I believe it should be the same rule of thumb we are accustomed to using elsewhere.
If we have a computer security question, we will defer to the people who have dedicated 30 years of their life to mastering computer security. Whatever their opinion is, it's statistically more likely to be correct than whatever opinion I have after 2 weeks of "research". Likewise for astronomy, neurosurgery, being a pilot, and any other complicated area of study. I can't fly a plane, I can't operate the LHC, and I don't know anything about vaccines, so in all of these areas I need to outsource my opinions, to an extent, to the people that know these things better than everyone else. It's not perfect, and we can call that imperfection faith, but I can't think of a better approach.
For the record - I personally agree with the findings of bodies like the IPCC. But I am not sure there is more than the aforementioned faith and some more indicators backing me up on that.