Three constituencies this argument depends on, and what each is owed.
The young
They did not choose the arrangement and will live longest inside its consequences. Two things are owed to them and both are usually withheld. Honesty: not managed optimism, not catastrophism, but the actual situation stated at the resolution the evidence supports. And agency: real decisions with real consequences, early, because a generation told it is the hope of the world and given no authority learns only helplessness.
The grief many young people feel about the future is not a disorder. It is an accurate response to accurate information, and it becomes destructive only when it has nowhere to go.
Science
Science is the most reliable method humanity has developed for finding out what is the case, and this book does not qualify that. Its authority rests on a genuinely spiritual discipline: the willingness to abandon a cherished belief when the evidence goes the other way.