magic space zombies?

    It has to be Magic Space Zombies since 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.

  45. 645

    BPL @ all of them:

    You can’t linearly regress a cyclical function, much less one that is chaotic.

    Try this — regress hourly temperature and cosine(incidence angle). Taking a naive approach to statistics, you’d have to conclude that “afternoon” causes high temperatures …

    (For even more fun, regress hourly =indoor= temperature on an un-air conditioned building and the cosine of the incidence angle — then you get “sundown” causes high temperatures …)

  46. 646

    Ray @ 643:

    Yes, it’s easy to be fooled by “noise”, but the probability that “noise” is unrelated to “signal” is reduced each time “noise” and “signal” correspond.

    Unless I missed something, this decade is not like the previous one. In this decade, people argue about how many more years before a new record means it’s “global cooling”. In the last decade, it was pretty obvious. In this decade, the sun is quieter than in a century or more. In the last decade, the sun was at record levels of activity.

    This relationship goes back for millenia — all other things being equal.

    This doesn’t mean that CO2 isn’t the =primary= climate driver at this point in time — point conceded, agreed to, acknowledged, supported, etc. But until you start acknowledging that the giant ball of hot gasses in the sky has gone quiet, and we’re not setting records like we used to, people are going to question your motives.

  47. 647
    Hank Roberts says:

    FCH, if you’d point to your sources it’d be easier to figure out what you’re talking about. I can’t match your descriptions with anything I find except stuff from last year.

    Here’s what I find when I look this stuff up. YMMV, but show me please.

    Sun? New cycle starting.
    Yes, we haven’t watched enough stars long enough to know what’s expected or if we’re right about the other stars we think are like ours in variability. But ours isn’t quieter than it was last year.

    Temperatures? Much warmer than last year. Click on Ch05 and redraw:

  48. 648
    Hank Roberts says:

    PS, FCH, closest I found to “quieter than in a century or more” was 90 years, as of April 1, 2009 — before the recent uptick:
    “We’re experiencing a very deep solar minimum” — Dean Pesnell of the Goddard Space Flight Center.
    “This is the quietest sun we’ve seen in almost a century” — David Hathaway of the Marshall Space Flight Center.

    New instrument will be orbited late this year; here’s Judith Lean:
    This is attribution: “We think changes in irradiance account for about 10 percent global warming at most.”

  49. 649
    Ray Ladbury says:

    FCH, At this point, I think it is even premature to say that the past cycle was anomalous. Previous cycles in the past century had similar lengths. And although Solar Min usually lasts 4 years, it’s not unusual for it to last as long as six. You have to realize that the heliomagnetic field is not really quite periodic. Rather, like all such dynamos, it loses strength in the dipole with energy going into other modes until the dipole flips and you get a new cycle. It’s more criticality than cyclicity.

  50. 650
    Jacob Mack says:

    600 I like natural gas better. Nuclear waste and a potential meltdown 3 mile island and chernobyl etc…lol have me in ongoing concern. Of course that is Russia. France seems to be handling nuclear energy well, but I am not in support of the US depending upon nuclear reactors more than we alreayd are now.