The credibility of scientists comes from two working premises:
1. They are bright people who do conscientious work to learn stuff, and
2. They're honest about what they find, and demonstrate their honesty by being transparent about their work.
If they can't do the above, they might as well shut up 'cuz nobody's buying it. We've already heard it all from the politicians. In other words, in the arena of science, scientists have the standing to speak with some authority. In the arena of politics, they're just another braying voice.