For the first time in history, the majority of humanity is living in cities. Until today, our notion of social progress has been almost synonymous with our ability to build and maintain these vast, elaborate settlements, which are living testaments to human ingenuity. But also for the first time, we are becoming aware of the mounting costs of this relentless urban growth on our planet's environment. The United Nations estimates that each week, some 1 million people around the world are moving to cities. At this pace, more than 60 per cent of us will live in ever-larger metropolises by 2030, putting severe strain on their infrastructure and quality of life the world over. UN Environment Programme chief Klaus Toepfer says city dwellers are insatiable consumers of fresh water, food and other natural resources, while disgorging a disproportionate share of household and industrial waste, contaminated water and greenhouse gases that afflict the ecology of whole countries and regions. The worst of such urban blight is found in developing nations. Rapid industrialisation and higher birth rates have triggered massive migration of rural inhabitants to cities from Brazil to China. Too often, though, their frenetic search for better jobs and life has given rise to ill-planned and heavily polluted mega-cities teeming with poor slum dwellers. So the UN's focus on the challenges posed by the rise of mega-cities for its World Environment Day on Sunday was a timely one. Gathering in San Francisco, mayors from around the globe adopted a 21-point action plan to promote cleaner, healthier and safer cities. Their proposals include a commitment to safe drinking water for all by 2015, zero landfill and incinerator waste by 2040, more use of renewable energy and public transport, easier access to public parks, a phasing out of the riskiest chemicals and 'greener' buildings. These are all worthy goals. But when it comes to environmental protection, corrective measures taken today may take decades or more to produce visible results. That is why sustained actions will speak louder than fleeting words. China's haphazard effort to combat pollution is a case in point. As the world's biggest and fastest developing country, China faces unprecedented environmental challenges. The 2004 Chinese Urban Environment Protection Report released on Sunday says severe smog is a problem in 290 of China's 500 large cities. More than 400 cities have constant water shortages and in some 50 cities, supplies are not of drinking quality. One reason: only a third of household sewage is treated. Chinese leaders know such trends are unsustainable and are issuing increasingly dire warnings. The report, however, notes that improving the urban environment remains 'an enormous and arduous task'. Still, China and the rest of the world must never let up on the effort.