Expert Judgment in Climate Science

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Submission Summary
Consensus is often regarded as an important criterion for laypeople or decision-makers to arbitrate between the opinions of experts. Other criteria include tracking record and unbiasedness of experts, as well as validity of evidence and soundness of arguments. Overall, these criteria aim to ensure that expert judgment is grounded in objective arguments and is not a mere subjective belief or expression of interests from experts. In particular, consensus is supposed to guarantee a certain intersubjectivity. In this paper, we argue that the subjective aspects of expert judgment, such as intuitions and values, which consensus and the other criteria are supposed to counteract, actually bestow epistemic power upon those judgements. For that, we explore the role of expert judgment in climate science. We show that expert judgment can be found throughout the scientific process, in model creation and utilization, model evaluation, data interpretation, and ultimately ending with the quantification and communication of uncertainties to policy-makers. We argue that expert judgment is used for the purpose of supplementing models and managing uncertainty. First, as no model can perfectly represent the target, expert judgment is used as an alternative cognitive resource in order to provide climate projections and associated probabilities. Second, expert judgment is used as a means for quantifying epistemic uncertainty surrounding both general theories and specific scientific claims. This is shown through the IPCC’s use of confidence and likelihood metrics for evaluating uncertainty. We further highlight that the production of an expert judgment is more epistemically opaque than computer simulations, as this production is partly internal, mental and thereby non-accessible. How then to justify that expert judgment can still supplement models and manage uncertainty? A pessimistic view would answer that expert judgment is simply a last resort: facing high uncertainty, one has no other choice than appealing to expert judgment. An optimistic view would rather recognize that there is some quality in expert judgment that makes it a precious cognitive resource. We contend that this quality stands in its subjective aspects. First, we argue that the trustworthiness of an expert judgment bears down on the expert being exceptionally well-informed, a not interchangeable rational agent, due to their education and professional experience; with experience comes tacit knowledge, and thereby insight and to some extent intuition. Second, we argue that, while values are possible sources of scientific disagreement, if we were to remove their influence from the expert judgments, we would be left in the same position as we started, a wealth of uncertainty and no practical way to overcome the challenge. Experts would indeed be left as mere databases, where information would be an input stored for later recall. Furthermore, under specific circumstances we define, value differences within an elicited group of experts can provide the condition of independence for rational consensus, as aimed by the elicitation methods which score, combine and aggregate expert judgements into structured judgments in the IPCC reports.
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