Globally, urbanization is rapidly increasing and as it does there will be a need for new infrastructure to accommodate the accompanying population growth. Cities and the urban environment are important drivers of greenhouse gas emissions as city design and infrastructure contribute to emissions. Reducing the carbon intensity of urban dwellers will play a large role in mitigation.

Cities already contribute to climate action. One example is the C40, a network of megacities collaborating to address climate change (http://www.c40.org/). Such leadership is welcome, and learning from others could greatly benefit rapidly expanding cities. However, there is no one-size-fits-all solution, with individual characteristics of cities affecting emissions reduction adaptations. An analysis of 22 global cities identified different approaches to fit different city types (C. A. Kennedy, N. Ibrahim & D. Hoornweg, Nat. Clim. Change 4, 343–346; 2014), suggesting cities must find solutions to fit their residents, climate and existing infrastructure.

In this issue, Felix Creutzig and co-authors (page 1054) discuss how existing and future urban infrastructure can play a role in urban climate solutions. They estimate buildings account for around 70% of urban infrastructure emissions, with the remaining 30% from urban transport. Savings from changes to existing infrastructure can be achieved, but these are limited. The biggest emissions savings will be associated with new-built infrastructure. Construction of infrastructure results in emissions but it is the locked-in emissions, related to the use of infrastructure due to design and efficiency, that will have the greatest long-term reduction in emissions. Planning for low-carbon cities could reduce emissions from transport and buildings by 27–57%.

A barrier to such adaptation is finance — infrastructure and development requires investment and currently there are barriers to low-carbon investment. A Perspective from Ilmi Granoff and colleagues (page 1065) explores these barriers and how they might be overcome. The authors conclude that green growth requires an overall increase in infrastructure investments, whilst transitioning existing infrastructure investment towards low-carbon investment.

The urban environment changes slowly, but in some regions rapid urbanization is necessitating construction at an unprecedented rate. Globally 54% of the population now lives in cities (Urbanization and Development: Emerging Futures; UN-Habitat, 2016), contributing 70% of global greenhouse gas emissions, meaning urban design and planning for a low-carbon economy needs to start now.