> Argument from authority is not invalid when it can be backed with fact.
Your line of argumentation doesn't make sense here. We can look for backing up an authority with facts which come from...that same authority, who we then have to believe or not, short of replicating their study. Or we can get someone else to replicate the study, and someone else to replicate that study, and...suddenly we're talking about scientific consensus again.
You do not have time to replicate and verify the work of every scientist. At some point you are going to have to choose to take some authority's word for it.
The person I choose to believe is the one who talks about radiation balance and p-values and confidence intevals. Then I expect to see peer revieve publication and some peer revieved confirmation. It's also possible to establish somekind of authority to conduct meta-study. This works nicely in medical field.
Someone talking about "99%" of scientists is just distracting. Link the IPCC document and call it a day.
It's often unnecessary, because you need the fact anyhow. In some cases it's justified to save time. Like in the military.