The document summarizes China's urbanization policies and trends over recent decades. It notes that the hukou system ties migrant workers to rural homes, but that China's 12th Five Year Plan calls for increased urbanization. Land in China is divided into urban and rural types, with 30-70 year leases. By 2015, 36 million new housing units were to be built, and by 2020 10 million rural residents will have moved to cities like Chongqing. Rapid urbanization has caused China's cities to emit most of its greenhouse gases.