When my great-grandfather was alive in the 1900s, roughly 10% of the world’s population lived in cities. The 50% mark was passed in 2007 – not that long ago. By 2050, around 75% of the world’s population will be living in some form of city or urban agglomeration. This is due to two things: the first is natural birth – more people in cities are giving birth to more children, and they’re living healthier lives. The second is national and international migration. Of course, migration is what cities have been about for thousands of years, attracting people because of jobs, opportunities and education.

As we reflect on urban growth and changing patterns around the world, we need to think about issues of scale, speed and geographical distribution. Most of the urban growth in the next 30 years will happen in parts of Asia and Africa, where the economy is not as advanced as in other parts of the world, but it’s growing and picking up speed very quickly. The economy of some of the countries in East Asia and South East Asia – China, Singapore, Malaysia or India – are, instead, strong and increasingly urbanised.

In vast, sprawling, low-density cities, like Lagos or Delhi, people commute long distances to and from work using cars, communal taxis, buses – vehicles often powered by noxious fossil fuels. Apart from causing congestion and exacerbating pollution, low-density sprawl not only has a negative impact on the environment and climate change but also contributes to social fragmentation and inequality. Higher density cities are kinder to the environment and, if well-designed, can promote social integration.

These social and environmental dimensions can be addressed in part through good urban design and management. If you plan and govern a city more effectively and control growth, you can determine the impact that that urban environment has in terms of its carbon footprint. If carbon emissions are reduced by 10%, 15%, or 20% – which is what is happening to cities around the world, both in the global north and the global south – there is an incommensurate positive impact on the health and environmental stability of the planet.

Each city has a particular spatial DNA: a pattern of streets, routes and connections that relates to its unique social and economic culture. In some cases, this urban infrastructure no longer responds to the dynamics of contemporary life – it becomes dysfunctional. For such cities to survive, a key issue is whether they can be upgraded and modernised – in effect, retrofitted, like a human body. Can you surgically intervene in the physical urban fabric and improve the way the circulation flows, breathing new life into aging organs?

City resilience is a response to this process of aging and dysfunctionality. I don’t believe that any city is too old to survive. I’m convinced that most cities, if they have the right DNA, can be adapted and retrofitted through good leadership and good planning. Certain structural problems – like aging populations in Japan or South Korea, where the birth rate has fallen dramatically – cannot be overcome by resilient planning and design. Urban retrofitting can help deliver environmentally and socially more balanced cities in the 21st century.