Sustainable urban development moved forward last year, when the United Nations adopted both the Sustainable Development Goal 11 on cities and the New Urban Agenda (see go.nature.com/2qz8ows). Unfortunately, these international policy documents interchangeably use two quite different concepts — sustainability and resilience. We are concerned that policymakers confuse the two because academics do, which hampers implementation.
Resilience is a property of a complex system. For example, the Internet is resilient because it continues to function even when major nodes collapse. Resilience may not always be desirable — witness dictatorships that are resilient across generations. It may also run counter to sustainability goals: for instance, efficiency reduces diversity and redundancy, both of which are key features of resilience.
This conflict is illustrated by high-density urban areas, which can be more efficient to run in terms of, say, energy distribution, communications and waste collection. However, these areas can also be vulnerable to extreme events such as flooding because they are less diverse (with few green areas, for example) and have few redundancies (in the form of back-up facilities and disaster-management processes).
The research community needs to be clear about the differences and synergies between sustainability and resilience. Only with clarity can such concepts be applied in policy and practice.