In an industrialised society, buildings and the movement of goods and people between them account for two-thirds of energy consumption. As a result, cities contribute to about 70 per cent of global CO2 emissions. But while this sounds like bad news, there is plenty of evidence to suggest that cities can be designed to be both productive and clean. Stockholm reduced emissions by 35 per cent from 1993 to 2010, but grew its economy by 41 per cent, one of the highest growth rates in Europe. Copenhagen has done much the same. If Beijing, Mexico City or São Paulo were to take their cue from Stockholm, Copenhagen or even London, the impact would be startling.