This paper examines carbon ‘lock-in’ risks from an urban planning perspective, comparing two scenarios of urban development over the next 15 years to gauge the emissions implications of different choices. Cities around the world are emerging as leaders in the fight against climate change, embracing low-carbon transport, high-efficiency buildings, renewable energy and other strategies to reduce emissions while building more vibrant urban communities. This paper looks at two scenarios of urban development over the next 15 years. It finds that, in the reference scenario, new, energy-inefficient urban development may substantially “lock in” future CO2 emissions, as roughly 30% of future CO2 emissions “committed” annually occur due to new, urban building and transport systems. However, in an aggressive “urban action” scenario, urban policy-makers can instead deploy the most energy-efficient technologies and urban design, and avoid committing about 45 Gt CO2 of cumulative future emissions.