For the vast majority of its 4.54 billion years, Earth has proven it can manage just fine without human beings. Then came the first proto-humans, who emerged just a little more than 2 million years ago—a fleeting moment in geological time. What is it that made this benevolent moment of ours possible? Ironically, it’s the very same thing that now threatens us—climate change. The conditions that allowed humans to live on this earth are fragile, incredibly so. Climate variability has at times created new niches that humans or their ancestors could potentially exploit, and challenges that at times have spurred innovation. But there’s a relatively narrow envelope of climate variability within which human civilization remains viable. And our survival depends on conditions remaining within that range.