Get used to it. The future is urban and in 50 years’ it may be weird to find people living in the countryside. The UN planet-watchers have found not just that we are becoming an urban species but that the world’s cities are growing and merging with each other, forming vast “megaregions.” These giants are already sprawling across borders and becoming homes to 100 million or more people. Today just over one half of us live in urban areas. By 2050 it will be 7 in 10 and eventually the city will take over.

UN-Habitat’s biennial cities report says urbanization is now unstoppable and we should prepare to live in “the endless city.” Look back a few hundred years, say the authors, and you can see how places like London, Manchester and Liverpool in England outgrew themselves and merged with towns and villages around them to form conurbations of a few million people. Now cities like Hong Kong, Shenzhen and Guangzhou in China are linked by urban corridors and are home to 120m people. Look ahead a bit and you may see Rio de Janeiro and São Paulo in Brazil, Delhi and Mumbai in India, and even the 77 separate major cities which now stretch from Beijing to Tokyo via Pyongyang and Seoul, all linked together.

Outwardly, the development of these megaregions makes economic and environmental sense. They cover just a fraction of the habitable surface of the earth but are home to nearly 20% of the world’s population. They account for 66% of all economic activity and about 85% of technological and scientific innovation. The top 25 account for more than half of the world’s wealth, and the five largest cities in India and China now account for 50% of those countries’ wealth. They lead development and provide work for the masses, and the money earned in them is sent back and supports the country areas. Cities, in short, are the cradles of culture and wealth and will allow earth to accommodate a further three billion people.

This view is backed by writers like British academic and author Owen Hatherley: “Supercities have always horrified environmentalists but they shouldn’t. With their relatively short distances easily served by public transport, they are in fact greener than the countryside; a recent academic report estimated that cities produce less than two-fifths of greenhouse gas emissions. What is worrying about the ‘endless city’ is that it may lack the public spaces and networks that make urban life superior. The cities of hypercapitalism, with their gated communities, ‘urban villages’, pseudo country villas and private transport, are malevolent because they try to simulate the countryside. The megalopolis need not be the cause for handwringing.”

But the reality is that the world’s megacities are becoming sprawling megaslums, with city authorities unable to keep up with their growth and increasing inequality. The same UN report found that 827 million people – nearly one in six people alive – now live in crowded, substandard housing often without safe drinking water and sanitation. The number is set to grow, especially in Africa, where nearly two-thirds of the world’s slum-dwellers live.

Besides, cities do not stop at their borders, says cultural anthropologist and Middlesex University urban studies professor Herbie Girardet, who calls for a complete redesign of how cities work. “Urban living depends on enormous resource consumption. Urban citizens use four times the energy that rural dwellers consume. The characteristic of a truly sustainable city is, first and foremost, that it powers itself entirely by means of renewable energy systems. In nature, waste materials are absorbed beneficially back into the local environment as nutrients. Cities don’t do that. They work by way of taking resources from one place and dumping them somewhere else causing damage to nature. We need to turn this linear process into a circular process instead.”

But what will it be like to live in the endless city? The answer, says British environmentalist Jonathon Porritt, depends not on the size but what on what kind of cities we build. In Europe you can travel across heavily urbanized areas without even being aware that you are in a megalopolis. A long history of parks, open space, civic responsibility and good public transport has not divorced people from the natural world. “Sustainability can certainly be achieved in urban areas. Cities actually have some distinct advantages [over rural areas] when it comes to energy use and transport,” says Porritt.

But life in the endless city would be psychologically intolerable without contact with nature, he says. The vast city disconnected from the natural world and impossible to leave becomes a vast prison with potentially terrible consequences for both human society and the planet itself.

“The key is the degree to which the cities of the future allow people to live high quality lives. Without access to green space sustainability is impossible. Life must include a connection to the natural environment,” he says.

–John Vidal is the Guardian’s environment editor.