With the global population expected to reach 9.5 billion in 2075, the world's giant cities are quickly filling up.

But experts believe that the brunt of migration will not be on big cities.

Instead, they say that most urban migration will be to smaller cities, that are not prepare for a population flood.

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Experts believe that most urban migration will be to smaller cities, that are not prepare for a population flood. Pictured is a Brazilian favela town near Rio de Janeiro

THE 100RC INITIATIVE The 100RC initiative is backed by a commitment of $164 million (£132 million) from the Rockefeller Foundation. The funding pays for cities to appoint a chief resilience officer and develop a strategy to deal with challenges such as climate change. The 100RC initiative aims to give 'mind space and oxygen' to the cities signed up so that they can prepare themselves for the future and 'think differently' over a horizon of 20 to 30 years. Advertisement

The warning comes from Eugene Zapata Garesche, Latin America and Caribbean director for the 100 Resilient Cities (100RC) initiative.

He said: 'The cities that are set to grow are those that are not prepared to grow - the medium and small-sized ones.

'The cities that got prepared to grow are not growing anymore as they are already big enough. That is where the whole challenge of urbanisation is sitting today.'

As megacities such as Brazil's Sao Paulo can no longer sustain more population growth, cities like Curitiba or Salvador, which still have room to expand, will be required to absorb rising numbers of people, said Mr Zapata.

'You have cities like Mexico City that aren't even growing any more population-wise, because the city has turned into a nightmare for commuting, and there's a lot of pollution,' he said.

'It's good news for Mexico City, but bad news for the cities around.'

The United Nations estimates the number of people living in medium-sized cities of up to 5 million inhabitants is expected to jump to 1.1 billion by 2030, up from 827 million in 2014.

And it predicts an estimated 27 percent of the world's population will be living in smaller cities of at least 1 million people by 2030.

In Latin America, where 80 per cent of people are urban dwellers, 17 cities - including Mexico City and Rio de Janeiro - have signed up to the 100RC initiative, which is backed by a commitment of $164 million (£132 million) from the Rockefeller Foundation.

POPULATION SPIKES OVER THE LAST 50 YEARS From 1960 to 2015, certain regions have experienced a startling rise in inhabitants, data from the World Bank reveals. This includes: Afghanistan - from 8.9m to 32m Bangladesh - 48m to 160m Brazil - 72m to 207m Egypt - 27m to 91m Ethiopia - 22m to 99m Indonesia - 87m to 257m Iraq - 7m to 26m Mexico - 38m to 127m Pakistan - 44.8m to 188m Syria - 4.5m to 18.5m Source: World Bank Advertisement

The funding pays for cities to appoint a chief resilience officer and develop a strategy to deal with challenges such as climate change.

'Resilience is not an easy thing to push forward. There has to be willingness and capacity... within the government,' said Mr Zapata, who will head up a regional office for 100RC opening in Mexico City later this year.

Making sure resilience is institutionalised by cities and supported by legislation is essential to guarantee the long-term continuation of the strategy, he said.

The biggest resilience challenges for cities are improving transport and waste management, and securing water supplies, along with social policy issues such as supporting migrants, he said.

The 100RC initiative aims to give 'mind space and oxygen' to the cities signed up so that they can prepare themselves for the future and 'think differently' over a horizon of 20 to 30 years, he added.

In 2011, a report, entitled Popluation: One Planet, Too Many People? outlined the issues we might face if populations continue to grow.

It suggests that without drastic changes there will not be sufficient resources to provide people with basic human needs such as water, food, energy and shelter.

Experts say that without drastic changes there will not be sufficient resources to provide people with basic human needs such as water, food, energy and shelter (stock image)

Climate change is likely to place even more stress on resources, resulting in as many as a billion people moving from inhospitable regions.

Water requirements are projected to rise by 30 per cent by 2030 while food resources will be stretched by a doubling of demand for agricultural produce by 2050.

Slum living, already forced on a third of the world's urban populations, will become even more widespread as cities became increasingly packed with people.

As a result billions could be at risk of hunger, thirst and appalling living conditions, creating tinderbox conditions that could ignite civil unrest and conflict.