Today a lot of people feel hopeless about the future due the climate emergency, how have people in the past dealt with perceived existential threats to civilisation?

I am sorry, this is no answer (I am not very familiarised with the policies of this sub), but could you specify a little bit, on the place/culture/era ? As it's not the same to compare how US citizens felt during Cuban Nuclear crisis in 1962 and a Roman citizen with Germanic invasions, social and economical crysis of the third century.