Chinese authorities are beginning the western phase of the historic and ambitious 'Great Green Wall', a 4,480-kilometre belt of forest across 551 counties and 13 provinces in north-west, central north and north-east China. Part of broader national environment programmes, it is the world's largest ecological development, and is designed to halt 2,460 square kilometres of land being lost annually to the expanding Gobi Desert due to overgrazing, deforestation and drought.