The most threatening change, from my perspective, is extermination of species. Several times in Earth’s long history rapid global warming of several degrees occurred, apparently spurred by amplifying feedbacks. In each case more than half of plant and animal species went extinct. New species came into being over tens and hundreds of thousands of years. But these are time scales and generations that we cannot imagine. If we drive our fellow species to extinction we will leave a far more desolate planet for our descendants than the world that we inherited from our elders. We will leave a world haunted by the memories of what was.Another unknown over the next few decades or centuries may be whether humans, or their successors, create or deliberately select for the evolution of new life forms, and if so what the consequences may be.
See also Toast, Ghosts and shadows and On the Origin (and Fate) of Species
P.S. Hansen's article appeared in The Observer here on 15 Feb.