The transition to a net-zero carbon economy will be straightforward: we just roll out cheap clean energy technologies, become more energy efficient every year, keep innovating and stop chopping down forests. What could be easier? The transition will also be brutally challenging: every sector of the economy will have to switch to new technologies, consumers will have to change behaviors, new supply chains will have to be built, and all this has to happen in every major economy, in just a few decades, and at the cost of a whole generation’s savings. What could be harder?