Cities are part of the system we’ve invented to keep people alive on Earth. People tend to like cities, and have been congregating in them ever since the invention of agriculture, 10,000 or so years ago. That’s why we call it civilisation. This origin story underlines how agriculture made cities possible, by providing enough food to feed a settled crowd on a regular basis. Cities can’t work without farms, nor without watersheds that provide their water. So as central as cities are to modern civilisation, they are only one aspect of a system. There are nearly eight billion humans alive on the planet now, and that’s a big number: more than twice as many as were alive 50 years ago. It’s an accidental experiment with enormous stakes, as it isn’t clear that the Earth’s biosphere can supply that many people’s needs – or absorb that many wastes and poisons – on a renewable and sustainable basis over the long haul. We’ll only find out by trying it.