Roads and urban sprawl are leading causes of ecosystem fragmentation; a serious reduction in car use would free up huge amounts of space. In many US cities, for example, approximately 60 per cent of municipal land area is dedicated to car use in the form of roads, car parks and easements. Even if energy efficiency means that carbon pollution from oft-demonized cars is not as great as one might have expected, reducing their use is important for reasons of land scarcity. Air travel will need to be rationed, too. Although planes have doubled their fuel efficiency since 1978, flying is the fastest growing sector of transportation and, in the short term, the greenhouse-gas pollution emitted by planes has an effect 20 times greater than all the world’s cars, because of the sensitivity of the atmosphere’s upper reaches. Substitutes, such as solar-powered planes, will not be able to compete with kerosene-driven rivals for many decades. Here, there is no technological fix in sight.