Dystopias — dark, funhouse mirrors of our fears — will always be with us. Nineteen Eighty-Four was the product of a time when Big Brother Stalin was on the march, and the Eloi and the Morlocks of The Time Machine reflected H. G. Wells’s anxiety about where the onrush of 19th-century capitalism could lead. So what to make of the success of a “young adult” trilogy set in a North America that has — here a shout-out to a fashionably green vision of global catastrophe — emerged after “the droughts, the storms, the fires, the encroaching seas that swallowed up …