The meteoric growth of megacities-there are now more than 25 in total-has brought with it huge environmental and social problems. Cities occupy just two percent of the land surface of the Earth but consume three-quarters of the resources that are used up each year, expelling the half-digested remains in clouds of greenhouse gases, billions of tons of solid waste, and rivers of toxic sewage. Their inhabitants are making ruinous demands on soils and water supplies for food and on forests for timber and paper.