A fully renewable system will probably occupy one hundred times more land than a fossil-fuel-powered one. In the case of the US, between 25 and 50 per cent of its territory, and in a cloudy, densely populated country such as the UK, all of the national territory might have to be covered in wind turbines, solar panels and biofuel crops to maintain current levels of energy production. While ongoing tinkering will improve renewable energy systems, they will never have the power densities of fossil fuels. It is land scarcity, rather than rare natural resources, that is the ultimate limit to economic growth: energy consumption must be cut.