A dialogue with the renowned Pedro B. Ortiz, Senior Urban Consultant at the World Bank, provides insight into the challenges of urbanisation.

According to the UN, 85 percent of the world's population will live in cities of more than 750,000 by the year 2050. What are some of the challenges of this unprecedented level of urbanisation?

Above: Madrid, a city that has benefited from the reticular planning method

Population growth and increased urban concentration are two of the biggest challenges we face. Currently, the world population is around seven billion and by 2050 it will reach 11 billion. In addition to population explosion, we face a huge migration of people from rural to urban centres.

The issue is that many of these developing metropolises (with more than 750,000 people), of which there are now 600, are growing at a rate of five percent per year. This means that cities such as London will double in size every 14 years. In Australia alone, the population will reach 42 million by 2050 and Melbourne will experience 50 percent growth.

The most dramatic impact will be as cities combine as they expand, notably around the Pearl River Delta (Hong Kong, Macau, Shenzen, Foshan and Guangzhou) where the urban population is expected to reach 120 million in that timeframe, but also similar conurbations of greater New York, Tokyo, Shanghai and Mumbai.

These are huge numbers to manage, particularly in terms of infrastructure and critical services such as water, energy and transportation. If we don't get it right, it could take up to 300 years to fix our mistakes.

For the past 30 years, the rationale for many governments has been to discourage people from leaving rural towns and moving to the cities. However, there are no social amenities in these towns and while there has been a focus on creating regional centres and jobs, it has proved to be an extremely expensive approach. It hasn't worked as people have still moved to the cities, so instead of building acceptable urban environments, we have ended up with slums or congested cities that are bursting at the seams. However, we are now starting to see governments accepting this inevitable migration and implementing policies to manage its impact.

Do the challenges of urbanisation differ between developed and developing nations?

Yes, for developed countries, 70-80 percent of the population lives in cities, whereas in developing nations it's about 30 percent. The issue for developing nations is that they are also growing at a fast pace with vastly different financial capabilities. When these cities get to the point of housing 70-80 percent of the population, it means they will have more than tripled in size, as the new extensions have lower sprawl density than the consolidated areas. For Latin America, Africa and Asia this will be particularly challenging.

In developed countries, 99 percent of growth is controlled, yet in developing nations, as much as 80 percent is uncontrolled. You can't plan for the uncontrolled, but you can invent a new process for planning in those cities that's based on having dialogues with the informal sector.

For example, Nigeria is now the second biggest film producer in the world after India, and Africa's biggest economy. Known as Nollywood, this is an industry that has grown rapidly in just 20 years because it produces films that address the specific challenges of Nigeria. It is this kind of adaptation that we need to bring to the planning of cities.

What are some of the solutions to deal with the impacts of urbanisation?

We have to rethink our approach. One of the ways to do this is to transform the model of metropolitan planning from orbital (growing outwards from the centre) to reticular, where every square on the city's grid has a key role to play. This is the same approach that enabled New York and Barcelona to grow so rapidly during the Industrial Revolution in the 19th century, when the need for rural labour was high and cities were unprepared.

I adapted this reticular approach for metropolitan Madrid in 1996, when the city was growing at a pace of 50 percent every 20 years. The concept, which I call the Metro-Matrix, transforms a rigid urban grid into a flexible framework at a regional scale. This allows a city to grow in a sustainable way by encouraging the creation of multiple centres – such as housing developments, commercial precincts and airports – all connected by a network of public transport and highways. It is certainly an approach that can assist both developed and developing cities today.

What is at the core of the urbanisation debate?

Collective intelligence is something we need to strive for. Let me explain. Cities that have 'human resources' can leverage the capacity of two people and by collaborating closely together the output will be the same as two and half people. This multiplier effect is what we call 'social resources' and when it is combined with governance, you can achieve a powerful, two-way dialogue (bottom up and top down) about a city's needs and plans for the future.

For example, a city that needs a light rail system but cannot afford to build one now should not scrap it from its planning opinion mandate but rather ensure it has earmarked a location for it. Sooner or later the funding will be available and at that time the rail system can be constructed along the planned corridor.

In developing nations, this is a much harder concept to achieve as social resources are closely related to the value system of that country. Unfortunately there are systems of value that do not promote working together to achieve win-win situations. The message here is that no matter how under-resourced a city is, upgrading a slum is three to nine times more expensive than doing it from the ground up, and it will never be as good.

How can professional services companies like GHD help public and private sector clients prioritise their projects to adequately address the future demands of urbanisation?

Essentially it comes down to helping your clients adopt a long-term vision wherever possible. It's also about being flexible in providing solutions that are adaptable and customised to the specific circumstances of a particular city, including climate, culture, socio-political and economic factors. By understanding the entire fabric of a city, you can create design solutions that are more inclusive and are therefore more beneficial in the long term.

Helping your clients open a dialogue at the highest levels of government is another way to elevate the urbanisation debate. The issue of course is that most administrations are only concerned with investments during their term of office, so discussing plans for the next 30 years is difficult. However, when politicians see the effects of urbanisation on their mandate they take notice. That's why focusing on three or four key priorities and making decisions based on a small scale is useful in this scenario. It's akin to acupuncture: you place your needle at the small scale, but you must have a vision of the entire body and the effects it will have at a larger scale.

Overall, the opportunity is to ensure your clients have a long- term vision with a sense of responsibility and a clear mandate for short-term priorities.

Can cities grow in a sustainable fashion?

Yes they can, provided we can allocate this growth rationally and use approaches (such as the reticular system that I mentioned earlier) to organise development in such a way that it can be improved in the future. The objective here is to plan the entire development in a sustainable fashion, and then prioritise projects based on available funding. Even if the infrastructure or services can't be constructed now, the most important thing is to protect rights of way for the future and start development in the right locations.

Sustainability is a key part of the urbanisation challenge, particularly when you consider the increased demands on water, energy and transportation.