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