This statement does what rhetoricians call establishing ethos, trying to create a persona that will appeal to a potential audience. Diamond is addressing a particular authorial dilemma. Right now, much of the reading audience in the United States is highly polarized, between "red" and "blue" camps, and reads material about such issues as geopolitics and the environment through intensely tribalist lenses. 

What is interesting about the rhetoric of Diamond's introduction and the prompt of the assignment is that it is framed in terms of and accepts this tribalist perspective, rather than the notion of scientific neutrality, of simply studying the scientific evidence and seeing what it suggests. 

My own sense of Diamond's rhetoric is that it is quite effective in weaving a broad sweep of historical events into a coherent narrative and making it accessible to a popular audience. In terms of audience, what helps in creating as broad a market as possible for this book is the way he aims at an inoffensive conclusion that although problems exist, they are can be solved. Although his book will not appeal to extreme climate change skeptics, and because of its broad sweep can overgeneralize, his generally reasonable and studiedly nonpartisan tone should make it appeal to a fairly broad audience.