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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. 51
    David B. Benson says:

    “Detection of Human Influence on a New, Validated 1500-Year Temperature Reconstruction” Gabriele C. Hegerl, Thomas J. Crowley, Myles Allen, William T. Hyde, Henry N. Pollack, Jason Smerdon, and Eduardo Zorita
    From the abstract: A new reconstruction using this method shows substantial variability over the last 1500 yr. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate.

  2. 52
    David B. Benson says:

    Anand (46) — Arrhenius’s approximation
    holds over all CO2 concentrations of interest. For more detail see the IPCC AR4 WG1 report.

  3. 53
    Jerry Steffens says:


    The “sharp” drop in CO2 on the graph looks large only because of the way the data is graphed; any fluctuation can be made to appear large if the vertical range of the graph is sufficiently small. See how significant that drop looks if the data is re-plotted with a lower limit of zero rather than 270 ppm.

  4. 54

    Anand 46: You cannot call “CO2 is the main climate driver” argument a denier strawman, and use a variant of it yourself!

    BPL: Read a book, okay? I mean one on climatology.

  5. 55
    Alexandre says:

    Prof. Mann,

    I´d like to learn more about how data infilling is done. Is there any online starting point you would suggest?

    [Response: You can find some discussion about this and some relevant references for further details, in the supplementary online information. The key reference is Schneider, Journal of Climate, 14, 853–871 (2001), available here. --mike]

  6. 56
    Edward Greisch says:

    20 John Mashey: Thanks much for the graph of CO2 from the ice core. The points you marked in red aren’t that big of a dip IMHO. But they do look like a small dip. Perhaps a plague could do that by allowing a lot of trees to grow.
    RC: Does this rise to the level of needing research, or shall we just leave it?

    [Response: It's not relevant to the topics Mike raises in the post, because as has been pointed out repeatedly now, the amount of CO2 involved relative to radiative forcing requirements for creating the LIA are completely insufficient, not to mention the other issues I mentioned. Why people are obsessed with this I have no idea.--Jim]

  7. 57
    Edward Greisch says:

    34 Scott A. Mandia: Just keep on trying to raise the math and science standards for those innumerate humanitology majors. We have been kicked around by them for too long. It is high time for ALL college students to be required to take the Engineering and Science Core curriculum at the very least. They need exposure to reality.

  8. 58
    Ike Solem says:

    Jim Bouldin says: “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.”

    Solar variations and supposed volcanism are also highly uncertain – are they enough to account for the observed temperature changes in those time periods? What’s the relative difference there?

    Note that published records of CO2 changes in this time frame dispute the claim of a “few ppm” changes:

    Atmospheric CO2 reconstructions are currently available from direct measurements of air enclosures in Antarctic ice and,
    alternatively, from stomatal frequency analysis performed on fossil leaves. A period where both methods consistently
    provide evidence for natural CO2 changes is during the 13th century AD. The results of the two independent methods
    differ significantly in the amplitude of the estimated CO2 changes (10 ppmv ice versus 34 ppmv stomatal frequency).

    Hence, dismissing this notion out of hand seems unsupportable – you can’t keep ignoring the carbon cycle – and given that the work seems carefully done, peer-reviewed and published, why do so? The authors aren’t claiming that this is the only cause, after all:

    Nevle and Bird don’t attribute all of the cooling during the Little Ice Age to reforestation in the Americas.
    “There are other causes at play,” Nevle said. “But reforestation is certainly a first-order contributor.”

    All in all, there seems to be decent evidence that indicates human activity was modifying the global climate – slowly – during the past thousand years – not via fossil fuels, but rather by land use changes. In addition, you should consider the amount of black carbon released via humans burning off grasslands and cutting down trees – it would have a certain warming potential, correct? So, if population crashes in Europe and the Americas reduced atmospheric black carbon and increased CO2 uptake (and that seems to have happened, based on carbon isotope ratio studies), then the combined effect could be significant over several centuries – and thus could play a role in explaining both the Medieval Warm Period and the Little Ice Age. This also raises the question of when the “Industrial Warm Period” really began – could that have been what terminated the Little Ice Age?

    On the black carbon issue:
    Brown Haze from Cooking Fires Cooking Earth, Too, 2007

    Given that future carbon cycle uncertainties will play major roles in the future climate trajectory, the carbon cycle deserves a bit more focus – such as:

    1) 1. Uncertain ocean responses with respect to future carbon uptake

    2) 2. Uncertain permafrost / soil carbon responses under warming scenarios

    As far as the LIA and the MWP, I know that realclimate authors & co-workers prefer the solar/volcanic hypothesis along with some kind of interaction from “shifting ocean modes” – but the evidence for such multi-decade/century-scale ocean modes is still based on statistical arguments, not on any well-defined mechanism, correct? I think much of it involves misapplication of time series data analysis to proxy records, in fact – and when you look for mechanistic explanations, you won’t find any. Many of those “modes” were discovered by researchers running time series analysis against some dubious proxy records (Pacific salmon catch records, for example?) – it just doesn’t seem very reliable – but this is the general argument being promoted:

    For example, Shindell et al (Science, 2001) showed model results that suggested solar forcing could lead to enhanced winter cooling over certain regions of the Northern Hemisphere such as Europe during the Late Maunder Minimum (the coolest part of the European “Little Ice Age”, the late 17th and early 18th centuries). The mechanism involves the large-scale dynamical response of the atmosphere to the estimated decrease in solar irradiance and subsequent stratospheric ozone change. The response was associated with a shift towards the negative phase of the so-called North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) which projects onto a pattern of regional warmth and coldness that tends to cancel out in an average over the entire Northern Hemisphere. There is supporting evidence for this idea based on widespread proxy evidence for a negative phase of the NAO during late 17th and early 18th century intervals.

    Nice idea, but I don’t think it justifies entirely ignoring the carbon cycle issues.

    I’d go into more detail on EOFs and time series analysis – but that topic’s been rejected before, hasn’t it?

    In fact, the evidence for decadal oscillations in the ocean is pretty weak. [edit--that's enough of your indiscriminate bashing of legitimate climate research efforts. we've allowed you to make these dubious arguments several times now. you can take it your own site if you like, but we've had enough of it here.]

    [Response: Nobody's ignoring the carbon cycle--the opposite in fact. The idea--at least as applied to the late 16th century, is not as far fetched as I first thought, and the Nevle and Bird paper is definitely one I'll be reading closely. It's highly likely that the drop in Indian populations increased carbon sequestration, but if you assume that the Law Dome data are "truth", then a drop of 7-8 ppm (=16 Gt C) over 35 years requires an enormous amount of land, greater even than their estimated 50 million impacted hectares--and also is not supported by the subsequent rapid increase in CO2 after that time--just at about the time you would expect a rapidly accelerating sequestration.--Jim]

  9. 59
    Sufferin' Succotash says:

    Re #31…

    Columbus sailed the ocean blue in 1492, but the Amerindian population losses were spread out over several centuries, starting mainly in the 1520s in Mexico. Europe’s 14th century demographic decline really begins 40+ years before the Black Death with the famine in 1315-1316(c. 10% population decline?)due to poor harvests caused by unusually cold wet weather.

    Jest sayin’…

  10. 60

    Mike, welcome back. I hope you get some more positive comments here. I posted a petition to support you and Phil Jones and had some good comments there too:

    It’s good to get back to a touch of realism about what we are doing studying the medieval climate. If arguing over who did what in a 1998 paper is the best the denial people can do, I wonder how they think science really works.

    [Response: Well put. Thanks so much Philip! --mike]

  11. 61
    Sili says:

    Even if depopulation after the Black Death played a role, one’d then also have to take into account the changes in agriculture brought about by the BD.

    The reduced population meant that labour intensive farming had to be cut back from lack of serfs. In place of it the number cattle of grew greatly since they can be kept by a single cowherd for extended periods of time. So one could even speculate that increased populations of cattle would have to give higher methane emissions with subsequent warming. I’m not aware of any evidence for such a hypothesis, though.

    Hasn’t there been claims that the MCA could in part be due to deforrestation of South America by the booming cultures there?

  12. 62
    flxible says:

    I’ll add my small vote of appreciation for the science post – and especially for one by a most excellent scientist. Heartening to see Dr Mann carrying on providing his insights. Thank you!!

  13. 63
    Anonymous Coward says:

    Ike (#58),
    One last time: the dates do not match. 14th century events can not explain the carbon cycle of the 13th century. If you want demographic catastrophes in the 13th century, look to Asia!

  14. 64
    Frank Giger says:

    Jerry, plotting from zero is as bad in demonstrating statistics as is plotting too narrowly, as it does the reverse – disguising changes in the trend.

    When I did trend analysis a lot of thought went into scaling; sometimes it was from zero, and other times it wasn’t. Typically we used double the top and bottom peaks, other times roughly one standard deviation from the mean, and others two standard deviations with markings for the bands.

    The charts in question are clearly marked to scale; it is assumed by the author that one is used to working with statistics and can interpolate how it fits on a zero scale. Or atleast I’m assuminging that what they’re assuming. ;)

    I also assumed good faith as there wasn’t any tomfoolery such as having the upward lines extend past the listed scaling, always a big tip-off.

    Let’s keep skepticism in the good faith of activists completely segregated from scientists working in good faith, as they should be considered mutually exclusive, IMHO.

    And remember that charts aren’t analysis any more than the pictures in a book are the story. They’re simply aids in helping illustrate main points of the text.

  15. 65
    caerbannog says:

    Just a little anectode (hopefully not too off-topic) that pretty much sums up the Mann-bashing denial movement…

    Had a brief discussion about global-warming with a very so-so engineer at work not long ago. A few minutes into the discussion, he treated me to a “Mann’s method generates hockey-sticks from random noise” song-and-dance routine.

    So I asked him, “How would a random-noise eivenvalue spectrum compare with Mann’s hockey-stick eigenvalue spectrum?”

    His reply was along the lines of, “Uhhhh…. what’s an eigenvalue?”

    Needless to say, that’s where the conversation ended.

  16. 66
    caerbannog says:

    arrgh… anecdote!

  17. 67

    Illegitimi Non Carborundum – Don’t let the bastards grind you down! Thanks for staying in the good fight. When the time comes for remembering who was right and who was wrong, your name will be on the side of good.

    OT: Just read this article in SciAm (yeah yeah, I know):

    Which has me wondering, how many non-temperature indicators are there of AGW? Is there a list? Just off the top of my head:

    GRACE showing decline in ice sheets
    Tundra greening, and Taiga browning
    Plants, animals, and insects shifting either towards the poles or higher altitudes
    Sea level rising
    Migrating animals returning sooner and leaving later, or not leaving at all
    More growing days

  18. 68

    Richard Hendricks @ 67:

    I went for several =years= without wearing a jacket.
    A/C stopped running in November, started running again in February.
    No peach harvest for =years= due to a lack of chilling hours.
    Had to mow the lawn (assuming it wasn’t a hard summer drought year, which is actually fairly normal where I live) well into the Fall, and started much earlier in the Spring many years.

    Someone needs to make a web page that collects these. They are much more “human” than forcings and error bars and satellite data.

  19. 69
    Witgren says:

    “68FurryCatHerder says:
    17 May 2010 at 12:10 PM
    Richard Hendricks @ 67:

    …Someone needs to make a web page that collects these. They are much more “human” than forcings and error bars and satellite data.”

    True, but then the denial crowd just changes course again and admits that while things might be warming, we can’t prove it’s because of human-produced CO2. :-P

    Of course, they never offer an alternative explanation, either…unless it’s just to hand-wave it away with “it’s natural”.

  20. 70
    Completely Fed Up says:

    “he treated me to a “Mann’s method generates hockey-sticks from random noise” song-and-dance routine.”

    It always seemed to me that this “random noise” thing required “random noise” to mean “something that more normally increases than decreases the value”, which has always had me wondering: isn’t this what an “upward trend” means?

    You know, just like the stock market. Or population. However, nobody says “the stock market is just random”, do they?

  21. 71
    Hank Roberts says:

    > nobody says
    Nobody says the stock market is _just_ random. Emphasis required.
    doi:10.2307/2330525 Cited by 208
    Beta as a random coefficient
    “… This would explain why the average NYSE stock has less than half of its total risk explained by market forces ….”

  22. 72
    Jacob Mack says:

    Indeed eric. Confidence intervals, level of confidence and the like in terms of probability/inferential statistics also plays important roles when high likelihoods are attached. It is erroneous for people to expect an exact point estimation of such complex system dynamics. I also want to second the earlier encouraging words to Dr. Mann: keep up the hard work in the scientific analysis.

    RC is a great resource even with some of those wikipedia references:)

  23. 73
    Frank Giger says:

    One has to be very careful along that track, CatHerder.

    We had a three year drought in Georgia. Every politician and activist talked it up as proof of global warming. Lake Lanier is as down as it ever has been! Act now to save the planet! (Pay no attention to the unreasonable demands placed on the water supply due to incredible increases in population as well as the drought).

    At the same time, other frustrated politicians tired of being told they had to fix the problem simply said the answer was to pray for rain.

    Then it broke, as it was more cyclical than anything else. Lake Lanier filled.

    And with it, all the credibility for the folks who had banked the bulk of their arguments on the drought as proof of global warming.

    Preachers, however, made much hay over the rains coming after the call for prayer.

    Both were dead wrong.

  24. 74
    Jacob Mack says:

    FCh: admittedly, there are limits to what any math alone can do. The measurements of central tendency,using error bars, and other legitimate methods within statistics, are still very effective in summarizing and explaining raw data findings. For example in genetics and elsewhere, the chi-square method is indispensable. Using weighted means and following the tenets of Chebshev’s theorem do matter. Real data is being used, with real empirical observations, with satellite data, with paleo-climate reconstructions, in consideration of the actual laws of physics, and there is no way this can all be analyzed without those “forcings” and “error bars.”

    Granted climate is non-linear and is also analysed using calculus with some facotors being considered noise out of necessity in such a complex system, however, the same can be said in Theoretical Physics, Physical Chemistry (with assigned probabilities in terms of the wave function)and many aspects of Developmental Biology. Epdemiological studies are analyzed and summarized using mathematics. When using X^2 or p hat analysis, for example, there are inferences made based upon the confidence interval (the former) and percentage found based upon the decimal quotient (the latter).

    With so much data the null hypothesis must be rejected; H0:mu = k. The global mean temperature has increased since the industrial age even when considering boundary conditions on the graph.

    This is a gross oversimplification in several ways,but I find it interesting that most people who seriously deny the statistics in climate science do not grasp or deny even Elementary Statistics.


    Bluman A., (2009) Elementary Statistics A Step By Step Approach.

    International Edition chapters 3,6,7, & 8.

    Storch H., & Zwiers F., (1999) Stastical Analysis in Climate Research.

    Chapters 1& 2.

    And yes Atkins as always:)

  25. 75
    Phillip Shaw says:

    FCH – here is another warming anecdote to add to the collection.

    My wife and I live in Austin, TX, and about three weeks ago at dusk we were treated to the sight of pairs and trios of large ducks flying to, and roosting in the oak trees adjacent to the restaurant we were at. Never having seen ducks perching in trees (about a km from the nearest body of water) we did some investigating and found that they are Black-bellied Whistling Ducks. Impressive birds. As recently as 2005 they were listed as a rare sighting by the local Audobon Society, but here there were about two dozen up in the oaks, calling to one another and staring down at the people below.

    What I found interesting in my brief investigation was that in a 70′s vintage North American bird guide they were not listed at all. Move forward to the 80′s and they were listed as a Central American bird occasionally blown north to south Texas. In the 90′s they were described as breeding in south Texas and occasionally seen further north. In the latest field guides and on the intertubes they are shown as breeding through much of the gulf states and have been sighted as far north as Pennsylvania.

    Any predictions on when Black-bellied Whistling Ducks will make it to Canada?

  26. 76
    Jacob Mack says:

    The stock market has plenty of random variation.

  27. 77
    SecularAnimist says:

    Frank Giger wrote: “And with it, all the credibility for the folks who had banked the bulk of their arguments on the drought as proof of global warming.”

    How do you know the drought in Georgia was NOT caused by global warming? Because it didn’t last forever? There is no reason to expect that every drought caused by global warming will be permanent. There IS reason to believe that global warming will cause more frequent, more intense and more prolonged droughts in many regions.

    Frank Giger wrote: “Then it broke, as it was more cyclical than anything else.”

    What “cycle” are you referring to? Is there a known “cycle” of drought in Georgia?

  28. 78
    Ike Solem says:

    By the way, here’s the kind of claims that are being made about the “Pacific Decadal Oscillation” (the one ‘discovered’ in the salmon catch proxy data) by Don Easterbrook:

    A prominent U.S. geologist is urging the world to forget about global warming because global cooling has already begun.

    Projected cooling for the next several decades is based on past PDO patterns for the past century and temperature patterns for the past 500 years. Three possible scenarios are shown: (1) global cooling similar to the global cooling of 1945 to 1977, (2) global cooling similar to the cool period from 1880 to 1915, and (3) global cooling similar to the Dalton Minimum from 1790 to 1820…

    The more itemized claims from Don Easterbrook are as follows:

    1. The PDO has a regular cyclic pattern with alternating warm and cool modes every 25-30 years
    2. The PDO has accurately matched each global climate change over the past century and may be used as a predictive tool.
    3. Since the switch of the PDO from warm to cool in 1999, global temperatures have not exceeded the 1998 high.
    4. Each time the PDO has changed from one mode to another, it has stayed in that mode for 25-30 years; thus, since the switch of the PDO from warm to cool in 1999 has been entrenched, it will undoubtedly stay in its cool mode for another several decades.
    5. With the PDO in cool mode for another several decades, we can expect another several decades of cooling.

    That’s from the linked study at (Revkin contrasts that claim to the warming East African lakes)

    Any critiques of Easterbrook’s claims from realclimate, or does that sound reasonable?

  29. 79

    Thanks, jl (#42), I’ll write it on my hand this time :)

    La nina, more Atlantic hurricanes, warmer anomalies for my area (that little point of Texas deep in the heart of Mexico).

    And I guess we don’t really know what to expect of the arctic oscillation — whether to expect more positives or negatives or about the same.

    So now I can definitively say I don’t like the implications of this post at all. More (or more intense) hurricanes, hotter climate over here. Pooh. And who knows, maybe more frequent (or the same) negative AOs to freeze out my plants in snap winter freezes. Just what I don’t want.

    But unlike the denialists who would bury their heads in the sand and kick sand up in the faces of the scientists & kill the messengers. I’m just going to have to redouble my efforts at mitigating and getting others to mitigate climate change. That’s all there is to it.

    Thanks Mike for the useful info.

  30. 80

    Richard Hendricks #67

    Which has me wondering, how many non-temperature indicators are there of AGW? Is there a list? Just off the top of my head:

    GRACE showing decline in ice sheets
    Tundra greening, and Taiga browning
    Plants, animals, and insects shifting either towards the poles or higher altitudes
    Sea level rising
    Migrating animals returning sooner and leaving later, or not leaving at all
    More growing days

    A few more:
    ocean acidification
    increasing toxicity of cassava
    direct observation of glacier retreat

    Long list for UK here:

    A few predictions are turning out harder to relate to climate change than expected (e.g. someone yesterday told me the historical epidemiology of tropical diseases suggests that spread of insects does not result in as rapid a spread of the diseases they carry as you’d expect — my guess is because e.g. mosquitoes carrying malaria have short lifetimes and new populations need infected hosts to spread the infection).

  31. 81

    Ike Solem @ 78:

    This is the type of information that I =wish= “the good guys” would talk up a lot more. I don’t know if its the PDO (could be) or my personal fave, GCRs (could be), but I do know that we seem to be moving sideways for about 10 years now, and both the PDO and GCRs are expected to change their direction of influence in the next decade or three.

    And when that happens, and it will, life on Planet Earth is going to be far more unpleasant than if people would just learn to understand that when the temperature goes up and down, and it goes up more than down, that going sideways or even a little down doesn’t mean it isn’t going to go up a LOT.

    People need to understand that action =now= is absolutely essential. It’s like the calm before the storm. The storm =is= coming, and we need to act =now= before we can’t.

  32. 82
    Snapple says:

    Dear Dr. Mann,

    I am sorry about these attacks. Do not underestimate these people.

    There is a Russian TV station called Russia Today (RT) that has given a lot of time to denialists like the 9-11 Truther Alex Jones and that other goofball Lord Monckton. You can search “Russia Today” “climate change” to see for yourself. RT is close to the Kremlin, I believe. See the RT clips on Youtube.

    I read that the famous Russian scientist Roald Sagdeev signed that open letter. Do you understand how huge this is for you? You should write about this on your site. Maybe you are young and don’t know about what Dr. Sagdeev did. You climate scientists should all thank him. He stood up to the KGB propaganda on behalf of the Soviet Academy of Sciences and now he stands up for you.

    In 1987, Dr. Sagdeev denounced the KGB propaganda about AIDS being made by crafty US scientists. Right in Isvestia. In 1992, the KGB chief Primakov said right in Ivvestia:

    Izvestiya (3-19-92) reported on March 19, 1992:

    “[Primakov] mentioned the well known articles printed a few years ago in our central newspapers about AIDS supposedly originating from secret Pentagon laboratories. According to Yevgeni Primakov, the articles exposing US scientists’ ‘crafty’ plots were fabricated in KGB offices.”

    KGB Chief Primakov actually said:

    “Articles exposing US scientists’ ‘crafty’ plots were fabricated in KGB offices.”

    I know a lot about that campaign, and the attacks on the alleged “plots” of “crafty climate scientists” strike me as very similar to the KGB AIDS propaganda which characterized AIDS as a plot by US scientists. This is all a variant on the old anti-Semitic canards about Jews spreading diseases, plotting world domination, etc.

    Here is my main article article about Sagdeev’s role in denouncing the AIDS propaganda. I think you all might learn from this.

    The denialist Marc Morano even linked to my obscure blog (article below) to mock me, but he doesn’t tell that Dr. Sagdeev signed that letter. This is different than my article about Sagdeev (above).

    These denialists are not going to stop. They just had a conference in Chicago. You might want to take note of the Russian media on global warming. It sounds exactly like the denialists and FOX News.

    Inhofe cites the Russian Andrei Kapitsa, who has been closely associated with Russian TV for decades and a Russian economist named Andrei Illarionov who has a think tank called The Institute for Economic Analysis in Russia as well as a position with the Libertarian Cato Institute. Illarionov used to be a big cheese in Gazprom.

    President Medvedev is the former Chairman of the Board of Gazprom. Many former KGB went into Gazprom. Those Gazprom guys want to be able to do whatever they want. Maybe that’s why they like Libertarians.

    The AIDS campaign probably began in the US according to the CIA.

    The KGB promote junk science in Russia, but most Russian scientists don’t agree with it. They are just afraid.

    For example the KGB used a fake mental illness called Sluggish Schizophrenia to hospitalize dissidents. Most Soviet psychiatrists didn’t want to cooperate.

  33. 83
    Jacob Mack says:

    # 80: quote: “(e.g. someone yesterday told me the historical epidemiology of tropical diseases suggests that spread of insects does not result in as rapid a spread of the diseases they carry as you’d expect — my guess is because e.g. mosquitoes carrying malaria have short lifetimes and new populations need infected hosts to spread the infection).”

    Hmmm, fromt the epdemiological research I am looking at and from several discussions with Microbiology and entomology professors, the spread is still quite rapid and widespread. Then again “as you’d expect,” is a vague and not specific modifier. How fast would one expect it to spread?

    For one some people resist such diseases like malaria better than others, there are insect nets, insecticides, sporulating phases, and insects who simply are die prior to behaving as vectors of disease (as you stated) Think of it this way: not every at risk sexual behavior with a HIV positive person results in HIV infection even in the presence of a high viral load.

    Then there is sickle cell anemia confering resistance to malaria as well. I do not think your guess is wrong, just that the issue is more multi-factorial than people expect, and there we also agree regarding the consequences of climate change. Still, if we can avoid the more severe detriments by simply being more responisble conservationists and using clean alternative energy sources we should definitely do so.

    I agree somewhat on predictions for the future with climate change and even attributions.

  34. 84
    Frank Giger says:

    SA, the drought was a period of reduced rainfall that wasn’t unprecedented, or even a record by any shot.

    While one can’t definitively state is wasn’t global warming, one can’t say it definitively was, either.

    Putting all one’s eggs in a weather condition is as wrong as the folks who yelled “fake” this winter due to snow. If we have another cool winter (two in a row!) and rains on par with what’s expected in a normal year will that disprove global warming? Of course not!

    Noting local trend change isn’t invalid in gauging climate. One just has to use caution in not exaggerating it to the point where it is no longer credible or appears to be the primary proof.

    I tell folks that climate change isn’t marked by Biblical style signs and portents. It’s far more insideous and subtle. It’s plants blooming a few days early…and the pollinators not ready for it, meaning reduced yields.

    That’s not nearly as sexy as pointing at Hurricane Katrina and pinning it on global warming, especially since it’s patently a lie to do so. The weasel words of “well, storms may become more intense” as an excuse when the error is pointed out makes it even worse.

    But it is really tempting (and was done) to do just that.

    Ditto every other phenomenon. The Icelandic volcano was NOT caused by global warming. Yet we see silliness and rhetorical nonsense suggesting it was (or, by proximal content, implying it). The oil slick in the Gulf is not going to heat the planet on a global scale. It’s serious enough by itself; no need to borrow trouble that’s not warranted.

    The scientists have done a really good job of trying to mitigate such mistakes by pointing them out (though there is a lot of room for improvement, IMHO), but are hampered by the same ignorance of activists and laymen that is displayed on the “denialist” side of the coin.

  35. 85
    Doug Bostrom says:

    By the way, here’s the kind of claims that are being made about the “Pacific Decadal Oscillation” (the one ‘discovered’ in the salmon catch proxy data) by Don Easterbrook:

    [PDO explains everything]

    That’s odd. If you read this interview, Easterbrook seemed to think it was -all- about solar variation, not a thing about the PDO. Now it’s -all- the PDO? Or is it both? Which marbles are we looking for today?

  36. 86
    MarkB says:

    Doug (#85),

    I think Easterbrook’s just trying to throw out enough junk at the political show, aiming for quantity (not quality). I don’t think he’s got any published studies on the matter. The “PDO” claim has been his primary bread and butter play, when he’s not drawing up weird graphs that place the LIA 1000+ years before its actual occurrence.

  37. 87
    Brian Dodge says:

    “Any critiques of Easterbrook’s claims from realclimate, or does that sound reasonable?” Ike Solem — 17 May 2010 @ 4:44 PM

    Already done

    “The AP interviewed Don Easterbrook, who claimed that “We started the cooling trend after 1998. You’re going to get a different line depending on which year you choose.” According to one of the statisticians, the fact that you have to choose 1998 as your starting point in order to observe a (statistically insignificant) cooling trend is part of the problem.”
    “This is what’s referred to in statistics as “endpoint sensitivity,” and it’s the main reason that climate disruption deniers like Easterbrook can appear and sound so reasonable when they’re actually misusing or misunderstanding the data.”

    Not to mention that if you start in 1999, the trend is up.
    If he is correct about statements 1,2,& 4, despite cocking up 3(hey, 3 outta 4 ain’t bad), then the fact that we are warming is additional evidence for AGW. Be sure to thank him for providing it.

    “Which has me wondering, how many non-temperature indicators are there of AGW? Is there a list?” Richard Hendricks — 17 May 2010 @ 11:46 AM

    Snow cover is declining, disappearing earlier in the spring, and reaccumulating later in the fall. The peak is higher, which may reflect that higher humidity and decreased latitudinal temperature gradients caused by global warming is causing heavier precipitation events [1] which fall as heavy snow when the temperature is cold enough. I downloaded data from and plotted the normalized snow cover anomaly compared to a 1967-1989 baseline here.

    [1] My area of NC is under a flash flood warming right now; it’s probably just weather, but if Tennessee, Arkansas, Texas, North Carolina, and Italy(it’s rained every day of the Giro di Italia) start flooding every spring, eventually it will become climate.

  38. 88
    Thomas says:

    Frank @84:
    I agree about the dangers of putting one’s rhetorical case into the “event X was caused by global warming” basket. However we have to deal with the fact that the probablity distribution of the occurrence of many of these phenomena is expected to be seriously modified as a result of warming. And I think a good bit of the human and ecosystem cost of climate change is going to be transmitted via the change in frequency/intensity of extreme events. So we have a real dilemma here, it is important to make the case that the frequency of damging events is increasing because of AGW, while trying to avoid the journalistic/rhetorical trap of attributing any single occurrence to AGW. This isn’t easy in a mathophobic world.

    Finally, you too strongly poo-poo the AGW volcanoe connection. There are paleo studies and theoretical reasons to expect that periods of rapid deglaciation will see an increase in volcanic activity in deglaciating volcanic regions. Conversely during periods of glaciation, it is expected that volcanic activity will be somewhat suppressed. This is similar to the case of hurricanes, where warmer sea surface temperatures are implicated in increased frequency and intensity, but are not a sole factor, so attribution of individual events becomes difficult.

  39. 89
    Edward Greisch says:

    Jim: Thanks for the response. It could be a temporal lobe problem. See Michael Persinger’s book.

  40. 90
    Hank Roberts says:

    What can we learn from the last 20-year maximum and minimum?

    I’m puzzled by a picture (well, graphed from the database).
    From this page, follow the pointer linked to the word “website”
    That takes you to this page
    Choose, as he suggests, the 14,000-foot (Ch05) chart
    The two greenish lines are for 2009 and for 2010-to-date.
    At the bottom, check the boxes for the 20-year record highs and lows.
    Click ‘redraw’
    Then click in all the years available.
    Click ‘redraw’

    Now try it for the “Near surface (Ch 04).

    It can get confusing, the colors used for each year are different for different elevation/bands.

  41. 91

    Brian Dodge @ 87:

    [1] My area of NC is under a flash flood warming right now; it’s probably just weather, but if Tennessee, Arkansas, Texas, North Carolina, and Italy(it’s rained every day of the Giro di Italia) start flooding every spring, eventually it will become climate.

    Well … we’ve had an exceptionally wet and not-much-sun past 8 months here in Central Texas, but I’m betting we’re experiencing a short-term set-back. Of the past 8 months, total solar radiation (of the kind I can turn into electricity) has been well-below average for 7 of those 8 months. And it isn’t just happening on my roof — I’ve read countless discussions by solar power people who are finding themselves looking for a Force Majeur clause they can use because the sun just ain’t shining as much as it has in the past. I’m 20% =below= where I should be and that’s what I’ve heard from quite a few other people in the biz.

  42. 92
    Edward Greisch says:

    Reference Chapter 8 of “Storms of My Grandchildren” which talks about CO2 and temperature going back 65 million years. I am now reading about how methane hydrates [clathrates?] melting kicked the temperature up in the PETM Paleocene-Eocene Temperature Maximum. Dr. Hansen published “Target CO2″ in the The Open Atmospheric Science Journal 2008
    This is where the target CO2 concentration of 350 ppm or less comes from. A target of 280 ppm CO2 would be better from the “What we can learn from studying the last millennium (or so)” article but more difficult to reach.
    I haven’t read far enough to find out when the methane hydrate-clathrate? feedback kicks in.
    Is a hydrate the same as a clathrate? Any comments on the critical conditions for that feedback? Maybe another article? It seems to me that the trigger point for methane release from the ocean bottom is something we want to avoid like 2 or 3 plagues.

  43. 93
    John N-G says:

    #77 SecularAnimist: “How do you know the drought in Georgia was NOT caused by global warming?” (see also #84 Frank & #88 Thomas)

    Seager et al., J. Climate,

    From the abstract: “…From the perspective of the historical record the recent drought [in the Southeastern United States] that began in winter 2005/6 was a typical event in terms of amplitude and duration….The recent drought, forced by reduced precipitation and with reduced evaporation, has no signature of model-projected anthropogenic climate change.”

    This is from the same lead author who said that the American Southwest was facing a permanent Dust Bowl due to global warming. When he says this was not an AGW drought, I tend to believe him.

  44. 94
    David Horton says:

    #80 Phillip “increasing toxicity of cassava” – it’s my understanding is that this is a prediction of a consequence of higher CO2 in the future, based on laboratory studies, not a real world observation right now (

  45. 95
    John Mashey says:

    re: #58 Jim
    1) If you really want to get into this, you may want to read Carbon pool and biomass dynamics associated with deforestation, land use, and agricultural abandonment in the neotropics, July 2009.
    “Ecosystem C pools of Neotropical primary forests minimally range from ~141 to 571 Mg/ha”. i.e., 141-571 tons/ha.

    50M ha X 141 tons/ha => ~7 GT
    50M ha X 571 tons/ha => ~29 GT
    (and of course, everything around it is more complicated, but at least the order of magnitude is in the ballpark.

    [Response: John, a couple of points. First, their 50 million ha estimate is heavily dependent on pre- and post-contact population estimates, combined with the estimated per-capita amount of cleared forest land used for agriculture (1ha/person)--all three of these are going to have high to very high uncertainties. Second, aside from the wide variations in various primary (i.e. old growth) tropical forest biomass estimates, we are talking about a 34 year sequestration period (1570-1604, the most abrupt decline in Law Dome record), a small fraction of the time required to reach max. carbon. If we use something like 350Mg/ha for primary forest C, and (generously) 25% of Cmax at t=34, you're talking about 7.6ppm * (2.13Gt/ppm)/(0.25 * 3.5 e-7 Gt/ha) = 185 million hectares of required land. That's a LOT of land.]

    2) But, you really want to keep an eye out for the Nevle, et al paper submitted to The Holocene, which is 2+ years later than the 2008 paper I think you are referring to.

    [Response: Definitely--thanks for the heads up.]

    3) It is clear that multiple groups of plausible people are trying hard to get bounds on the multiple sources of uncertainty as to attributions of {reforestration, volcanoes, irradiance changes, feedbacks, and of course timing), and a lot has been going in just the last year or two.

    4) I still think, that for this thread, it makes relatively little sense to debate whether or not this happened (especially if people haven’t tracked references like that, or the earlier Nevle&Bird, or recent Ruddiman work, etc.)

    I still think the interesting question for this is to apply what-ifs back to Mike’s original post and ask if one would expect any differences in regional fingerprints or not.

    [Response: I agree.--Jim]

    Re: temperature scale for Law Dome
    Needless to say, 0-origin atmospheric CO2 scales are not easily found in the literature :-), given that ~180 is about as low as it gets, and since it’s stayed in the 270-285ish band for ~4,000 years. One picks scales so that relevant features are visible but not overemphasized. I suppose I could have started at 250, which would have covered all of human history. If an indicator sticks within a ~5-pt range for 550 years, and then seems to drop ~7 ppm, and then stays 3-4 ppm for 200 more, a chart ought to show that.

  46. 96
  47. 97
    Completely Fed Up says:

    “SA, the drought was a period of reduced rainfall that wasn’t unprecedented, or even a record by any shot.

    While one can’t definitively state is wasn’t global warming, one can’t say it definitively was, either.”

    Can you say it would have been as bad if we weren’t warming?


    In fact the processes involved can be shown to be exacerbated by the current warmer climate.

    If a one-in-a-hundred event starts turning up one-in-ten times, then you KNOW that something has changed. Even though your one-in-a-hundred event is still not unprecedented (it happened before, roughly once every hundred) and it’s nowhere near a record (it would be less severe than a one-in-a-thousand event).

    Denialists throw out the subtlety in making their claims and use it again to throw out explanation of events by the science.

  48. 98
    Completely Fed Up says:

    The malaria pathogen exists in the Thames Valley in the UK. Mosquitos also exist there. But the climate is too cool for the pathogen to transfer from the mozzies to the human population.

    In a warming climate, this will no longer be the case.

    Sometimes there’s no need for spread.

  49. 99
    CM says:

    [OT] Edward Greisch #57, on behalf of us innumerate humanities graduates: sorry, we didn’t know we were oppressing you.

  50. 100
    Lawrence Coleman says:

    Re: Hank Roberts,
    Thanks for the reply in the previous post. Given the vast volume of arctic ocean and the relative depth I think I would agree that the ph would indeed stay much the same at least for now. Your comment that at a 1m depth sediment is converted to methane or methane hydrates amongst other compounds is very interesting..cheers!

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