What is “green” about this growth is how it is stimulated: through urban activities (including policies and programmes) that reduce either: 1) negative environmental externalities, such as air pollution and C02 emissions, or 2) the consumption of natural resources and environmental services, including w'ater, energy and undeveloped land. These effects are in part the result of more readily identifiable interactions at the urban level among economic efficiency and environmental objectives. By focusing on growth, this definition recognises that polices to reduce environmental impact can only be sustained over the long term if they generate wrealth (OECD, 2013).