Joan Clos said even though the rate of population growth is decreasing, the UN projects that in the next 30 years the global population will increase from 7 billion to 9 billion and the urban population will grow between 2.5 billion and 3 billion people.
"In all human history we have reached 3.5 billion of urban settlers and in the next 30 years we are going to have 3 billion more," Clos said.
With 96 per cent of the growth of cities expected in poorer developing countries, he said, there are going to be huge demands on land, resources and services for urban residents.
Clos, a former mayor of Barcelona who is now executive director of the UN Human Settlements Programme known as UN-Habitat, spoke at a news conference promoting the agency's upcoming World Urban Forum from April 5-11 in Medellin, Colombia which will focus on growing inequalities in urbanisation worldwide.
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Currently, Clos said, the world is experiencing "the highest rate of urbanisation in human history," and national and local governments don't have the capacity to address key issues including organisation, governance, finance and the provision of services.
In recent decades, he said, inequalities in urban areas have led to protests and unrest as cities have faced difficulties integrating a big influx of migrants.
"This is why we are very worried, because the number of people living in slums is increasing," Clos said.