Verheggen’s Consensus: Not 97%, not 47%. It’s 66%.

Update: After an exchange of comments with Bart Verheggen, he has pointed out that he did indeed reference the headline figure I said he was ignoring in his paper in EST–the 66% who said half or more of recent warming was caused by human emissions of GHGs. It did not appear in his blog post or previous writing about the survey and it is only mentioned once in his paper. I apologize for the error.

I have written several times about the 2012 survey by Bart Verheggen and the Netherlands Environmental Survey. However, the climate blogosphere has started to look at it and is coming to some incorrect conclusions. But that’s okay, Bart himself is pushing a skewed interpretation of the results, so let’s look at it again.

Fabius Maximus has posted that the Verheggen survey shows the consensus of scientists believing half or more of recent warming is caused by human emissions of greenhouse gases is only 47%. This got picked up by several blogs and is being used to counter John Cook’s nonsense about the consensus being 97%. Cook’s figure is nonsense. The figure reported by Fabius Maximus is not nonsense, but it is a somewhat tortured reading of the data. Bart Verhegeen himself is turning a blind eye to the headline results and writing solely about the opinions of scientists who have the most publications under their belt.

Let’s show why the true consensus on the issue is not 97% as Cook reported and not 47% as Fabius Maximus reported. It is a very legitimate 66% as the survey data shows very clearly.

In the Spring of 2012, the Netherlands Environmental Assessment Agency PBLheld a survey among 1868 scientists studying various aspects of climate change, including physical climate, climate impacts, and mitigation. The main results of the survey were published in an article in Environmental Science and Technology (ES&T) in August 2014: “Scientists’ views about attribution of global warming”. It showed that there is widespread agreement regarding a dominant influence of anthropogenic greenhouse gases on recent global warming. This agreement is stronger among respondents with more peer-reviewed publications.”

On to the survey findings. Starting with the sexiest topic first, the question of attribution was explored in the survey. 66% of the respondents said that 50% or more of global warming since the mid-20th century can be attributed to human induced increases in greenhouse gases. As 19% responded ‘don’t know’ or ‘unknown’, it is clear that only a small minority has the opinion that GHGs caused less than 50% of recent warming. In fact, only 12% indicated that GHGs caused between zero and 50% of warming since the middle of the 20th century.

An interesting follow-up question was asked of those who felt AGW caused more than 50% of recent global warming. “What confidence level would you ascribe to the anthropogenic GHG contribution being more than 50%?” Similarly, those who ascribed less than half of recent warming to GHGs were asked about their level of confidence.

Those who think GHGs caused more than half of recent warming are far more confident in their perceptions than those who think GHGs caused less than half the warming.

Eighty-nine percent (89%) of those who attribute more than half of recent warming to GHGs said it was ‘virtually certain,’ ‘extremely likely’ or ‘very likely’. In sharp contrast, only 45% of those who felt that GHGs had caused less than half of recent warming expressed similar levels of certainty.

Why Fabius Maximus Is Over-Reaching

Fabius Maximus gets a lower percentage because he feels that the true ‘consensus’ consists only of those who were extremely confident of their opinion out of the 66% who believed half or more of recent warming is caused by human emissions of greenhouse gases. That would be in line with the level of confidence asserted by the IPCC, which says it is 95% certain that half or more of recent warming is caused by humans.

Fabius Maximus writes, “Now for the second part of the statement: what is the certainty of this finding? That the IPCC gives these answers is one of its great strengths. Of the 1,222 respondents to the PBL survey who said that the anthropogenic contribution was over 50%, 797 (65%) said it was 95%+ certain (which the IPCC defines as “virtually certain” or “extremely likely”).”

But the IPCC offers a definition of 95% certainty in their publications. This definition was not presented to survey respondents prior to asking them about how certain they were. Some may have used the IPCC definition of certainty, some may not. In any case, without knowing how they defined certainty it is not possible to assert that only 47% of respondents agree with the consensus.

We do know that 66% agree. We know that they are more confident than those who disagree. That is the percentage that should be offered as the Consensus. It is exactly the same percentage as found in a survey by Bray, von Storch et al in 2008 (although the question was worded differently), which should give us more confidence in the figure.

Why Bart Verheggen Is Under-Reaching

In all of his writing on the results of the survey, Bart Verheggen has focused exclusively on the higher agreement with the IPCC shown by scientists with more publications.

Indeed, in a paper written with John Cook, among others, Verheggen does not even report the top line statistic of his finding–that 66% of the scientists he surveyed agree that half or more of recent warming is caused by human emissions of greenhouse gases.

This is astonishing. That a report on a survey would not show and discuss the headline finding is bizarre. It would be like writing a report based on what Donald Trump’s supporters think of Mexican immigrants without even reporting what the overall population thinks.

It is legitimate to focus your analysis on the numbers of interest to you–even if that interest is only sparked by the fact that the entire numbers don’t support your position. But it is not at all legitimate practice to fail to report the top line data.

Verheggen’s failure to report the real numbers of his survey in my mind invalidates the rest of his paper. I know Bart Verheggen slightly through extended conversations at his blog and via emails. He has always been fair and honest with me. I find this incident bizarre, but the participation of John Cook in this paper may well explain it. John Cook is the author of a paper that should by now be completely discredited that reports the consensus at 97%. It is junk science and should be treated as such.

As for the position adopted by Verheggen’s report, it may well be that scientists with more publications are a better judge of attribution than scientists with fewer articles to their credit.

But we don’t know that. Verheggen et al make no attempt to show why that would be the case and they don’t even really make the argument. There is nothing in the survey that would show greater expertise on their part and there are a number of reasons to question that assumption, for assumption is all it is.

Scientists working in the private sector may have equal or even superior expertise than academics who must publish or perish. New scientists may be even more conversant with new information and data than older scientists who may not feel they have to keep up with the latest data. Scientists who get their name added to 10 multi-author publications may have no more expertise to offer than their colleague at the next desk who writes only one paper.

In any event, Verheggen et al don’t even try to make the case. They simply ignore the 66% consensus shown by all the respondents to his survey and report only on the differences in opinion shown by those with differing numbers of publications.

So at the end of the day, what a surprise. Advocates for skeptics and alarmists look at the same data and come up with different answers. Neither advocate tells an untruth. Both strain at gnats and swallow camels to come up with an answer that they like. Both the answers that they like are gross distortions of reality.

The consensus is real, although it looks more like a majority. It is not the nonsensical 97% shown by John Cook. It is not the 47% shown by Fabius Maximus. Verheggen doesn’t even show it at all.

It is 66%.

15 responses to “Verheggen’s Consensus: Not 97%, not 47%. It’s 66%.

  1. Tom, I will concede that this consensus™ may be “real” – whatever the most accurate “percentage” might be.

    However, in your examination of these various and sundry surveys, did you happen to notice how many of the respondents had the required statistical expertise – and actually used such expertise – to independently examine the underlying data and conclusions prior to formulating their respective “opinions”?

    Or did they simply choose to go with the oh-so-frequently-repeated (and IMHO) superficial IPCC/UNEP/WMO/NGO-generated propaganda flow?!

    • Hi Hilary

      I have asked Bart three times for the data since 2012 and still haven’t received it, so I can’t really answer your questions.

      More broadly, because of the very close agreement between the results of Bray, von Storch et al 2008 and Verheggen et al in 2012, I think 66% is a percentage I can live with without problem.

      • I think 66% is a percentage I can live with without problem.

        Yes, I had gathered this from your post, Tom:-)

        Nonetheless … While you may not agree (although I cannot imagine why;-)), I still believe – particularly considering the history among the “climate concerned” from the original Mannian “hockey-stick” through to the Cook-Lewandowski crocks, not to mention the more recent contributions from the Cowtan & Way crew – that my questions above are … well … valid.

        At the very least … Would you not agree that the answers to the questions I posed may well shed a somewhat different light on this consensus™?

        Even Mike Hulme (teflon-coated as he strikes me as being) long ago conceded, during the course of an explication of an earlier clarification**, that:

        This specific statement from the IPCC AR4 was initially crafted by a small team of detection and attribution experts, then evolved under review from other experts and then was further reviewed, amended and finally approved by governments.

        ** vis a vis the so-called “scientific consensus on man-made climate change”

        Furthermore …YMMV, but “crafted” doesn’t strike me as being particularly … well … “scientific” – regardless of the specific, but unknown, “expertise” of the “crafters”😉

      • What does Mosher say about science papers that do not share data?

  2. Hi Hilary

    Without a doubt the consensus has been redefined, distorted and lied about from the beginning. When the statement is ‘crafted’ in one way the consensus is solid. The climate has warmed, humans have contributed. But that’s not what the alarmists present as the consensus.

    So, umm, I agree with you.

    • I don’t think it is a consensus in the first place if say 30 per cent of relevant scientists are not willing to sign what IPCC says:

      ‘It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.’

      We must also be careful not to mix total anthropogenic and anthropogenic GHG effects. Many scientists think that anthropogenic CO2 effect is largely hidden by anthropogenic aerosols. So this question (and IPCC statement) can be understood in different ways pretty easily.

  3. Now this is all fine assuming that the 30% respondents is a representative statistical sample of the total scientist population. But I am certain that there is a huge convenience sample bias in play here.

    • Hi Hans,

      I imagine there is some sample bias. However, Verheggen reached out to the skeptic community by emailing potential respondents gathered from Jim Prall’s despicable database of skeptics. So it might not be as bad as you think.

  4. Now we have to consider if the surveyed scientists understand the RCP8.5 case used as “business as usual” by the ipcc is a bogus case?

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