Even if everybody agreed on the basic facts (which is only possible if they're all drinking from the same well, eg impossible if the press is free) there would still be huge disagreement on what political course of action to take in response to those agreed upon facts, because different people have different values.

Take your global warming example, and suppose we have a magic wand to make everybody agree that it's happening, that humans were causing it, that its happening fast enough to cause massive extinctions, and that action now might still prevent this. With all of these given as universally held beliefs, it should be easy to resolve right? Well no, because in this scenario the magic wand aligned just about everything except values. Does somebody really care about the long term ecological impact of the thing more than they care about how environment austerity would impact them and their family personally? Some will, some won't, so the political debate remains standing. In fact, many of those selfish people will probably decide to stubbornly insist on a narrative that global warming isn't real, even though they know it is (thanks to the magic wand), so you'll be left wondering if your wand even worked at all.