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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. 1
    Jim Redden says:

    It would seem that an increased energy retention in an H2O phase state modulated system, such as the Earth, can result in greater observed temperature differentials (and more kinetic activity and extrema in general) without necessarily raising the average measured surface temperature all that much, especially in brief geologic time periods (before Oceans fully respond, etc).

    ENSO has been proposed as a self-regulator of temperature that increases in convective activity in response to temperature differential. For example, see The Diabatic and Nonlinear Aspects of the El Nino-Southern Oscillation: Implications for its Past and Future Behavior (Sun 2009). Perhaps it is altogether possible that the actual climate sensitivity of the Earth is greater than currently proposed as feedbacks such as increased convective cooling, as well as cloud mediated reflective aerosol cooling reduces surface temperatures in a limited response range. Moreover, at some point of greater perturbation and forcing, new energy relationships will emerge and a gross change in average temperature will be observed.

    So bandying about theories of temperature regulation to dismiss concerns of anthropogenic climate change isill advised; critical biological systems stand at great risk, such as the carbon dioxide oxygen plankton cycle. Like other large scale perturbations in systems, climate forcing is certain to unfold in ecological and biological systems in very surprising ways as unknown limits are passed beyond a point of no return in human time scales…

    Life forms, and their intrinsic ecological feedback webs seem adapt a predictable range of climate periodicity and extremes in a limited range that once exceeded results in extinction; humans and our dependencies are no exception. It would seem a conservative prudent approach, and a change in our ways as a specie is in order…

  2. 2
    Tom S says:

    Hey, finally a science article again from the webs leading AGW site. What a relief, I was going to stop coming here if the last couple months kept up.

    It’s pretty relevant as far as I am concerned. The modeling is a sore point in my view, both the over-confidence in them by the media and the more “committed” followers of CAGW.

    Performance of the models going forward is the key obstacle to bringing in the more technically minded skeptics in the public IMO. Back-casting performance isn’t very interesting when the models are tuned as they are, although I certainly recognize how important back casting is to creating the model.

    I’ve brought this up before here, but I have watched hurricane season forecasts closely for over a decade here in FL, and it still is effectively a random number generator, devoid of skill. I’m not equating these two technologies, only the futile early attempts on prediction of these complex physical interactions.

    So I have no more confidence in climate prediction then these other forecasts, and when yet another model run shows “it’s worse then we thought”, it has zero impact on me. These type of articles are much more useful.

    Unfortunately for everyone, these things take decades to prove out and the model iteration rate is very slow with new data. Lack of accurate data other than temperature going back hundreds of years makes this even more difficult, and error prone.

    You can use the d-word for me if you like, but I totally support the modeling efforts as they could be very useful when they are proven accurate, particualarly on the regional basis.

    [Response: I would by no means disagree that a model prediction saying "it's worse than we thought' is meaningless. On the other hand, to the extent that any of us at RC have used this sort of phrase (if we ever have) it is in response to observations of phenomena (such as sea level rise) that are greater than the models have predicted, not the other way around as you imply. As for model we, and the IPCC, have always been careful to use 'projection' to avoid the impression that a prediction is being made. Projections are about ranges of possibility (sometimes with likelihoods attached, but still very much model-dependent and explicitly so). I would agree with you that the mainstream media, the deniers, and yes, some scientists confuse these issues quite badly.--eric]

  3. 3
    Anne says:

    “…. 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.”

    The “MCA” (Medieval Climate Anomaly) could mean that, rather than a CO2 driven AGW, we could be experiencing a “Modern Climate Anomaly” of 300+ years.

    We got another 150 years to go without having to blame CO2.

    [Response: If and when you can show that the "MCA" was globally similar to the current warming in its spatial pattern (which no one has any evidence for), and when you can show that the MCA was entirely due to internal variability (which might be right, but certainly isn't proven), then you will begin to have a point. Until then, don't fall into the trap that just because anthropogenic CO2 can't have been the cause of previous climate anomalies, it can't matter now. Think about it! Do you think that because previous market crashes have happened, the recent market crash can't be due to subprime mortgages (which didn't exist in the 30s, after all!).--eric]

  4. 4
    Anonymous Coward says:

    Discussion of medieval and early modern CO2 levels was censored in another thread this week because it was off-topic. Perhaps Mike will accomodate it.

    There is an 7.9 ppm swing between the second-highest (MWP) and second-lowest (LIA) readings of the Law Dome CO2 concentrations. I discarded the highest and lowest readings because I figured they might be flukes.
    Assuming for the sake of the argument that this swing was caused by a fall in global temperatures and using the median carbon cycle sensitivity value from the Frank et al. recent Nature letter, a 1.03 C global cooling would be implied.
    Surely this very crude approach of mine can only be a first approximation at best. I imagine that only a fraction of these 8 ppm can reasonably be explained by the temperature changes but on the other hand I assume that there was not enough time for the full impact of the cooling to be reflected in CO2 concentrations. I also imagine that the regional variations Mike emphasized might well be more relevant than global average. I would nevertheless be interested in Mike’s opinion: would a 1C global cooling (with a huge uncertainty) between the extremes of the MWP and the LIA be more or less coherent with the temperature record?

  5. 5
    Jeff Id says:

    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”).

    I wonder what your opinion is of the obvious variance loss caused by your EIV and CPS style regression methods?

    Since the loss is known both at blogs and literature to reduce historic signal in comparison to the calibration period, wouldn’t uncertainty be the wrong word to use?

    [Response: I'll assume you're simply unaware of the fact that extensive tests of precisely this issue were provided in the previous article by Mann et al (2008) (covered by RealClimate previously) which used both "RegEM with TTLS" and "CPS" to reconstruct hemispheric mean temperatures. Tests were provided in the Supplemental Information of that article for both of the methods used, under conditions that closely emulate the situations encountered with the actual proxy data at hand. More extensive tests have been the subject of a number of previous papers referenced therein. The tests indicate that low-frequency variance loss is likely to be minimal in both cases, and particularly with RegEM with TTLS which is designed, like other similar methods that have been developed for this problem (see e.g. Hegerl et al (2006)), precisely to deal with that very issue. As the technical issues have been dealt with in this previous work, and are not the focus of this current post--which is addressing issues regarding the regional patterns of past variability and the factors that may explain them--all similar comments rehashing this and related talking points, will be considered OT. Lets keep the discussion on the topic at hand, and the scientifically-interesting issues that remain to be resolved. -mike]

  6. 6
    Byron Smith says:

    The “MCA” (Medieval Climate Anomaly) could mean that, rather than a CO2 driven AGW, we could be experiencing a “Modern Climate Anomaly” of 300+ years. We got another 150 years to go without having to blame CO2.
    Anne, that might be the case if there were not an adequate understanding of the natural forcings (levels of solar activity and vulcanism) causing the MCA and of anthropogenic forcings (greenhouse gases) largely causing what you call the Modern Climate Anomaly.

  7. 7
    AlCrawford says:

    To Tom S (#2)

    I would like to echo your comment on the fact that it is very welcome to have an actual science article here. I, too, am tiring of political battles and would like to learn more about Climate Science.

    I would, however, like to point out that the “it’s worse than we thought” projections start out by noting that the current measurements of the variables that effect climate are currently at the worse case projections of the latest IPCC forecasts. Thus it really is “worse than we thought” a few years ago — not based on models but rather based on actual measurements.

    Second, it has been point out many times a major difference in forecasting weather and forecasting climate.

    With climate forecasting we are looking at average climate over a period of time. For example, what will be the average temperature of the earth during the decade 2100-2109? And we can make these predictions with relative accuracy based on the amount of GHGs man puts into the air between now and then (that is an unknown since it is not science but rather policy.) And these projections are based on physics.

    On the other-hand, I can understand that you would like to know exactly (say to the hour) when a hurricane would hit, where it would hit, and how strong it will be. And you would like to have this information several days out. And, as you point out, that is not within the capability of weather forecasting at this time.

  8. 8
    Deech56 says:

    Anne, for your possibility to be true, the climate sensitivity would need to be low – which, if I understand the science correctly, would be contradictory with the changes in climate during the LIA.

    It is important to understand the causes for changes in past climate so we can anticipate the changes due to a higher CO2 forcing.

    Very welcome post, BTW. It is good to read about the context of the 2009 Mann, et al. paper.

  9. 9
    RichardC says:

    2 Tom says, “You can use the d-word for me if you like, but I totally support the modeling efforts as they could be very useful when they are proven accurate, particualarly on the regional basis.”

    and 3 Anne says, “We got another 150 years to go without having to blame CO2.”

    You’re both setting up impossible standards. If AGW is sound, there is no time to waste. Prudence says to leave something alone when it may bite. You’re inverting the philosophy of prudence, which says to stop adding CO2 as quickly as can be done practically, until/if we know CO2 is “benign”.

  10. 10
    Ike Solem says:

    There have been several papers published on an alternative explanation to the solar forcing – volcanism model for the Little Ice Age – namely, a terrestrial carbon uptake model based on the depopulating effects of the black plague in Europe and smallpox in the Americas:

    Europe’s “Little Ice Age” may have been triggered by the 14th Century Black Death plague, BBC, 2006

    Dr Thomas van Hoof and his colleagues studied pollen grains and leaf remains collected from lake-bed sediments in the southeast Netherlands. Monitoring the ups and downs in abundance of cereal pollen (like buckwheat) and tree pollen (like birch and oak) enabled them to estimate changes in land-use between AD 1000 and 1500.

    There were a few objections – one, that the changes in carbon emissions and uptake were too small to make a real difference – however, the same argument also applies to solar forcing and volcanism. There are other effects of reforestation that can alter temperatures, as well – reduced albedo, cooler soil temperatures, etc. – particularly over several centuries.

    It’s possible that the ocean would have equilibrated with atmospheric CO2, reducing the effect. However, there could have been a significant lag time (say, 300 years?) There are also indications that similar pandemics in America played a collaborative role, meaning that it wasn’t only European carbon uptake:

    New World Post-Pandemic Reforestation Helped Start Little Ice Age, Say Scientists
    Dec. 19, 2008

    They concluded that reforestation of agricultural lands—abandoned as the population collapsed—pulled so much carbon out of the atmosphere that it helped trigger a period of global cooling, at its most intense from approximately 1500 to 1750, known as the Little Ice Age.

    “We estimate that the amount of carbon sequestered in the growing forests was about 10 to 50 percent of the total carbon that would have needed to come out of the atmosphere and oceans at that time to account for the observed changes in carbon dioxide concentrations,” said Richard Nevle, visiting scholar in the Department of Geological and Environmental Sciences at Stanford.

    Hence, carbon-cycle feedbacks linked to biological activity appear to have played at least some role in the climate variations of the past thousand years. Predicting how such feedbacks will play out in the next 300 years will certainly require good predictions of regional climate change – not just temperature, but also precipitation, snow pack, soil moisture, etc.

    Only by including carbon cycle feedbacks and longer time periods than 100 years in your projections can you come up with realistic long-term temperature estimates. So, you have to take the carbon cycle into account – hence, if the model ignores the carbon cycle effects…

    We interpret the resulting reconstructions in the context of results from climate model simulations forced by estimated past changes in natural (solar and volcanic) radiative forcing. – Mann et al. 2009

    Shouldn’t you at least take a look at forcing those models with plausible shifts in atmospheric carbon (not only in CO2, but also in black carbon from fires) due to a human population crash and associated reforestation? It’d be nice to see some kind of model-based estimate of the possible effect.

    [Response: Very unlikely scenario--CO2 didn't vary more than a few ppm over that time period, and the amount of land cover change possible at the time (minimal) is in concert with that. Further, albedo generally goes down with increasing tree cover, not up, albeit countered by evapotranspirational increases.--Jim]

  11. 11

    Tom S 2: Back-casting performance isn’t very interesting when the models are tuned as they are…

    BPL: Precisely how do you believe the models are “tuned?”

  12. 12

    Anne 4: The “MCA” (Medieval Climate Anomaly) could mean that, rather than a CO2 driven AGW, we could be experiencing a “Modern Climate Anomaly” of 300+ years.
    We got another 150 years to go without having to blame CO2.

    BPL: It’s called “radiation physics,” Anne. You might want to research it. There are physical reasons for thinking CO2 is responsible. The theory is not based on correlations in climate data.

  13. 13

    Anne 3, I meant, not 4. Sorry.

    Question for the scientists: if it were to be discovered that volcanic aerosols make the spread of drought worse, but industrial aerosols slow it down, what would account for the difference? What is different in the effects of the two?

  14. 14
    Anonymous Coward says:

    Ike (#10),
    Law Dome CO2 concentrations peaked before the “black death”, which is not to say it had no effect but it was no trigger. The temperature reconstructions I’ve seen (no doubt a small subset, and possibly not representative) peak centuries earlier.
    This event has been given unreasonable weight by the way. It took place during a protracted period of high mortality in many regions of Europe. This, and not a single disease, explains the demographic setback. So it’s more likely (though very speculative) that climate change caused chronic famines and therefore the “black death”.

    Note that, going by the ice cores, the fall in CO2 concentration between the MWP and the LIA was quite small. It’s not much of a forcing compared to the circulation changes described in Mike’s post.

  15. 15
    Edward Greisch says:

    9 Ike Solem: That is very interesting, but the Black Plague Hypothesis implies an enormous and local sensitivity. If it is right, we are in much deeper trouble than the deep trouble we thought we were in. Problem: CO2 effects couldn’t be limited to regions. CO2 disperses worldwide. I would like to hear from RealClimate people on that.

    3 Anne: NO!!! Too dangerous! We can’t take chances that might result in the extinction of the human race.

    RC: Thanks much. I downloaded the Guiot et al paper.

    I am on page 122 of “Storms of My Grandchildren” by Jim Hansen. I strongly recommend this as a popular book. Are there climate textbooks to download for free?

  16. 16
    Hank Roberts says:

    > Edward Greisch
    > plague … implies an enormous and local sensitivity

    Doesn’t make sense. CO2 is well mixed over a relatively short time.
    If you infer local sensitivity, what’s your basis for the inference?

  17. 17
    David B. Benson says:

    Mike — Very clear. Thank you.

  18. 18
    Completely Fed Up says:

    “What a relief, I was going to stop coming here if the last couple months kept up.”

    Whew, we dodged a bullet there, guys…

  19. 19
    David B. Benson says:

    From the 75 year smoothed data in
    ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/law/law_co2.txt
    CO2 concentration varies over the intervals in question:
    1010 AD 279.5
    1185 AD 284.0
    1250 AD 281.9
    1330 AD 283.1
    1395 AD 280.3
    1440 AD 280.9
    1465 AD 280.6
    1530 AD 283.2
    1630 AD 275.6
    1730 AD 277.0
    1850 AD 285.2

  20. 20
    John Mashey says:

    1) An issue of The Holocene coming out later this year will have a bunch of papers relevant to this.

    2) Here’s a Law Dome CO2 plot.

    The chart shows 500 years where CO2 jiggles around ~280:284. Even allowing for the uncertainty of dates, *something* odd clearly happened in 30-50 years (~1570-1604 plotted dates) to drop CO2 sharply, and it didn’t return back to the 1000AD-1570AD range for 200 years…

    3) Regardless of what was going on earlier with the Black Death, evidence has been piling up for a 50M+-person dieoff (not the 5-10M of earlier guesses) in the Americas, with massive reforestration that covered previously-cleared land in Central America and the Amazon region. This was on top of any volcanoes and solar irradiance changes that occurred… but it’s hard to see anything comparable.

    Here’s a related article.

  21. 21
    Anand says:

    Dear BPL, You said:
    “It’s called “radiation physics,” Anne. You might want to research it. There are physical reasons for thinking CO2 is responsible. The theory is not based on correlations in climate data.”

    The best CO2 proxy estimates (for eg., see Frank et al, 2010)for the early part of the millenium have high values for atmospheric CO2. If it were all due to “radiation physics”, we would have had a medieval warm period and not just medieval climate regional anomalies, wouldn’t we?

    Which is the question raised/discussed by the post.

    Regards

  22. 22
    fhsiv says:

    Can anyone give me a reference or two to a paper, website or whatever that describes the details of the techniques employed in the actual drilling/coring, sample recovery/preservation and sub-sampling of ice cores like the one taken at Law Dome?

    [Response: Start here.-Jim]

  23. 23
    barry says:

    I am reminded of a web page regularly posted by skeptics in the blogosphere, showing a world of medieval warming.

    The Medieval Warm Period – A global Phenomenon

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

    Slightly more than cursory examination reveals that the warm peaks at various of the locations around the world on that map are offset by as much as 500 years. I guess the skeptics just look at the headline, unaware that they are providing corroboration for the lack of sync and sign between regions during the putative MWP much discussed in the literature.

  24. 24
    Hank Roberts says:

    Anand, presenting “best CO2 proxy estimates” would likely be a full research paper — more than this Letter seems to even claim to present.
    You’re going beyond what the authors seem to be claiming.

    Letters are: http://www.nature.com/nature/authors/gta/#a1.2
    http://www.nature.com/authors/editorial_policies/peer_review.html

    It’s certainly interesting, going by the Abstract and supplement and figure that are available to the public (the actual Letter is of course paywalled). I’ll be curious to read James Annan’s comment on it. Is there one yet?

    Have you read the actual Letter, or are you posting someone else’s opinion? What’s your source for claiming this Letter contains the “best” estimates? Why do you consider your source reliable on this point?

    Your description of what’s in the Letter doesn’t seem to match the Figure they do make public there, though. Have you looked at it?

    Some links:

    http://www.ncdc.noaa.gov/paleo/pubs/frank2010/frank2010.html
    http://www.nature.com/nature/journal/v463/n7280/suppinfo/nature08769.html
    http://www.nature.com/nature/journal/v463/n7280/fig_tab/nature08769_F1.html#figure-title

  25. 25
    Timothy Chase says:

    It would appear that Peter Sinclair’s Climate Denial Crock of the Week has won. He took the lead about 3:00 AM East Coast Standard and by a little before midnight he was roughly 800 votes ahead. At end of count it was 5674 to 4859. Congratulations to everyone that was involved. As far as I am concerned we all won!

    However, now perhaps is a good time to start getting people to add http://climatecrocks.com to their blog lists. The posts may be videos, but the scripts are posted, certainly measure up and they are weekly.

    If they have any doubts, I would remind them what two leading climatologists had to say of his work:

    “…arresting graphics and straightforward explanations to point out what the science really says, how the contrarians distort and misinform.”

    -Gavin Schmidt, Goddard Institute for Space Studies, NASA; and,
    Micheal Mann, Director, Earth System Science Center, Penn State University

  26. 26
    Edward Greisch says:

    16 Hank Roberts: I agree with you. I don’t see how CO2 could be local either. But the Black Plague hypothesis requires local CO2. So I was asking if I missed something. I think RC Jim answered it: “[Response: Very unlikely scenario–CO2 didn’t vary more than a few ppm over that time period, ……” Also look at the numbers 19 David B. Benson provided. He didn’t show every year, but it looks like nothing much happened with the CO2 over those years.
    Thanks for the link:
    http://www.nature.com/nature/journal/v463/n7280/fig_tab/nature08769_F1.html#figure-title
    which shows it even better. Holocene normal seems to be 280 ppm. Variation is quite small until the 20th century begins.

    This is turning out to be quite interesting.

  27. 27
    SoundOff says:

    A skeptic made the following argument to me with respect to the significant warming period 1985-2000:

    Skeptical Science – The albedo effect says that “A change of just 1% to the Earth’s albedo has a radiative effect of 3.4 W/m², comparable to the forcing from a doubling of CO2”. A reasonable statement.

    And RealClimate – Cloudy outlook for albedo? says “the ISCCP estimate shows a decreasing albedo trend of 1-2% in the 80s and 90s”. Not sure.

    Using ΔRF = 5.35 ln(CO2/CO2_orig), the no feedbacks forcing of CO2 added during the 15-year period was 0.34 W/m² based on 346 ppmv in 2000 and 369 ppmv in 1985. Even if all earlier forcing since industrialization (1.48 W/m²) is added we still only get 1.82 W/m² (I added this since most of the CO2 is still there, he didn’t).

    The skeptic’s conclusion is that CO2 is a minor player in climate change (0.34 W/m²). Rather, an albedo decrease (3.4 W/m² to 6.8 W/m²) was driving the warming for the period in question. This can’t be right since this is equivalent to a 2.5°C to 5.0°C temperature increase in 15 years (didn’t happen).

    Can someone explain the fallacy here?

    [Response: They are assuming instant equilibration. The planet does not respond instant;y to increases in forcing - mainly due to the heat capacity of the ocean which requires decades (and more) to rise. Secondly, the ISCCP trends are mostly due to satellite coverage and calibration issues and are far smaller than the ISCCP precision would justify. - gavin]

  28. 28
    Completely Fed Up says:

    Now, to be serious for a change, John, I would only call the drop odd. That it took 200 years to recover is not odd, merely expected: after such a large drop, you would only get a quick recovery if there was an unusual and sudden increase.

    I would also like to point out that that date requires you take the graph as without error. But the line doesn’t have to *pass* a figure to return there: it has to be close enough to be indistinguishable given the error bars for that line.

    It would be better to use the peer reviewed work by someone skilled in this area and guild from it (or, if you wish to show the work incorrect, show its errors), rather than start with analysis on your own. Because not only has someone already taken time to consider what *ought* to be said of a graph, there’s a lot more information in a paper than you can fit here in a blog posting.

  29. 29
    Anonymous Coward says:

    Anand (#21),
    Do the (very basic) math: how much cooling would one you get out of a fall of 5 to 10 ppm in atmospheric CO2 from about 280 ppm with 3 C of sensitivity per doubling?

  30. 30

    Anand 21: The best CO2 proxy estimates (for eg., see Frank et al, 2010)for the early part of the millenium have high values for atmospheric CO2. If it were all due to “radiation physics”, we would have had a medieval warm period and not just medieval climate regional anomalies, wouldn’t we?
    Which is the question raised/discussed by the post.

    BPL: What was the Earth’s albedo back then? The cloud cover? The aerosol loading? Other greenhouse gases? Volcanic activity?

    It’s not all a one-to-one relationship between CO2 and temperature. That straw man exists only in the minds of denialists.

    [Response: Also note that the 'high' CO2 values are only a few ppm above the values in the LIA. The radiative forcing from 5ppm is 0.094 W/m2 - which would imply a temperature rise of 0.1 deg C at most (even assuming a very high end sensitivity). The GHG changes are taken into account when modelling the last millennium but they aren't dominant until you get to the 20th C. - gavin]

  31. 31
    Dennis says:

    John’s graph of the Law Dome CO2 levels is quite remarkable. Apparently,
    “In 1492, Columbus sailed the ocean blue”,
    spread new diseases to the Americas,
    which decimated the native Americans,
    allowing large scale reforestation,
    which sucked CO2 out of the atmosphere,
    which measurably reduced CO2 levels,
    possibly causing The Little Ice Age.
    It certainly makes sense: last I heard Native American population fell by up to 90% prior to the Jamestown Colony in 1609. Since many tribes did slash & burn agriculture, that’s a lot of reforestation.

    [Response: Let's not get carried away with conjecture here. As Mike noted, we should stay focused on the suite of (very interesting and) important scientific questions raised by this post--especially those related to the idea of spatial/temporal patterns of climate data in relation to concepts and models of their likely physical causes. Although the small but abrupt CO2 drop referred to represents a lot of potential forest carbon, it's insignificant as a global climate forcing. And there are large uncertainties wrt pre-historic land use changes and the non-atmospheric components of the carbon cycle.--Jim]

  32. 32
    Martin says:

    Tom S (#2)
    AlCrawford (#7)
    I agree. A scientific article was long overdue. Hope to see more posts like this one at RC.

  33. 33
    Anonymous Coward says:

    Ed (#26),
    The CO2 “hockey stick” arguably starts in the 1870s when the second industrial revolution was in full swing rather than in the 20th century. There was a pause afterwards but the rate of increase in atmospheric CO2 around 1880 would not be matched until the 1920s and not supassed until the 1950s (going by the Law Dome).

  34. 34

    Dr. Mann,

    I am pleased to see that you are still banging away with science education despite the numerous personal attacks you keep taking. You are a real pro.

    I am reminded of a quote one of my chemistry colleagues told me whan I kept losing my battles to raise the minimum math standards for non-majors science courses:

    The sign of a true pioneer is the number of arrows in his/her back.

    Scott A. Mandia, Professor of Physical Sciences
    Selden, NY
    Global Warming: Man or Myth?
    My Global Warming Blog
    Twitter: AGW_Prof
    “Global Warming Fact of the Day” Facebook Group

    [Response: Thanks for the kind words Scott. --mike]

  35. 35
    Dennis says:

    Yes, it’s a beautiful speculation (that Columbus caused the Little Ice Age), but only speculation. A glance at the millennium temperatures seems to show a pre-existing cooling trend, and there’s a lot of variability between estimates and over time. Any temperature changes caused by a 3% drop in CO2 will be lost in the noise.
    Still, some temperature reconstructions show a dip at the expected time. (I’m looking at wikipedia’s graph http://en.wikipedia.org/wiki/File:1000_Year_Temperature_Comparison.png).
    Unfortunately, it’s no more speculative than a lot of putative cause&effect chains that are floating around in the noosphere. Maybe I’ll use it the next time I debate a climate cynic, since they love such spurious arguments.

  36. 36
    Dennis says:

    (#29) “Do the (very basic) math: how much cooling would one you get out of a fall of 5 to 10 ppm in atmospheric CO2 from about 280 ppm with 3 C of sensitivity per doubling?”
    Ans: 3% x 3C = 0.09C, which is consistent with the temperature changes in that time span (some of the reconstructions seem to show a change of up to 0.2C, but none show an increase). (using the graph in wikipedia)
    (Caveat: such a small change is lost in the noise. If the graphs would be black with error bars, if it showed them)

    [Response: A further point here is that the Law Dome data are more extreme wrt the CO2 drop in question, than are other high res. ice core data which show a smaller, and less abrupt decline at the end of the 16th century.--Jim]

  37. 37
    Anonymous Coward says:

    #36: Some reconstructions seem to have drops of as much as 0.6 to 0.9 C (eyeballing) depending on the period you’re considering. Others feature smaller changes. I don’t know which ones are most credible, hence my question in #4.
    In any case it seems that changes in heat distribution and insolation (see Mike’s post and Gavin’s comment) among other plausible factors had more impact than CO2. So the carbon cycle sensitivity to temperature should be more relevant than the climate sensitivity to CO2… or maybe not after all, considering that the ppm/C ratio around 280 ppm seems to be an order of magnitude higher when CO2 is a forcing rather than a feedback.

  38. 38
    Thomas says:

    I’d like to throw out one concerning anthrogenic climate forcings during the period in question (last millenia roughly). We know from the CO2 proxies that the global forcings were too small to be important. However if there were major changes in land use -perhaps driven by population diebacks, might not that produce some regional climate drivers (albedo of forest versus field, etc.)? Since these drivers are not globally diffuse, might they have some impact on atmospheric circulation, and hence regional climate shifts? I haven’t seen anyone in these comments talk about methane forcing, does the proxy dta show that the change in methane forcing was alaso too small?

  39. 39

    Okay, what I want to know is (I keep forgetting), what might a more la nina response mean for me in the Rio Grande Valley of Texas (about 50 miles west of the Gulf of Mexico).

    Does it mean more or less frequent/intense Atlantic hurricanes?

    Does it mean cooler or warmer weather?

    After the strongly negative arctic oscillation brought freezing weather from the arctic down to my doorstep here this winter and killed my tomato seedlings & lots of my tropical plants in Dec & Jan (most of which are growing back pretty well, and we planted more tomatoes), I’d like to know.

    I’d been thinking all along how utterly horrible AGW is, esp all the death and destruction it is and will be causing, but at least we wouldn’t get freezes here anymore, or they’d be much less frequent.

  40. 40
    Frank Giger says:

    Lynn, my understanding is that there is no solid answer to your tomato question.

    “Climate is what one expects; weather is what one gets,” to nearly get the Mark Twain quote correct (I haven’t bothered to look it up for word for word accuracy).

    So a shift of climate doesn’t mean it won’t ever frost in Florida. It just means it will become more unusual than it is now.

    The pain comes in the establishment of the “new normal” in the climate. Crops and timing of planting and harvest will change, as will a host of secondary effects. I had a friend from Canada ask about the HVAC unit we had installed (one of those high efficiency deals), and was curious that I was rather disinterested on how well it heated the house. Here in Alabama we’re far more interested in cooling and pulling humidity out of the inside air, which isn’t such a big concern for folks in the Great White North.

    We may well see where the importance of units that cool as well as heat moves with the adjustment of climate – and the need for heat to move south.

    On plagues and climate, let’s remember that the unclimate like cold of Europe was driving folks inside, furthering the spread of the Black Plague, and that the return to warmer climates (particularly on the north Med), lead to the flowering of technology (along with a lot of great plants Columbus brought back), aided by a reduction of population pressure.

    Likewise, the plague didn’t strike uniformly. Some cities in Europe were decimated while others in relatively close proximity weren’t touched at all. And it wasn’t just one sweep; there were several waves of the Black Death that happened with large gaps between them.

    I’m very encouraged that RC has returned to science!

  41. 41
    John Mashey says:

    re: 28
    This started with the Black Death, and I mostly pointed out the Law Dome data because I already had that chart around to show that if there were a plague-induced die-off that actually made a noticeable difference, it was likely to be the later one in the Americas. I am quite familiar with “Correlation is not causation”, and fact that ice-core records differ, and dates aren’t accurate to a year. I urge people to keep on eye on The Holocene for an issue that I think will have a bunch of interesting papers.

    Does anyone think that (currently) reforestration/forest preservation can have the slightest useful effect on CO2? If your answer is NO, then for sure, then the 1500s speculation is irrelevant. If your answer is YES, then it just might be possible that a much larger reforestration (land reclaimed by high-density, fast-growing Neotropical biomass from that used by 50M people doing low-density agriculture) might have had an effect…
    One more time: the relevant-to-this-post interesting scientific question is whether or not a cooling that certainly happened would have had different *regional* fingerprints depending on the relative contributions of:

    a) Changes in volcanoes + solar irradiance
    b) Changes in CO2 from massive reforestration
    c) Feedback from lower

    I suggest, to avoid hijacking this thread (and I don’t have time to drag out the various references I’ve studied, and which I suspect not everyone has), that for this thread, the relevant issue to debate is NOT whether the die-off caused this, but whether different combinations of the above would have showed different fingerprints. After all, people do lots of “what-if” models for various scenarios, and this doesn’t seem any different.

  42. 42
    jl says:

    for lynn V
    http://www.esrl.noaa.gov/psd/enso/compare/
    keep in mind there are many other factors going on
    like http://www.ncdc.noaa.gov/teleconnections/
    or the State Of the Climate report http://www.ncdc.noaa.gov/bams-state-of-the-climate/
    hope this helps
    PS
    to RC keep up the good work

  43. 43
    Smitty says:

    Is it true that what we currently label the MWP, used to be called
    the “Medieval Optimum?” Maybe the intention of bringing clarity and understanding by renaming events, might conversely create greater confusion.
    Just sayin’

  44. 44
    Kate says:

    I second Scott Mandia’s comments on your courage, Mike – many people would turn and run away, rather than continue to fight misconceptions, when they were subject to the kind of attack you have been.

    Keep up the good work, and thanks for this great overview, it was a very clear and interesting post.

    [Response: thanks :) --mike]

  45. 45
    Completely Fed Up says:

    “Does anyone think that (currently) reforestration/forest preservation can have the slightest useful effect on CO2?”

    You would have to search for the inevitable consequence of reforestation at the level to produce that effect.

    Rather like the precession of the orbit of mercury was a consequence of general relativity.

    If that much CO2 was taken out by tree building, what other effects would occur?

    Albedo changes? O2 changes, Rainfall runoff and alluvial basin thinning?

    How much land would have changed, and where could that have been?

    Think of the things that would be the consequence of your theory to explain CO2 changes but are not themselves the CO2 nor things that could change CO2, else you have a self-referential theory that cannot distinguish between itself and other theories that reduce CO2.

    Only some of the alternate theories have the same consequence, and you can thin the herd.

    If that consequence doesn’t appear, then you’ve removed that from being the major cause.

  46. 46
    Anand says:

    Hank Roberts:
    Thanks for your comments

    Yes, I have read the paper. I discussed this paper in Jim Bouldin’s thread over a three-week period some time back at RC. There is a thread “Good news for the earth’s climate” (or SLT).

    The paper incorporates the ‘best’ possible estimates for early millenial CO2 – those are not great considering how far back we are trying to project, but one could argue the same about temperatures as well. We have to do with what we have.

    Gavin:
    I read R. Lindzen’s explanation that the CO2 GHG effect yield is highest when its concentrations are lower i.e., doublings at lower CO2 conc. produce greater GHG effect than at higher concentrations.

    If this were true, the CO2 fluctuations during the MWP/LIA should have had a greater effect than seen with the higher concentrations seen today. Is this correct? Especially with the emergence from the LIA.

    Another thing is – if we set up the argument that the present CO2-predominant regime is very different from any situation hitherto seen, how and what can we ‘learn’ from the climate of the recent past?

    Michael/Jim
    Is it the right approach to look at or calculate the temperature anomaly as an indicator of global change, or, is it simply sufficient to have high resolution regional histories right? And what drives what?
    The apparent circularity of arguments (at-least to me) is beyond my ability to break out of.

    Dr Mann, You should fight the good fight. :)

    BPL:
    You cannot call “CO2 is the main climate driver” argument a denier strawman, and use a variant of it yourself!

    Regards

  47. 47
    mike roddy says:

    We don’t need to do millenial timescale reconstruction for certain ecosystem responses. In 1994, The Oregon State Board of Forestry published a collection of over 200 papers called “Cumulative Effects of Logging in Oregon”. Siltation and ecosystem simplification were noted to be serious and long term legacies, but measurements of microclimate temperature alterations were fascinating.

    Both air and adjacent stream temperatures were noted to be several degrees or more higher after industrial logging, to the point that habitable temperature bands for andromonous fish were compromised, and aquatic food webs altered.

    These changes persisted and in some cases were long term, because even if the trees were reestablished, stream ecosystems had headed into different trajectories.

    Similarly, after the forests of Greece and Turkey were destroyed several millenia ago, hotter and drier microclimates prevented their reestablishment.

    This is slightly OT to this post, but there is detailed data under our noses about changing microclimate effects on ecosystems that is relatively recent. If you can find the book in the interlibrary lending system, that is- the timber industry suppressed this detailed report soon after it appeared. Tim Hermach of Native Forest Council has hard copies for those who are interested.

  48. 48
    John N-G says:

    “Medieval Climate Anomaly” is nicely value-free, but implies that the Medieval climate was somehow anomalous compared to other periods. Is there any evidence that the MCA departed farther from some suitable multi-millenial mean than other carefully-chosen 500-year intervals?

    The immediate pre-industrial period may be the basis for our sense of normal, making medieval climate seems anomalous, but it seems to me that over a multi-millenial horizon the “Little Ice Age” is much more of a climate anomaly than what some prefer to call the Medieval Climate Anomaly.

    In the broadest sense, the natural forcings appear to exhibit sufficient variability to cause climate to be nothing more than a sequence of anomalies. Just as the weather is never normal, neither is the climate.

  49. 49
    David B. Benson says:

    (1) Law Dome CO2 records are of the highest quality. (1.1) Law Dome was chosen to obtain an ice core for exactly this reason. (1.2) Law Dome records overlap the Keeling cuvre until 1978 CE. The two records agree to within 0.25% (0.0025) up until 1972 CE and then the data for 1972, 1972, 1977 & 1978 agrees to within 0.5%. (I suspect these small variations are due to being at the end together with “20 year smoothing”. (1.3) If other Antarctic ice cores disagree with Law DOme I’ll take that as evidence of the inferior quality of such other cores; the usual difficulty is dating, a problem which Law DOme does not suffer from.

    (2) While
    http://en.wikipedia.org/wiki/Charles_C._Mann
    is not an anthropolgist, his scientific journalism is of high stand and his
    http://en.wikipedia.org/wiki/1491:_New_Revelations_of_the_Americas_Before_Columbus
    is generally on the right track, as shown more recent reasearch in, for example, the Amazon Basin (see a recent issure of Scientific American).
    One concludes a tremendous decline in agriculture throughout the Americas in the century following Columbus.

    (3) The conclusion that CO2 concentrations declined to a minimum around 1610 CE then follows, in agreement with Law Dome data.

    (4) But, by itself, that decline in CO2 concentrations appears insufficent to explain LIA and other factors, inlcuding the Maunder Minimum, are known.

    (5) More puzzling are the temperature profiles during LIA from locations in the southern hemisphere.

  50. 50
    David B. Benson says:

    John N-G (48) — Exactly so.

    While central Greenland palotemperatures (GISP2) only indicate the general trends in that region (northern North Atlantic), looking at Richard Alley’s GISP2 data over the Holocene suggests nothing at all remarkable about MCA; temperatures wobble on all time scales.


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