A common claim among various kinds of pseudoscientists such as creationists or climate change denialists is that appealing to scientific consensus is either an appeal to the popularity of a position or an appeal to an authority and that therefore, appealing to scientific consensus is a logical fallacy.
However, appealing to scientific consensus is not the claim that “the scientific community is an authority or that it is a popular position, and therefore correct”, but rather, there is an additional premise in the appeal to scientific consensus that does not normally exist in the average appeal to authority. To elucidate the difference, let us see how this plays out.
P1. There is a scientific consensus on X (evolution, global warming, HIV causing AIDS or whatever).
Now, had we gone straight from this to the conclusion that X is true, it would have been an argument from authority or appeal to popularity. However, let us not forget our additional premise.
P2. If there is a scientific consensus on X, then it is probably the case that X is a reasonable scientific conclusion supported by most different lines of converging evidence.
In general, the scientific community as a whole is very conservative in making strong statements, because as we all know, making categorical statements may come back to haunt you. So we can be reasonably sure that, in the majority of cases, a consensus position is at the very least support by most currently known evidence. It is easy to see how the following conclusion follows.
C. It is probably the case that X is a reasonable scientific conclusion supported by most different lines of converging evidence (from P1 and P2 by modus ponens).
Too be sure, the scientific community is not infallible or always right. However, when the majority of the evidence available supports a position, it is reasonable to hold it as a tentative conclusion regardless.