Page:Final Draft of the 2017 Climate Science Special Report.pdf/11

 CSSR 5OD: FINAL CLEARANCE 1

Front Matter

results or on expert judgment).

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The terminology used in the report associated with these metrics is shown in Figure 2. This language is based on that used in NCA3 (Melillo et al. 2014), the IPCC’s Fifth Assessment Report (IPCC 2013a), and most recently the USGCRP Climate and Health assessment (USGCRP 2016). Wherever used, the confidence and likelihood statements are italicized.

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Assessments of confidence in the Key Findings are based on the expert judgment of the author team. Authors provide supporting evidence for each of the chapter’s Key Findings in the Traceable Accounts. Confidence is expressed qualitatively and ranges from low confidence (inconclusive evidence or disagreement among experts) to very high confidence (strong evidence and high consensus) (see Figure 2). Confidence should not be interpreted probabilistically, as it is distinct from statistical likelihood. See chapter 1 in IPCC (2013a) for further discussion of this terminology.

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In this report, likelihood is the chance of occurrence of an effect or impact based on measures of uncertainty expressed probabilistically (in other words, based on statistical analysis of observations or model results or on expert judgment). The authors used expert judgment based on the synthesis of the literature assessed to arrive at an estimation of the likelihood that a particular observed effect was related to human contributions to climate change or that a particular impact will occur within the range of possible outcomes. Model uncertainty is an important contributor to uncertainty in climate projections, and includes, but is not restricted to, the uncertainties introduced by errors in the model's representation of the physical and biogeochemical processes affecting the climate system as well as in the model's response to external forcing (IPCC 2013a).

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Where it is considered justified to report the likelihood of particular impacts within the range of possible outcomes, this report takes a plain-language approach to expressing the expert judgment of the chapter team, based on the best available evidence. For example, an outcome termed “likely” has at least a 66% chance of occurring (in other words, a likelihood greater than about 2 of 3 chances); an outcome termed “very likely,” at least a 90% chance (or more than 9 out of 10 chances). See Figure 2 for a complete list of the likelihood terminology used in this report.

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Traceable Accounts for each Key Finding 1) document the process and rationale the authors used in reaching the conclusions in their Key Finding, 2) provide additional information to readers about the quality of the information used, 3) allow traceability to resources and data, and 4) describe the level of likelihood and confidence in the Key Finding. Thus, the Traceable Accounts represent a synthesis of the chapter author team’s judgment of the validity of findings, as determined through evaluation of evidence and agreement in the scientific literature. The Traceable Accounts also identify areas where data are limited or emerging. Each Traceable Account includes 1) a description of the evidence base, 2) major uncertainties, and 3) an assessment of confidence based on evidence.

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28 June 2017