Size now matters: in a single human lifetime we have become a big world on a small planet.This simple fact is as profound as Copernicus’s conclusion that the Earth revolves around the sun or Darwin’s evolution by natural selection. A crucial difference though, is that the need to act on this information – as global citizens and as world leaders – has become critical.Moreover, this fact means our economic schools of thought, from Marxism to neoliberalism and everything in between, are obsolete. Given this knowledge, perhaps future economists will view their followers as no wiser than the medieval academics who counted angels on pinheads. We have built our global economic logic on the assumption that our planet is endless and can deliver natural capital for free and take abuse – such as abrupt climate change and mass-extinction of species – without sending any invoices back to the economy. A reasonable assumption until two decades ago. Not any longer. Now we have ample scientific evidence that we are hitting the ceiling of our planet’s capacity to support world development, without triggering extreme shocks, such as droughts, floods and heatwaves.