Expert Credibility in Climate Change – Responses to Comments

Guest commentary by William R. L. Anderegg, Jim Prall, Jacob Harold, Stephen H. Schneider

Note: Before Stephen Schneider’s untimely passing, he and his co-authors were working on a response to the conversation sparked by their recent paper in the Proceedings of the National Academy of Sciences on climate change expertise. One of Dr. Schneider’s final interviews also addresses and discusses many of the issues covered here.

We accept and rely upon the judgment and opinions of experts in many areas of our lives. We seek out lawyers with specific expertise relevant to the situation; we trust the pronouncement of well-trained airplane mechanics that the plane is fit to fly. Indeed, the more technical the subject area, the more we rely on experts. Very few of us have the technical ability or time to read all of the primary literature on each cancer treatment’s biology, outcome probabilities, side-effects, interactions with other treatments, and thus we follow the advice of oncologists. We trust the aggregate knowledge of experts – what do 97% of oncologists think about this cancer treatment – more than that of any single expert. And we recognize the importance of relevant expertise – the opinion of vocal cardiologists matters much less in picking a cancer treatment than does that of oncologists.

Our paper Expert Credibility in Climate Change is predicated on this idea. It presents a broad picture of the landscape of expertise in climate science as a way to synthesize expert opinion for the broader discourse. It is, of course, only a first contribution and, as such, we hope motivates discussion and future research. We encourage follow-up peer-reviewed research, as this is the mark of scientific progress. Nonetheless, some researchers have offered thoughtful critiques about our study and others have grossly mischaracterized the work. Thus, here we provide responses to salient comments raised.

Definition of groups: The first of four broad comments about our study examines the relevancy of our two studied groups – those Convinced of the Evidence that much of the warming of the last half century is due in large part to human emissions of greenhouse gases, as assessed by the IPCC, which we term “CE,” and those who are Unconvinced of the Evidence (“UE”). Some have claimed that such groups do not adequately capture the complexity of expert opinion and therefore lose meaning. To be sure, anthropogenic climate change (ACC) is an immensely multi-faceted and complex area and expert opinion mirrors this complexity. Nonetheless, society uses simplifications of complex opinion landscapes all the time (e.g. Democrat versus Republican for political views) that don’t “lose their meaning” by ignoring the complexity of nuanced differences on specific topics within these broad groups.

The central questions at hand are: are these groups (1) clearly defined, (2) different in views of ACC, (3) reasonably discrete, and (4) in the main mutually exclusive? Our definition of groups, based entirely in the case of the UE group on their self-selected, voluntarily signed statements and petitions expressing various versions of skepticism about ACC, is clearly defined in the published paper. The strongest evidence indicating that our CE and UE groups satisfy the second and third criteria is that only three of 1,372 researchers fell into both groups—and in two of those cases, the researcher unwittingly added themselves to a statement they did not in fact support. Thus, if only one researcher of 1,372, or 0.07%, legitimately falls into both of our groups, this suggests that the two groups both differ starkly and are discrete. Any statistical analysis would be only trivially altered by having three redundant members of the cohort. Furthermore, the CE and UE groups are coherent, as around 35% of signers in each group also signed another statement in that set.

Page 1 of 4 | Next page