Time is big, too—even if we just think on the timescale of a species. We’ve been around for approximately 300,000 years. There are now about 8 billion of us, roughly 15 percent of all humans who have ever lived. You may think that’s a lot, but it’s just peanuts to the future. If we survive for another million years—the longevity of a typical mammalian species—at even a tenth of our current population, there will be 8 trillion more of us. We’ll be outnumbered by future people on the scale of a thousand to one.

What we do now affects those future people in dramatic ways: whether they will exist at all and in what numbers; what values they embrace; what sort of planet they inherit; what sorts of lives they lead. It’s as if we’re trapped on a tiny island while our actions determine the habitability of a vast continent and the life prospects of the many who may, or may not, inhabit it. What an awful responsibility.