What is the purpose in speaking of al gore about climate change and trends toward extremes

Al Gore is a very public figure, although less so than he once was, and so Al Gore talking about climate change is a way to associate a movement with a person who can bring attention to the subject by the mere fact that THEY are talking about it.

appeal to ethos is your answer.

"a" because he is appealing to logic as oppose to appealing to the audience's emotions.