A generation ago, humans eluded nuclear annihilation; with luck, we’ll continue to dodge that and other mass terrors. But now we often find ourselves asking whether inadvertently we’ve poisoned or parboiled the planet, ourselves included. We’ve also used and abused water and soil so that there’s a lot less of each, and trampled thousands of species that probably aren’t coming back. Our world, some respected voices warn, could one day degenerate into something resembling a vacant lot, where crows and rats scuttle among weeds, preying on each other. If it comes to that, at what point would things have gone so far that, for all our vaunted superior intelligence, we’re not among the hardy survivors?