[QuotedForTruth]

How certain would the experts have to be to get such coordinated action?

With close approaches, it seems (as a layman) that there are always pretty large ranges of just how close they will be. I dont think you'd know "a comet is headed directly at us" until way too late. Instead you'd get some warning that there is a 10% chance or something. Over time that would go up to a near certainty. Maybe I'm pessimistic, but I would predict something more similar to climate change with deniers and paralysis.