More on realclimate on Cohn and Lins

Let’s look again at what Rasmus was saying before Gavin sent him to the end of the bench. He argued that Cohn and Lins were sucking and blowing by calibrating the autocorrelation on instrumental records, which themselves contained a trend. Gavin endorsed this position. On the face of it, this seems like a plausible criticism. However, this is not what Cohn and Lins either did or said. (One of the irritating features of realclimate authors is that they seldom quote directly from their adversaries and frequently set up straw men. You always have to check their characterizations. I try to avoid the same problem by quoting extensively as much as possible.)

Here is Rasmus arguing against using instrumental records to provide a benchmark long-term persistence for null distributions:

When ARIMA-type models are calibrated on empirical data to provide a null-distribution which is used to test the same data, then the design of the test is likely to be seriously flawed. To re-iterate, since the question is whether the observed trend is significant or not, we cannot derive a null-distribution using statistical models trained on the same data that contain the trend we want to assess. Hence, the use of GCMs, which both incorporates the physics, as well as not being prone to circular logic is the appropriate choice.

Or again,

I do not believe that statistical models are appropriate because i: they are used to test a null-hypothesis where no antropogenic forcing (of just solar volcanoes) is assumed, ii) they are trained on empirical data subject to forcings (be it anthopogenic as well as solar/volcanic)

So I think that I’ve characterized Rasmus fairly. Now let’s see Gavin’s endorsement of this position:

It may help you to follow if you actually read what is said. …Why you appear to think that GCMs should not be tested against the real world is beyond me.

Oops, that was Gavin being snarky to a civilian. People complain about me being sarcastic, but how often have I been snarky with a civilian? Not very often. OK occasionally with Lambert, who’s hardly a civilian, but not with Dano who was always civil here. Also I hope that there’s a difference in tone – I like to think that I’m ironic as opposed to merely bitchy.) Be that as it may, here’s Gavin on the same issue:

Maybe I can interject. First, I think we really all agree that statistics and physics are both useful in this endeavour. The ‘problem’ such as it is with Cohn and Lins conclusions (not their methodology) is the idea that you can derive the LTP behaviour of the unforced system purely from the data. This is not the case, since the observed data clearly contain signals of both natural and anthropogenic forcings. Those forcings inter alia impart LTP into the data. The models’ attribution of the trends to the forcings depends not on the observed LTP, but on the ‘background’ LTP (in the unforced system). Rasmus’ point is that the best estimate of that is probably from a physically-based model – which nonetheless needs to be validated. That validation can come from comparing the LTP behaviour in models with forcing and the observations. Judging from preliminary analyses of the IPCC AR4 models (Stone et al, op cit), the data and models seem to have similar power law behaviour, but obviously more work is needed to assess that in greater detail. What is not a good idea is to use the observed data (with the 20th Century trends) to estimate the natural LTP and then calculate the likelhood of the observed trend with the null hypothesis of that LTP structure. This ‘purely statistical’ approach is somewhat circular. Maybe
that is clearer?

So their concern is circularity. Fair enough, although they don’t seem in a big hurry to test for circularity elsewhere. Now let’s see what Cohn and Lins actually said. They do not use instrumental temperature records to benchmark autocorrelation – contrary to the allegations of both Gavin and Rasmus. Did someone once say: "It may help you to follow if you actually read what is said" Just my imagination.

Here’s what Cohn and Lins actually said:

The statistical significance, or p-value, associated with an observed trend, however, is more difficult to assess because it depends on subjective assumptions about the underlying stochastic process [von Storch and Zwiers, 1999; Woodward and Gray, 1993; Weatherhead et al., 1998]. In this paper, we consider the idea introduced by Hurst [1951] and discussed by others [Mandelbrot and Wallis, 1969a; Klemeàƒ’€¦à‚⟬ 1974; Lettenmaier and Burges, 1978; Potter, 1976; Potter and Walker, 1981; Hosking, 1984; Bras and Rodriguez-Iturbe, 1985; Vogel et al., 1998; Koutsoyiannis, 2000] that [hydroclimatological] records are realizations of physical processes whose behavior exhibits long-term persistence (LTP). …The purpose of this paper is not to evaluate claims related to LTP, but rather to explore what LTP, if present, implies about the significance of observed trends.

and again later:

The question remains whether natural [hydroclimatological] processes in fact possess LTP. The idea was introduced more than 50 years ago by Hurst [1951], and has been debated ever since [Mandelbrot and Wallis, 1968; Klemeàƒ’€¦à‚⟬ 1974; Potter and Walker, 1981; Hosking, 1984; Loucks et al., 1981; Koutsoyiannis, 2000, 2003]. Hurst’s fundamental finding has neither been discredited nor universally embraced, but persuasive arguments have been presented (for discussion and additional references, see Koutsoyiannis [2003]). Given the LTP-like patterns we see in longer HC records, however, such as the periods of multidecadal drought that occurred during the past millennium and our planet’s geologic history of ice ages and sea level changes, it might be prudent to assume that HC processes could possess LTP.

As I noted before, the data sets in the original references are long proxy series – not instrumental temperature series. I’m familiar with some, but not all of the Cohn and Lins’ references. Hurst discussed Nile River levels; Mandelbrot and Wallis discussed tree rings, varves, etc. So the issue arises because one sees long-term persistence in proxy records and was not raised because of autocorrelation in instrumental records. So the argument from Gavin and Rasmus is a total red herring. It doesn’t look like Rasmus or Gavin bothered to read the references or even read Cohn and Lins itself very carefully – if you’ll excuse the snark.

Additional to the series mentioned in Cohn and Lins is, of course, the Vostok series, which has pronounced long-term persistence as observed by Pelletier [2002] and discussed here. The Pelletier graph is re-stated here – see the left-hand side of the figure below.

Original Caption: Left: Fig. 1. Power-spectral density estimated with the Lomb periodogram of the temperature inferred from the deuterium concentrations in the Vostok (East Antarctica) ice core. The power-spectral density S is given as a function of frequency for time scales of 500 yr to 200 kyr. Right Fig. 2. Average power-spectral density of 94 complete monthly temperature time series from the data set of Vose et al. (8) plotted as a function of frequency in yr^-1. The power-spectral density S is given as a function of frequency for time scales of 2 months to 100 yr.

While we’re at it, I’d like to draw your attention to a deep-sea proxy series by Carter and Gammon [2004] here , which also shows remarkable long-term persistence and ties in neatly with Vostok. The figure below shows the tie-in between ODP site 1119 and Vostok (see caption for explanation):

Original Caption: Fig.2. Comparison between climate signals from ODP Site 1119 and Vostok, Antarctica, over the last 0.37 My. (A) Insolation curve for latitude 65°N. (B) Deuterium isotope ratio (dD) for the Vostok ice core (15). (C) Natural gamma ray signal from ODP Site 1119 (the scale is reversed) (13). Black triangles indicate age tie points between Site 1119 and Vostok; diamond bullets, the location of warm gamma anomalies in MIS 2 to 3 and 6 (the latter anomaly also occurs in the oxygen isotope record at DSDP Site 594) (17, 18). These features are site-specific and result from seaward movement of the STF and its probable merging with the SAF into an intense combined frontal zone, driven by the lowered sea level at glacial maxima (11). Termination I, III, and IV cold reversals are labeled T-I-cr, T-III-cr, and T-IV-cr, respectively.

Finally, here is a remarkable graphic from Carter and Gammon showing a 3.9 million year proxy. So far, to my knowledge, no one’s analyzed this for long-term persistence, but it would be worth doing. (I might discuss this with Carter who was the first person to encourage my interest in climate.) Carter finds it impossible to distinguish present variability from the variability in his series (C below). All of these series demonstrate long-term persistence and support the validity of the form of inquiry of Cohn and Lins. How one demonstrates significance of present warming as against the variability contained in these proxies is a serious question, which Gavin and Rasmus have not answered. Can a GCM replicate the natural variability of any of these series? Not at 25 model years per calendar day.

Original Caption: Fig.4. Summary history of atmospheric and marine climatic cyclicity over the past 3.9 Ma. (A) Core retrieval from ODP Site 1119 (average recovery, 89%), which controls the completeness of the stable isotope record. (B) Carbon (upper) and oxygen (lower) isotopic measurements for Site 1119C. (C) Natural gamma ray record from ODP Site 1119 (reversed scale), produced by merging onboard MST measurements with downhole log measurements (14). The age scale is derived by matching climatic cycles at the points indicated by crosses to their MIS equivalents at Vostok and at ODP Sites 758 and 1143. Selected isotope stage equivalents are numbered; warm and cold climatic extrema are indicated by W and C above and below the gamma ray curve, respectively; basal triangles indicate incremental 50-m depths on the revised meters composite depth scale (14); and the right-hand scale indicates the possible Antarctic polar plateau air temperature equivalent for Site 1119 cyclicity, based on scaling the amplitude of the MIS 5 to 6 difference to the Vostok and Mt. Fuji deuterium records (15, 22). (D) Sample resolution of gamma ray measurements for Site 1119 (logarithmic scale). MPT, Mid-Pleistocene Transition. (E to G) Benthic oxygen isotope records for the Pacific (Site 1143), Atlantic (Site 659), and Indian (Site 758) oceans, respectively (14).

References: Robert M. Carter* and Paul Gammon, 2004, New Zealand Maritime Glaciation: Millennial-Scale Southern Climate Change Since 3.9 Ma Science 304, 1659 here

Robert M. Carter, C.S. Fulthorpe and H. Lu, 2004. Canterbury Drifts at Ocean Drilling Program Site 1119, New Zealand: Climatic modulation of southwest Pacific intermediate water flows since 3.9 Ma Geology; 32,. 1005–1008; doi: 10.1130/G20783.1; here


  1. Stephan Harrison
    Posted Dec 22, 2005 at 3:07 PM | Permalink

    Interesting stuff as always but the issue here is not whether the present warming is unprecedented (I’m assuming that all credible sceptics now believe that warming is actually taking place) but to what we can attribute this warming. Anyway, happy Christmas and New Year to you all.

  2. TCO
    Posted Dec 22, 2005 at 3:57 PM | Permalink

    If it were unprecedented, that would make the supposition that it is AGW, that much stronger. Even without that, the coincidence of a warming trend with a CO2 rise is still a point towards beleiving AGW is occuring.

  3. joshua corning
    Posted Dec 22, 2005 at 4:17 PM | Permalink

    “Even without that, the coincidence of a warming trend with a CO2 rise is still a point towards beleiving AGW is occuring.”

    actually my first guess would be that becouse of GW CO2 is not being absorbed into the ocean as quickly as it has when it was cooler.

    GW causes CO2 accumulation in the atmosphere no the other way around.

  4. TCO
    Posted Dec 22, 2005 at 4:21 PM | Permalink

    what about all the fuel burning?

  5. Paul Linsay
    Posted Dec 22, 2005 at 5:27 PM | Permalink

    #3, please explain “becouse of GW CO2 is not being absorbed into the ocean as quickly as it has when it was cooler.”

  6. joshua corning
    Posted Dec 22, 2005 at 6:01 PM | Permalink

    “#3, please explain “becouse of GW CO2 is not being absorbed into the ocean as quickly as it has when it was cooler.”

    sun heats ocean, ocean absorbs co2 slower, co2 accumulates in atmosphere. See?

  7. Armand MacMurray
    Posted Dec 22, 2005 at 6:24 PM | Permalink

    Re: #6
    Could you explain the 2nd step: “ocean absorbs CO2 slower”? Does this take into account the biosphere? Is it based on modeling, or empirical observations?

  8. jae
    Posted Dec 22, 2005 at 6:42 PM | Permalink

    Re: #7 The solubility of gases in water decreases with an increase in temperature. Even I know that.

  9. Paul Penrose
    Posted Dec 22, 2005 at 8:07 PM | Permalink

    Every time I take a close look at 20th century temperature records and CO2 levels it looks to me like most of the alleged warming occured before the majority of the CO2 increases. How can the effect precede the cause? Maybe Joshua is correct, I don’t know, but something is fishy with the standard explaination of CO2 causing the warming.

  10. Armand MacMurray
    Posted Dec 22, 2005 at 10:06 PM | Permalink

    But the ocean is not just water. For example, plankton etc are converting CO2 into carbohydrates; I can certainly imagine a slight warming of the ocean potentially increasing the plankton population, thus increasing the fixing of CO2. Since much of this becomes part of ocean sediments, it’s a CO2 sink. It’s not clear to me that solubility of CO2 in the ocean is a limiting factor here. That’s why I asked about the biosphere, and whether there is observational data of some sort to support this.

  11. Dave Dardinger
    Posted Dec 22, 2005 at 10:44 PM | Permalink


    And, since the upper layer of the ocean, where photosynthesis occurs, is well mixed by tide and wave action, It will constantly be recharged with CO2. The real question is just how much more productive the surface waters can be, given that there are other nutrients in short supply such as iron and I believe phosphorus. Of course this also means that there’s a gigantic possibility for humans fertilizing parcels of ocean and raising fish, shrimp, etc.

    BTW, does anyone know how much shrimp farming is being done? I keep seeing so much shrimp in all-you-can-eat chinese buffets at reasonable prices, I have to believe that some of them are farmed shrimp.

  12. Ray Soper
    Posted Dec 23, 2005 at 5:14 AM | Permalink

    Re 1: Sorry Stephan, given all of the discussion here in various threads re the difficulties of measuring average temperatures etc, I don’t quite understand how you can say “the issue here is not whether the present warming is unprecedented (I’m assuming that all credible sceptics now believe that warming is actually taking place)”.

    The issues involved in establishing credible averages for the grossly heterogeneous system (earth) that we are dealing with is more than a little challenging, and the issues involved in excluding UHI effects not convincingly resolved so far as I can see. Maybe you can provide detailed evidence of your assertion “that warming is actually taking place” that will pass muster on this site.

    And, you won’t mind will you if I ask you to provide some sort of detailed evidence to support your assertion that “all credible sceptics now believe” that warming is actually taking place. You have polls do you? You have a way to distinguish the “credible sceptics” from the other sceptics? I would be interested in how you do that. “All” credible sceptics doesn’t seem to me to allow for the shades of scepticism that clearly exist. Also, I find it interesting that you use the word “believe” here, which to my understanding largely means that “I don’t know for myself, but I will take on trust, and adopt, what somebody else tells me about the issue”.

    I would be pleased to learn about your scientific support for your assertions which, I’m sorry to say, in the absence of such support, I for one find difficult to accept.

  13. Posted Dec 23, 2005 at 7:17 AM | Permalink

    I think you characterized Rasmus more than accurately. He’s not right when he says that we cannot derive a null hypothesis from the datasets themselves.

    This is what we’re doing in many situations in science – every time when an actual nontrivial check of our theories is provided by reality. Let me give you an example. We have thousands of such examples in physics.

    When the cosmic microwave background was discovered, no one had a complete theory. It was determined from the data that the microwave background was thermal and what was its temperature. Namely 2.7 kelvins. Of course one had to know that “being thermal” is a natural answer about the structure of radiation; in fact, the CMB is still the most accurate natural thermal blackbody curve we have seen so far.

    The fluctuations of the temperature were determined from the data, too. Their dependence on the scale was also found and the spectrum was seen to be approximately scale-invariant. Finally, deviations from the scale invariance are also observed from the data.

    The main conclusions – thermal curve; scale-invariant fluctuations; violations of scale invariance in a particular direction; various correlations etc. – are derived directly from the observed data.

    Then you independently pick your Big Bang theory and you see that it naturally explains the thermal distribution because everything was in equilibrium 300,000 years after the Big Bang when the radiation was created. Also, inflation much before this era – a fraction of second after the Big Bang – explains scale invariance. And some more detailed calculations that depend on the inflationary model also predict some deviations from the scale invariance, and many models may be falsified in this way. In fact, the last observation – the deviations from scale invariance – do not yet have a generally acceptable theoretical description even though people can, of course, fudge their models to get an agreement, much like the climate modellers are doing so.

    What I want to say is that there must exist separately conclusions derived from the experiments; and conclusions derived just from the theory. And these two sets of conclusions must be compared. If someone is showing an agreement simultaneously by twisting the data according to the theory and fudging the theory according to the data, merely to show that there is a roughly consistent picture, then it is no confirmation of “the” theory. It’s not science. This is how the priests in 15th century argued that the real world is consistent with the Bible.

    A correct scientific theory must be able to make predictions of some feature of the observed data before the data is observed – this is why it is called a prediction – and the same thing holds vice versa. Nontrivial experimental facts must be determinable and describable without the ultimate theory before this theory is found, otherwise they cannot be used to determine the theory. In other words, it must always be a historical coincidence whether the theory or the experiment was the first group that gave the result.

    Of course I am not saying that the actual evolution of science is decoupled to theorists and experimentalists who don’t talk to each other. What I am saying is that they should not be talking to each other when they try to determine whether a theory agrees with some particular observations.

    In this particular case, whether or not some heating is a natural persistence or an effect caused by XY is, of course, an important scientific question. It is much more likely and “default” that it is caused by long-term persistence because if it were not, there are still very many factors XY that could be really causing it. If we don’t have an observation that would suggest that the persistence does not exist (for example accurate enough observations of the 15the century temperature), we should not assume that it does not exist. Of course that it probably does, and a goal of the scaling papers is to find phenomenological laws that would help to determine the color of the noise – and henceforth also the persistence at various time scales – from the data, regardless of some additional effects caused by anyone else.

    The qualitative question whether the persistence exists is quite clear. It does. The noise exists at all scales. The real question is a quantitative one.

    It is extremely important to know what is the “natural background” if we try to figure out whether there is a new “effect”. Some people like RB just don’t want to study the natural background at all – they immediately want to get effects – which is why I think that they are crackpots. One of the defining features of crackpots is that they want to make big discoveries before they learn what is the science describing the “simpler” phenomena before their discovery.

  14. Posted Dec 23, 2005 at 7:31 AM | Permalink

    re 6:

    If I open a tap in my swimming pool and the water rises corresponding to half the amount I put in , the tap is not causing the rise, because I don’t know how fast my circulation pump is working, is that what you mean?

  15. Posted Dec 23, 2005 at 7:33 AM | Permalink

    btw there is a rule of thumb that 1K temperature rise causes 10 ppm CO2 rise. Where does the rest come from?

  16. Posted Dec 23, 2005 at 7:36 AM | Permalink

    Let me say why their research is defective in one more way.

    Whenever we try to design scientific theories that describe something, we must know which quantities in reality will be described by our theories and we must be able to isolate them.

    By isolating them, I mean both theoretical as well as experimental isolation. In theories we must know – or at least feel – that the effects we have neglected do not change our predictions too much. In experiments we must know – or at least have rational reasons to believe – that the effects we observe are not caused by something else, something “more ordinary”.

    The climate modellers almost never try to follow these lines. They have a completely vague, sleeky theories that predicts anything and everything – warming, cooling, bigger variations, smaller variations, more hurricanes, less winds, increased circulations, diminished circulation, more ice in Antarctica, less ice in Antarctica, and so forth – and then they’re arguing that the data agree with these predictions.

    Of course there is no direct way how one can ever construct a scientific framework out of this mess. To do science, one must focus on a limited class of questions that are sufficiently well-defined and that have a chance to be “cracked” by a theory.

    When we try to argue that the humans are suddenly dictating the climate trends – after 5 billion years when they were dictated by other, more natural things – it is a rather extraordinary conjecture that deserves extraordinary evidence. For getting any evidence, it is absolutely necessary to understand how the climate was behaving for 5 billion years before the hypothetical “revolution” occured around 1917. We must know what were the fluctuations and how they depended on the time scale. Only once we know the background, we can study additional effects.

    Studying additional trends above a background that we don’t need to understand is equivalent to the Biblical literalism.

  17. john emery
    Posted Dec 23, 2005 at 10:09 AM | Permalink

    Lubos: Right on! Brilliant! Alas, someone who understands science!

  18. TCO
    Posted Dec 23, 2005 at 10:24 AM | Permalink

    The tendentious behavior of the climate scientists is much more suited to the media or to politics than to hard science. However, while I think they overstate their case seriously and exhibit several dishonest behaviors in argument, the concurrence of warming with increased fuel burning and the plausible explanation of CO2 being a GHG causing the effect (with feedback) makes it more than 50% likely (in my oddsmaker head) that GW is occurring. What I don’t know is to what effect it will occurr. I think it more likely than not that the effect will diminish vice run away. And I also don’t see the big deal about a few polar bears sinking. I hate the cold.

  19. joshua corning
    Posted Dec 23, 2005 at 1:20 PM | Permalink

    “If I open a tap in my swimming pool and the water rises corresponding to half the amount I put in , the tap is not causing the rise, because I don’t know how fast my circulation pump is working, is that what you mean?”

    not to say that my model is perfect and without flaws but it does eliminate some the problems in AGW models…such as why surface temperatures are warmer then atmospheric temps,(solar global warming) and why GW predeeded CO2 increases,(gw causes co2 accumilation) and why most accumilation of CO2 happend in the latests quarter of the current warming period. (gw causes co2 accumilation) Or why 50% of the estimated human contribution of the CO2 has disapeared. (the oceans are still cold enough to absorb it)

  20. john emery
    Posted Dec 23, 2005 at 2:52 PM | Permalink

    Re: 18. OK, but the point is that we simply do not know, yet; and I don’t think we should be betting our economy on something so uncertain.

  21. Posted Dec 24, 2005 at 9:59 AM | Permalink


    If you make an average over the previous ice ages, where the temperature changes clearly are leading the CO2 changes, the temperature changes caused CO2 changes in the order of 8-10 ppmv per K. Even in the Eemian when the average global temperature (trees growing far more north in Alaska) was some 3 K warmer than today, CO2 levels were not higher than 290 ppmv. During the past (pre-industrial) millenium, CO2 changes were not more than 10 ppmv in ice cores, which followed temperature changes with some 50 years lage (Law Dome ice core). Thus the current CO2 levels, based on natural fluctuations, would be below 280 ppmv, while they are near 370 ppmv.
    Second proof of the increase of CO2 due to fossil fuel burning is in the isotope ratio between 13C and 12C. Plants tend to use more of the lighter isotope to build their cellulose, which at last is incorporated in all fossil fuels. Thus if lots of fossil fuels are burned, the “normal” ratio of 13C/12C will be thinned, which is what is observed. The detection method for a trend is accurate enough to even see the seasonal changes with plant growth. See Keeling under “Trends”.
    Despite the increase of CO2 from burning, some halve of it is absorbed by the oceans, this is because a higher partial pressure in the air shifts the equilibrium between CO2 in the air and water, until the vapour pressure of CO2 above the surface again is in equilibrium, just like (the other way out) unscrewing a soda bottle gives you a lot of CO2 bubbles, because the pressure above the surface changed.

    Of course, more is happening, as CO2 in the oceans again is in equilibrium with bicarbonate and carbonate ions, which may be used in biological processes. But temperature of the oceans doesn’t, neither biological processes explain the increase of CO2 in air since the start of the industrial revolution.

    In the same time period (1900-2000) as the observed warming, we have a sudden rise in solar activity. The problem now is which one (solar and/or CO2) is the main cause of the warming, or both…

    A Merry Christmas to all…

  22. beng
    Posted Dec 26, 2005 at 8:44 AM | Permalink

    This is a bit OT — didn’t know where else to post it, but there are interesting technical discussions on the surface-air temp trends (among other topics) on Pielke Sr’s blog:

  23. Steve McIntyre
    Posted Dec 26, 2005 at 9:09 PM | Permalink

    Luboà…⟠further criticizes Rasmus here

%d bloggers like this: