This paper provides a response to the question of how we might achieve greater sustainability, and from that sustainable development. An engineering approach, or applied science (physical and social), integrated within a multi-stakeholder partnership, is proposed. A road map to a partial, or 'shadow agreement' is proposed, that would hopefully serve as the start of a global process leading to comprehensive sustainable development. An argument is made why cities are the most likely actors to design and bring about such an agreement. An agreement among the world's larger cities (those urban areas with 5 million or more residents by 2050) is possible, and is likely a necessary, but not sufficient condition to achieve sustainable development. Each city is viewed as a unique system as well as collectively within a 'system-of-systems,' and more broadly within local and global ecosystems and economies. Global boundaries and objectives such as the Sustainable Development Goals are down-scaled and applied at a metro-city level.Population projections are provided for the world's larger cities (in 2025, 2050, 2075 and 2100). Along with the hierarchy of sustainable cities and urban metabolism, two new tools are developed: (i) a cities approach to physical and socio-economic boundaries, and: (ii) sustainability costs curves. These two tools underpin the shadow agreement and are powerful planning and engineering tools in their own right. Case study application of the tools is provided for Dakar, Mumbai, Sao Paulo, Shanghai, and Toronto.