Skepticism, Science & Pseudoscience

First of all, the IPCC is not 'a scientist', it is a group of scientists assigned to review the current body of scientific literature and draw conclusions concerning what the current consensus is on the various aspects of climate change. The IPCC has defined its terminology and outlined when specific terms like 'low confidence' should be used and what they mean. The definitions are here: http://www.ipcc.ch/pdf/supporting-material/uncertainty-guidance-note.pdf

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8) Use the following dimensions to evaluate the validity of a finding: the type, amount, quality, and consistency of evidence (summary terms: “limited,” “medium,” or “robust”), and the degree of agreement (summary terms: “low,” “medium,” or “high”). Generally, evidence is most robust when there are multiple, consistent independent lines of high-quality evidence. Provide a traceable account describing your evaluation of evidence and agreement in the text of your chapter.
• For findings with high agreement and robust evidence, present a level of confidence or a quantified measure of uncertainty.
• For findings with high agreement or robust evidence, but not both, assign confidence or quantify uncertainty when possible. Otherwise, assign the appropriate combination of summary terms for your evaluation of evidence and agreement (e.g., robust evidence, medium agreement).
• For findings with low agreement and limited evidence, assign summary terms for your evaluation of evidence and agreement.
• In any of these cases, the degree of certainty in findings that are conditional on other findings should be evaluated and reported separately.

9) A level of confidence is expressed using five qualifiers: “very low,” “low,” “medium,” “high,” and “very high.” It synthesizes the author teams’ judgments about the validity of findings as determined through evaluation of evidence and agreement. Figure 1 depicts summary statements for evidence and agreement and their relationship to confidence. There is flexibility in this relationship; for a given evidence and agreement statement, different confidence levels could be assigned, but increasing levels of evidence and degrees of agreement are correlated with increasing confidence. Confidence cannot necessarily be assigned for all combinations of evidence and agreement in Figure 1 (see Paragraph 8). Presentation of findings with “low” and “very low” confidence should be reserved for areas of major concern, and the reasons for their presentation should be carefully explained. Confidence should not be interpreted probabilistically, and it is distinct from “statistical confidence.” Additionally, a finding that includes a probabilistic measure of uncertainty does not require explicit mention of the level of confidence associated with that finding if the level of confidence is “high” or “very high.”

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The very fact that in making this statement the working group is reversing its previous findings is telling. Generally it takes a great deal of evidence for people to admit what they said before was wrong, and in this case the evidence was overwhelming. As many people who specialize in this field pointed out, the previous IPCC reports that claimed that global warming would result in more extreme weather events were made by people that had little expertise in the field and did not fully understand what the consensus actually was.