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A peer-reviewed response to McLean's El Nino paper

Posted on 18 March 2010 by John Cook

A paper published mid-2009 claimed a link between global warming and the El Nino Southern Oscillation (ENSO) (McLean et al 2009). According to one of its authors, Bob Carter, the paper found that the "close relationship between ENSO and global temperature, as described in the paper, leaves little room for any warming driven by human carbon dioxide emissions". This result is in strong contrast with two decades of peer-reviewed research which find ENSO has little influence on long-term trends. Why the discrepancy? A response has now been accepted for publication in the Journal of Geophysical Research (Foster et al 2010) explaining why McLean 2009 differs from the body of peer-reviewed research.

First, let's examine how McLean et al arrived at their conclusion. They compared both weather balloon (RATPAC) and satellite (UAH) measurements of tropospheric temperature to El Niño activity (SOI). To remove short-term noise, they plotted a 12 month running average of Global Tropospheric Temperature Anomaly (GTTA, the light grey line) and the Southern Oscillation Index (SOI, the black line).


Figure 1: Twelve-month running means of SOI (dark line) and MSU GTTA (light line) for the period 1980 to 2006 with major periods of volcanic activity indicated (McLean 2009).

The Southern Oscillation Index shows no long term trend while the temperature record shows a long-term warming trend. Consequently, McLean et al found only a weak correlation between temperature and SOI. Next, they applied another filter to the data by subtracting the 12 month running average from the same average 1 year later. The comparison between the filtered data for El Nino and Temperature are as follows:


Figure 2: Derivatives of SOI (dark line) and MSU GTTA (light line) for the period 1981–2007 after removing periods of volcanic influence (McLean 2009).

From this close correlation, McLean et al argued that more than two thirds of interseasonal and long-term variability in temperature changes can be explained by the Southern Oscillation Index. This result contradicts virtually every other study into the connection between ENSO and temperature variability, particularly with regard to long-term warming trends. Past analyses have found ENSO was responsible for 15 to 30% of interseasonal variability but little of the global warming trend over the past half century (Jones 1989, Wigley 2000, Santer 2001, Trenberth 2002, Thompson 2008). Why does McLean come to a different result? This question is examined in Comment on "Influence of the Southern Oscillation on tropospheric temperature" by J. D. McLean, C. R. de Freitas, and R. M. Carter (Foster et al 2010).

Foster et al examine the filtering process that McLean et al applied to the temperature and ENSO data. This filtering has two steps - they take 12-month moving averages then take the differences between those values which are 12 months apart. The first step filters the high-frequency variation from the time series while the second step filters low-frequency variation. The problem with the latter step is it removes any long-term trends from the original temperature data. The long-term warming trend in the temperature record is where the disagreement between temperature and ENSO is greatest.

Why do McLean et al remove the long-term trend? They justify it by noting a lack of correlation between SOI and GTTA, speculating that the derivative filter might remove noise caused by volcanoes or wind. However, taking the derivative of a time series does not remove, or even reduce, short-term noise. It has the opposite effect, amplifying the noise while removing longer-term changes.

To further illustrate how the filtering process increases the correlation between SOI and temperature, the authors construct an artificial "temperature" time series as -0.02 times the SOI time series. They then add white noise and a linear trend. This has the effect of creating a temperature time series with a long term warming trend. The correlation between the raw artificial temperature series and the SOI series is very low (R2 = 0.0161). However, when the McLean et al filters are applied to both time series, the correlation is now very high (R2 = 0.8295). This is because the filtering removes low frequency elements such as the long term warming trend.

Artificial temperature time series vs Southern Oscillation Index - filtered and unfiltered
Figure 3: (a) Southern Oscillation Index (SOI) data (black) versus artificial data proportional to the SOI, and with normally-distributed white noise and a sinusoidal signal added (red). (b): Filtered versions (using the McLean et al procedure) of the series in (a).

Despite the extreme distorting effect of their filter, McLean et al consistently refer to the correlations as between SOI and tropospheric temperature. They draw no attention to the fact that the correlations are between heavily filtered time series. This failure causes what is essentially a mistaken result to be misinterpreted as a direct relationship between important climate variables.

Another interesting feature of McLean et al 2009 is a plot of unfiltered temperature data (GTTA) against the Southern Oscillation Index (SOI) to illustrate the quality of the match between them. However the temperature signal is a splice of weather balloon data (RATPAC-A) to the end of 1979 followed by satellite data (UAH TLT) since 1980. RATPAC-A data show a pronounced warming trend from 1960 to 2008 with the temperature line rising away from the SOI line. This warming trend is obscured by substituting the weather balloon data with satellite data after 1980. It is especially misleading because the mean values of RATPAC-A and UAH TLT data during their period of overlap differ by nearly 0.2 K. Splicing them together introduces an artificial 0.2-degree temperature drop at the boundary between the two. Unfortunately, the splicing is obscured by the fact that the graph is split into different panels precisely at the splicing boundary. This splicing + graph splitting technique is an effective way to "hide the incline" of the warming trend.

 
Figure 4: Seven-month shifted SOI with (a) weather balloon RATPAC-A temperature data 1958–1979 and satellite UAH temperature data (b) 1980–1995. Dark line indicates SOI and light line indicates lower tropospheric temperature. Periods of volcanic activity are indicated.

It has been well known for many years that ENSO is associated with significant variability in global temperatures on short timescales of several years. However, this relationship cannot explain temperature trends on decadal and longer time scales. McLean et al 2009 grossly overstates the influence of ENSO, primarily by filtering out any long-term trends.

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Comments 51 to 68 out of 68:

  1. re HR 18:11; 24 March, 2010

    Interesting. That clarifies why the editor(s) chose not to publish the McL et al response to Foster et al's comment. McL simply didn't address the points raised by Foster et al. In fact the McL response reinforces the fallacies of their original paper that Foster et al highlight, since they admit that the large apparent contributions of ENSO to variance in the temperature records (72% of variance in MSU atmospheric temperature; 68% of variance in RATPAC radisonde atmospheric temperature), doesn't actually apply to these variances at all, but to the smoothed and derivatized data....

    ...which is obvious and exactly the point that Foster et al made, and having admitted that, there isn't really anyrhing left of substance to their paper that differs from previous analyses.

    ...and of course, McL et al didn't address Foster et al's critique of the non-sequiter of McL et al that "mean global tropospheric temperature has for the last 50 years fallen and risen in close accord with the SOI of 5–7 months earlier shows the potential of natural forcing mechanisms to account for most of the temperature variation."

    [Note that McL et al reintroduce a similar but rather more blatant non-sequiter in the abstract of their blog response, and that in itself should render their response unpublishable, at least in that form.]

    There isn't really the "right of reply" that McL consider they were denied in their blog pamphlet you linked to. Editors would certainly be expected to invite a rsponse to a comment on their paper. But this response should conform to basic scientific standards, the foremost in these circumstances being that it should address the points in the comment. McL et al. didn't do that, but chose to restate the erroneous points in their paper, assert their correctness and to bluster instead.

    All this doesn't matter from a scientific point of view. We're pretty much back where we always knew we were, various bits of people's time having been wasted. I suspect McL et al consider they've done a pretty good job altogether....
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  2. Looking at the Mclean reply HumanityRules pointed out, they spend a lot of time defending their derivative operator, and argue the coherence of their results with the known temperature variations. Great - I have some arguments with what their bandpass filtering method actually shows, and the Foster paper (submitted) has a strong argument that this bandpass fits arbitrary data equally well, but those are all parts of reasonable scientific discussions.

    In the paper McLean et al in fact say;

    "Fea10 state that the method of derivatives that we employed would minimize long-term trends. We completely agree..."

    However, in their abstract and conclusions they then state something else:

    "We explain that there are natural mechanisms that might account for the strong coherence of Southern Oscillation Index and mean global temperature. Our research did not set out to analyse trends in mean global temperature, but, should any such trend exist, it follows from our analysis that in most part it could be a response to the natural climate mechanisms that underlie the Southern Oscillation." (Italics added for emphasis)

    This statement about long term temperature trends is, by their own admission, unsupported. There is NOTHING about long term trends discussed. It has no place in either an abstract or conclusion for this paper.
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  3. A quick note about my last comment - I'm commenting on the abstract, content, and conclusions of Mclean's (submitted, rejected) reply, not the original paper.
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  4. KR 07:54 AM on 25 March, 2010

    ".....those are all parts of reasonable scientific discussions."

    I don't think that's quite right KR, although I agree with you post overall.

    The contrived "disagreement" over the derivates isn't a matter of opinion or point of view. McLean et al. are categorically wrong, and I expect that they know it.

    If you take a function (say temperature in relation to time) that has a trend, then the derivative eliminates the trend.

    You could reproduce the Mclean et al artefact simply by considering a linear trend with a gradient of (say) 1 unit per year. The derivative has zero slope and an amplitude of 1.

    If you have two data sets:

    (i) Southern Oscillation Index (SOI) which has no long term trend

    (ii) the tropospheric temperature (RATPAC for example) that has a long term trend (around 0.4 oC over the period of interest)..

    ...the SOI may account for some of the variance in the temperature series (that which corresponds to short term variation), but won't account for any of the variance that corresponds to the long term trend (since the SOI has essentially zero long term trend).

    So the contribution of the SOI to the RATPAC variance will be smallish.

    Now take the derivative of each of the data sets (McLean et al did this by subtracting the 12 month running average from the 12 month running average 1 year in advance, as can be seen from their paper). Now each of the SOI index series and the RATPAC troposperic temperature series has zero long term trend (that's what happens when you take the derivative of the data).

    Lo and behold (!) the apparent contribution of SOI to the tropospheric temperature has magically increased.

    It's a crude and blatant arithmetical fudge. McLean et al sooo want to pretend that the SOI underlies long term temperature variation (as in the quote from their "reply to comments" that you reproduced in italic in your post above).

    It's got very little to do with "reasonable scientific discussion" sadly...
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  5. HumanityRules writes: Here is Mclean et als reply to the Foster comment. Warning: this is not peer-reviewed (this becomes obvious very quickly). It also contains reference to the climategate emails which I know some people are a little sensitive about.

    Hmmm. It looks like McLean et al. went through the CRU emails and found the anonymous reviews of the Foster et al. comment.

    They quote a couple of sentences allegedly from one of the reviews ("But as it is written, the current paper almost stoops to the level of 'blog diatribe'. The current paper does not read like a peer-reviewed journal article. The tone is sometimes dramatic and sometimes accusatory. It is inconsistent with the language one normally encounters in the objectively-based, peer-reviewed literature.")

    I'm a bit disturbed by the idea of going through someone's email to find and selectively quote comments from the peer review process, which is supposed to be anonymous and confidential.

    If McLean et al. are going to do that, I think that they ought to make the reviews of their own (rejected) comment similarly public. And in both cases they ought to show the full text of all the reviews, not selectively chosen snippets from one review.

    What else did this reviewer have to say about the Foster et al. comment, besides a couple of sentences complaining about the tone? It's hard not to suspect that if there had been any more substantial criticisms, McLean et al. would have quoted those parts, rather than the fairly mild complaint they did quote.
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  6. Ned, McLean et al neglected to reproduce this part of the review:

    Accept pending major changes (mainly in style not scientific comment)

    The real mystery here, of course, is how the McLean et al. paper ever made it into JGR. How that happened, I have no idea. I can't see it ever getting published through J Climate. The analyses in McLean et al. are among the worst I have seen in the climate literature. The paper is also a poorly guised attack on the integrity of the climate community, and I guess that is why Foster et al. have taken the energy to contradict its findings.

    So the current paper (Foster et al.) should certainly be accepted. Someone needs to address the science in the McLean et al paper in the peer-reviewed literature. But the current paper could be - and should be - done better. That's why I am suggesting major changes before the paper is accepted. All of my suggestions have to do more with the tone and framing of the current paper, rather than its content.


    An account can be found in
    RabettRun
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  7. Well, chris, I was trying to be nice. I agree - the bandpass operator they use is pretty worthless, as shown by getting equal matches to randomly generated data; I just wanted to point out that big internal consistency flaw between their paper and their (unjustified) conclusions.
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  8. John Cook, Stephan Lewandowsky (ABC, Drum Monday, 29 March,2010), Foster et al. and others seem to be unable to read. The findings reported in McLean et al refer to the TROPICAL values of the MSU data, NOT to GLOBAL temperatures.

    By the Way, John is a very skilled computer scientist who has been working on climate science matters for about four years, in which time he has had the support of internationally renowned climate scientists such as Bill Kinninmonth, one time head of our own, Australia's BOM. John is enrolled as a PhD student and has started out well.

    It is a measure of the validity of his claims that so many interested persons have sprung out of the woodwork to criticise him and his work, which challenges the “consensus” without ONE single word of science to back up their arguments. Once upon a time, any one worth his scientific salt relished a controversy and welcomed criticism of his/her more than praise, because it made you think! Not any more it seems. Any challenge is seen as a threat to the comfort of conformal thinking – actually conformal unthinking! I have not yet looked up Cook's or Lewandowsky’s crededentials but have glanced through their articles in which they have both demonstrated their obvious inability to refute John McLean's arguments.
    The paper by Foster et al., which I have studied very closely, does nothing but present a mixture of first/second university theory of the Fourier Transform, including what they thought was a clever simulation – a few random numbers, a linear trend – looks impressive and oh so clever, but all quite well known and absolutely of no consequence here. McLean et al.'s taking of the first derivative ONLY to determine very nicely the seven month time lag shown in Fig 3 is perfectly legitimate and confirms other rougher and less accurate work as is appropriate in a paper of this nature and of this quality. The key graph, is their Figure 7, which Foster et al. have quite mistakenly claimed was detrended. No such thing. It is a plot of ENSO or SOI against the TROPICAL MSU data without any differentials being taken. As tudent in about grade 9 could see that from the caption! It just goes to show that many people who claim to be "scientists" are not only incapable of understanding science but also incapable of reading.
    As Dean of Science at an Australian University years ago, I used to be particularly interested in students ability in English, as it is a good measure of how well they will perform in science, even the hard sciences such as my own Optical gas spectroscopy and atomic physics and mathematics, geology and chemistry. A careful comparison of the plots in McLean's Figure 7 with other graphs in the paper shows immediately that the FORMER are differentials of the data time series shown in 7.
    John Nicol PhD (Physics)
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  9. Albatross
    You seem unable to understand that the key graph in McLean et al. is Figure 7. This is a plot of the raw data (NOT differentiated data as claimed by Foster et al.) relating SOI, as defined in the paper, against the TROPICAL lower troposphere temperature (LTT)measured from the microwave emissions in the lower troposphere MSU in the band 20 degress North to20 degrees south - not the whole globe. However, since the sun is ocverheasd in the tropics, most of the energy causing higher temperatures on earth are exprienced in the tropics. The point made in McLean's paper is correct and is being totally, perhaps deliberately or if not ignorantly, misinterpreted. The AGU/JGR has broken a long tradition of the right of reply to any criticism of a published topic. This is no surprise in fact since McLean'spaper and the blocking of a reply to any critique by manipulating the editors and reviewers in JGR is openly discussed in the CRU emails recently released.
    John Nicol.
    P.S. Why do all these criticisms come from un named authors - are people ashamed to show their real name because they have no confidence in what they are saying. JN
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    Response: I've reproduced the Mclean Figure 7 graph here:

     

    If you look carefully, you'll find that the top box uses RATPAC weather balloon data while the bottom box uses satellite MSU data. Eg - it's not one continuous datasets but two data sets spliced together. Most importantly, the mean values of RATPAC-A and UAH TLT data during their period of overlap differ by nearly 0.2 K. Splicing them together introduces an artificial 0.2-degree temperature drop at the boundary between the two.

    The most curious feature of this graph is that the graph is split into different panels precisely at the splicing boundary. This obscures the difference between the weather balloon dataset and the satellite dataset. It's a misleading way to display a graph and yet is obviously effective in persuading people into thinking that the long term trend in ENSO correlates closely with the long term warming trend.

    For the record, I don't hide my name nor does Stephen Lewandowsky. However, the people who really matter are the authors of Foster et al 2010 who are all named.
  10. jonicol, here's a quote from the abstract of McLean et al. 2009:

    The results showed that SOI accounted for 81% of the variance in tropospheric temperature anomalies in the tropics.

    Now, is it your understanding from that sentence that McLean et al. showed:

    (a) SOI explains 81% of the variance in tropospheric temperature anomalies in the tropics full stop, or

    (b) SOI explains 81% of the residual variance in tropospheric temperature anomalies in the tropics after removing the long-term trend, or

    (c) something else?

    It seems to me that McLean have been rather coy about this. It also seems to me that (b) is the correct answer. There is only a single reference to "81%" in the McLean et al. manuscript, in paragraph 19:

    [19] Using the same technique of derivatives with data exclusions for periods of volcanic activity the line of best fit is

    dLTT(trop) = -0.0311*dSOI + 0.0252, [Equation 3]

    where R2 = 0.81.


    Note that that regression model is based on the derivative data, not the raw data.

    Given this, what do you think about the following remarks from the Conclusion of McLean et al. 2009?

    We have shown that the Southern Oscillation is a dominant and consistent influence on mean global temperature. [...] [This] study has shown that natural climate forcing associated with ENSO is a major contributor to variability and perhaps recent trends in global temperature

    There are two obvious problems here. First, as you yourself emphasized, McLean et al. did not analyze global temperature data, despite the repeated reference to "global" in their conclusions. Second, as shown in Paragraph 19, their calculations were done on a transformed version of the temperature data from which the long-term trend had been removed. Thus, it is not possible to draw any conclusions from their work about whether SOI is a "dominant" influence or whether it is a contributor to "trends in global temperature."

    A paper demonstrating that SOI has an effect on short-term interannual climate variability would not be very interesting, as everyone already knows that. A paper demonstrating that SOI has an effect on long-term trends in climate would be very interesting, but McLean et al. didn't do that.

    Finally, jonicol writes: Why do all these criticisms come from un named authors - are people ashamed to show their real name because they have no confidence in what they are saying.

    The authors of Foster et al. 2010 are all listed at the top of the paper, along with their institutional affiliations.
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  11. Nobody disputes that ENSO has a major influence on global temps variations. McLean et al can not use their analysis to even remotely suggest that it can explain any trend since their analysis in essence removes the trend. That's all there is to it. The tiny, sleazy "perhaps" in the conclusion would have been enough to reject the paper.

    As far as the right to respond, the buck stops when the bulls**t becomes too glaringly obvious. Science journals do not have to endure that kind of nonsense.

    Here is an Australian's take on this all McLean fiasco:

    http://www.abc.net.au/unleashed/stories/s2858332.htm
    He describes the all thing very well.
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  12. I missed the previous reference to Lewandowsky, so I did not realize that my previous comment did not introduce anything new, except a link to his piece. It's every bit as worthy of reading as McLean's complaining.

    Jonicol, you complain, unjustifiably, about people not using their real names, yet don't reveal yours?
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  13. Philippe Chantreau writes: Jonicol, you complain, unjustifiably, about people not using their real names, yet don't reveal yours?

    Actually, at the end of his first comment here he does sign it "John Nicol PhD (Physics)."
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  14. Ok, apology for that, I missed it.
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  15. While I'm thinking about it. Where do you guys stand on the research published in a NASA supported document from Remote Sensing Systems in 2009 where the joint information from NASA Satelites and their globe wide drifting, or Moored buoy's have shown a steady decrease in oceanic temperatures over the last ten years?

    My first comment was removed without any response overnight, so I don't expect much cooperation on these two comments either.
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  16. skepticstudent - it would seem to believe things (like broken hockey stick and ocean cooling that are not true. Perhaps you might like to check on that and comment if you have fresh evidence in the appropriate place (not this thread).

    Antartica - models predicted antartic would grow period. When it is not, then obviously it becomes focus of attention. But it appears that East antartica may be losing mass as well. See antarctic ice which would be a matter for concern. What happened in past isnt that relevant - no one disputes that natural climate change happens (slowly), but the forcing that act in the past arent acting now (or acting to cool but not).
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  17. I will admit that my level of science expertise is not as august as those that surround me. However at the age of 50 I’ve been around the block a few times and I’m able to put 2 and 2 together and see through a few holes.
    However I would like to make some comments and ask some questions out of full seriousness not being trite.
    I believe I did offer some evidence in regards to oceanic cooling. The end all be all of scientific focus on the side of global warming seems to be from James Hansen and the NASA Goddard institute, and Michael Mann’s Hockey stick. However let’s just focus on Oceanic Temperatures for now. Since this thread is talking about whether or not the oceanic temperature frame could be cooling or stable and related to Antarctic Ice melt is at the core of that As I mentioned earlier a joint NASA Remote Sensing Systems paper shows the combined science of satellite imagery and oceanic buoy readings which show a general oceanic temperature decline over the last 10 + years. http://www.remss.com/
    If you look at their website it shows In Situ Data collocations for the most recently completed day. It shows a group of buoys placed all over the oceans of the world.
    Now as to your comment of forcings then, versus forcings now, and the forcings that were around 10million years ago that are not active today. What is your basis in fact in this comment?

    There is ice core evidence and lake bed sediment evidence that during the last 10 million years there have been numerous periods where the ice in the Antarctic has done the same thing as it is doing now, what forcings were in play then that aren’t now. I have heard that said over and over but yet never proven to my satisfaction.
    One of the major contentions against the McLean/Carter et al.’s paper is that El Nino has been around since about 1895 even though that’s almost a century earlier than what I have heard for the last two decades but I’ll just go with the flow for this argument, the other part of the contention is that McLean/Carter et al. are trying to deny ongoing activity. Well how can you say on one hand this has been going on for a very long time and then on the other hand say that there is no evidence that the forcings 10milion years ago aren’t continuing today? You are robbing from Peter to pay Paul in your argument.
    I think one of the major errors that people who believe one thing and one thing only are missing is that there is a multiplicity of things causing fluctuations of temperature.
    Also this is a regional thing not a global thing. Also there has been a major ignoring of the fact that there is evidence over eons, that when the Arctic ice has a major growth pattern, the Antarctic recedes and vice versa. The Arctic ice pattern has had some major growth patterns since 2008. Now if anyone wants to debate me on the amount of snow and ice and windstorms in the state of Alaska in 2008 and 2009 I lived up there and I will be glad to debate you on “warming” in Alaska. I won’t argue that perhaps the Antarctic ice has melted a little more than expect in the last two years. I won’t argue that most likely the Pacific Decadal Oscillation would most likely not be the single culprit of Antarctic melting. But I would also stand strong and deny that any amount of anthropogenic CO2 output is the sole cause of any amount of warming.
    I’m trying to stay to the theme of this thread but it is rather difficult, without bringing in corollary evidence.
    You might be asking yourselves where this guy is getting his information from if he admits that he is not a scientist per say. I am a scientist of sorts. I am a Network Engineer and I have to understand electronics and a million other things and piece things together one step at a time forming theories and hypotheses as to what might keep one part of a communications network from functioning properly with another. If that is not the heart of the scientific method I don’t know what is. I have also studied for years about weather and astronomy.
    I also happen to have two friends who are a retired meteorologist expert for the US Weather Bureau, and another is a climatologist.
    As far as whether or not the McLean/Carter et al. paper is fit for peer review, I would say that what they were saying is that you have to look at the facts and see that there are other things besides mankind’s contribution to carbon footprint globally. I believe they proved that. Did they prove that the Pacific Decadal Oscillation was the sole corollary cause of global warming? No but having read the paper I don’t think that was the intention. As in so many areas I believe that when science can’t be refuted you attack the scientist.
    I believe since their paper is stating that other things are at play in any temperature fluctuation and that is the main bone of contention I believe it is fair to quickly discuss the fact once again, that during the carboniferous period there was 7000 to 14000 ppm of CO2. Who drove cars then? What factories were putting out CO2?
    Was the temperature any different than it is now? (At different periods of time yes at others no) On the opposite side of the coin, there were times when CO2 was at around 400ppm the equator was nearly frozen solid. So the main quest of their paper was to show that CO2 really isn’t the cause of any warming past, present, or future.
    I’m not going to get into the other areas as they are not within the confines of this thread and I’m trying to play within the rules of this blog since I’m not the author.

    Ps... I just got an email from Mr. Bob Carter and I advised him of this blog, I'm sure he will be thrilled beyond description over the excitement and enthusiastic discussion of his hard work.
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  18. Interesting to see that J. Salinger, one of the authors of the rebuttal piece is at the very same university department as Chris de Freitas (School of Geography, Geology and Environmental Science, University of Auckland). That has to make for some awkward moments in the lunch room...!
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