Rick Santorum appeared on Bill Maher and, when asked about climate change, claimed that there was a recent survey showing that 57% of scientists don’t agree that CO 2 is the main control knob for our climate. The survey that he is referring to appears to be the one by Verheggen et al. (2014), which you can read about here. It should be patently obvious that their conclusion is not that a majority of scientists disagree that CO 2 is the main climate control know. If anything, it’s the opposite of that; a majority of scientists agree that the dominant cause of our recent warming is anthropogenic.

Bill Maher responded to Rick Santorum with I don’t know what ass you’re pulling that out of. Unless I’m mistaken, I think this claim originates from Fabius Maximus who re-analysed Verheggen et als. survey to claim that only 43% of scientists agree that we are 95% sure that man-made CO 2 is the dominant driver of climate change.

Well, I’ve discussed this before and you can read Bart Verheggen’s explanation for why this is wrong here. Essentially, Fabius is arguing that only 43% of those in the survey agreed that more than 50% of the warming is anthropogenic AND that this was extremely likely (> 95%). One problem is that the question that his claim was based on had a very high fraction of respondents (22%) who didn’t actually respond with what they thought the human GHG contribution was. Verheggen et al discuss this at length in their paper, and in the blogpost, and conclude that including them skews the result to – probably – a lower value than is reasonable. This conclusion is based on what respondents mentioned themselves, and on comparing results to another survey question about the causes of global warming. This is in addition to Verheggen et al. going out of their way to include contrarians, which also probably produces a slight under-estimate of the actual level of consensus.

More fundamentally, though, Fabius’ analysis illustrates a lack of understanding of attribution studies. An attribution study is really a null hypothesis test. In this case, the null hypothesis is that more than 50% of the warming could be non-anthropogenic. This is rejected at the 95% confidence level, resulting in the conclusion that it is extremely likely that more than 50% of the warming is anthropogenic. Asking individual scientists what confidence they have is not the same as a formal attribution study. Also, the consensus is simply that more than 50% of the warming is anthropogenic. The extremely likely is the confidence we have in this consensus position. What’s relevant is the level of agreement with this consensus position, not how confident individual scientists are in this position. Or, more correctly, arguing that only 43% of scientists personally think that it is extremely likely that more than 50% of the warming is anthropogenic, is not the same as only 43% agreeing with the consensus that more than 50% of the warming is anthropogenic. It is clear from the Verheggen et al. study that a large majority of those surveyed agree that more than 50% of the warming is anthropogenic.

If you’ve read much of Fabius’s blog, you might notice that he spends a reasonable amount of time commenting on how the public debate about climate science is broken. Well, yes, this may well be true. However, a good deal of this can be attributed to the mis-information being spread by those who would rather we didn’t take this issue seriously. That his re-analysis of Verheggen et als. study seems to have contributed to this seems remarkably ironic. This may not have been his intent, but that he has done so seems clear; well, unless Rick Santorum is referring to some other analysis that also produced a value of 43%. If so, I haven’t come across it.