A consensus claim is an appeal to authority . . . Scientists involved in the IPCC began the association of the FAR with a consensus claim. . .University of Arizona (and Miami) prof. Keith Lehrer, "Social Consensus and Rational Agnoiology," 31 Synthese 141, 141 (1974):
[T]he consensus claim created opportunities for opponents to object that the IPCC's emphasis on consensus was distorting the science itself. Once the consensus claim was made, scientists involved in the ongoing IPCC process had reasons not just to consider the scientific evidence, but to consider the possible effect of their statements on their ability to defend the consensus claim. Sluijs et al. have pointed out that the "climate sensitivity" (i.e., the temperature increase predicted for a doubling of CO2) has been reported as between 1.5 and 4.5 C since Working Draft 10 1975, with a "best estimate" of 2.5 or 3 C (an observation that still holds with the Fourth Assessment Report). They have suggested that one reason for this "anchoring" was that changing the figure would "embarrass" the IPCC and undermine its credibility, and that scientists were therefore forced (among other things) to "disqualify" results that did not conform . . . The IPCC and its defenders had to reply to these objections as well.
"Consensus" is a strong claim, and it opens a wide argument space; that is what I have been trying to suggest in the above sketch. By representing their work as a "consensus," the scientists of the IPCC essentially legitimated the objections of those commonly labeled as "denialists," and committed themselves to a twenty year process of replying to them.
Succinctly stated, the concern [of science and rational inquiry] is to obtain truth and avoid error. We shall argue that consensus among a reference group of experts thus concerned is relevant only if agreement is not sought. If a consensus arises unsought in the search for truth and the avoidance of error, such consensus provides grounds which, though they may be overridden, suffice for concluding that conformity is reasonable and dissent is not. If, however, consensus is aimed at by the members of the reference group and arrived at by intent, it becomes conspiratorial and irrelevant to our intellectual concern.(via Climate Etc.)