So, if we continue the thought experiment, and say thousands of people have been making observations of the comet for five years and have come to roughly 50% impact probability estimate, with multiple independent methods. And that impact would likely kill maybe a billion people.

And you say it is unacceptable for them to advocate doing anything?

http://neo.jpl.nasa.gov/risks/a410777.html

Current impact risks overview is here. 50% chance of a large object would certainly be unprecedented.

http://arxiv.org/pdf/0804.1126.pdf

Now, replace the comet watchers with paleoclimate researchers finding out what happened when the planet last time crossed 450 PPM (the other way around).