Just as 'sustainable farming' might be a contradiction in terminology
Futures Forum: Sustainable intensification of agriculture: an oxymoron?
... so too might the notion of a 'sustainable city':
Is ‘sustainable city’ an Oxymoron? - Local Environment :: Taylor & Francis Online
Sustainable cities: Oxymoron, utopia, or inevitability?
Sustainable Cities: Oxymoron or the Shape of the Future?

The UN has just published its report on population and city growth:

The planet's urban population – which overtook the number of rural residents in 2010 – is likely to rise by about 2.5 billion to more than 6 billion people in less than 40 years, according to a UN report.

Future development targets should focus on creating inclusive cities with adequate infrastructure and services for all residents, said John Wilmoth, director of the UN's population division. "Managing urban areas has become one of the most important development challenges of the 21st century," he said. "Our success or failure in building sustainable cities will be a major factor in the success of the post-2015 UN development agenda."

The UN cautions that sustainable urbanisation requires cities to generate better income and employment opportunities, and "expand the necessary infrastructure for water and sanitation, energy, transportation, information and communications; ensure equal access to services; reduce the number of people living in slums; and preserve the natural assets within the city and surrounding areas".

Urban population boom poses massive challenges for Africa and Asia | Global development | The Guardian

The problem is how an expanding city can 'preserve its natural assets'.

When cities expand, they tend to build on agricultural land:
Can we stop farmland losses? California Agriculture Online
Urbanization and its implications for food and farming | Philosophical Transactions of the Royal Society B: Biological Sciences
Sydney Must Not Destroy Market Gardens | Greens NSW
Urban expansion and loss of agricultural land

Looking at an example closer to home, the 'urban development' brought by the London Olympics also had an impact on agricultural land:
Manor Gardens Allotments: a Scandalous Legacy | Games Monitor
Manor Gardens - londongardenstrust.org

Some would even say the the city per se is an unsustainable entity:

Forget Chinatown, Get the Real Story of California's Most Famous Water War | Los Angeles | Artbound | KCET
Los Angeles, City of Water - NYTimes.com

Ancient Cities Were Unsustainable Too

January 9, 2014 JONATHAN NETTLER

Over the last several decades, researchers have examined how our cities deplete natural resources and change the climate and ecosystems of their surrounding areas. But new evidence shows that such impacts aren't a purely modern phenomenon.

"Writing for Nature’s Scientific Reports, a team led by David Kaniewski showed that the development of Akko, a port city along what is now Israel’s northern shores, coincided with a collapse of the local ecosystem, with dense coastal forests transforming into a dry, shrubby grassland," writes Colin Schultz for Smithsonian.com. Researchers conclude that residents of Akko, one of the world’s oldest cities, taxed the local water supply and produced an urban heat island effect that helped transform the area's climate and ecosystem.

Ancient Cities Were Unsustainable Too | Planetizen: The Urban Planning, Design, and Development Network

Are Cities Sustainable?

16 November, 2009

A reporter from Dubai phoned last week and asked, "Can Dubai become a sustainable city?" and specifically, "Could the tourism industry be sustainable?" In the age of global warming and declining fossil fuels, the entire airline industry is probably not sustainable. Dubai, of course, is not even remotely sustainable.

The sustainable cities are small, modest, usually poor, semi-rural centres, closely linked to local food and energy sources. One of the most ecological western consumer cities is Lingköping, Sweden. In the 1980s, Lingköping's seven political parties agreed to pursue a non-partisan 'Environment Path'. They replaced oil and coal heat with electricity from municipal waste and reduced city CO2 emissions by 40 per cent. The city offers free recycling, public transportation that runs on electricity and waste-biogas, bicycle paths, and reduced taxes due to income from the public waste-energy utility.

Even so, Dubai, Lingköping and all cities rely on goods, services, energy, and resources from around the world, delivered by fuel-guzzling transport. We hear a lot these days about 'sustainable cities', but let's look at the reality.
Cities in history

Hunting and gathering is a sustainable lifestyle. We know this because all animals live this way, and humans lived this way for several million years. Early human fire-making hunters caused local extinctions and disturbed natural habitats, but the real problems with sustainability began with urban concentration.

Four thousand years ago, Sumerian cities on the Euphrates river plains required intensive agriculture and irrigation, causing erosion and salt accumulation. Sumerian texts describe barren soils and 'earth turned white'. The communities migrated north along the river seeking new fertile soils, leaving abandoned cities to disappear under the sand.

By 500 BC deforestation and soil erosion had left most cities gasping for food and resources. In 460 BC, as the population of Athens swelled with war refugees, filth piled up, and a plague (probably typhus) killed over a third of the population. Cities everywhere began to experience similar plagues, and the human population growth rate began to decline for the first time in history.

Forty thousand years earlier, in Cro-Magnon communities, human population growth remained extremely slow, a few thousandths-of-one per cent each year. But this rate climbed steadily, and by 500 BC the growth rate reached 100 times higher, over a tenth of one per cent - about 0.13% -a year. However, cities became population drains, and by about 200 AD, the population rate had dropped below zero, and total human population decreased for the first time in history.

This growth rate did not recover to the 500 BC level for two thousand years, until about 1750 AD. During those two millennia, cities - centres of filth, disease, toxic smoke, and conflict - killed off more people than they produced. Lewis Mumford explains in The City in History that small, rural Mediaeval towns remained relatively clean and functional, but between 1200 and 1500 AD, large cities became centres of death and human population dropped incessantly. Meanwhile, burgeoning empires required ever more resources from distant lands.

Deep Green: Are Cities Sustainable? | Greenpeace International
Futures Forum: Lewis Mumford: "The physical design of cities and their economic functions are secondary to their relationship to the natural environment."

However, cities are a reality, and perhaps the 'solution' is to be 'smart'...

The Open University in Milton Keynes has some technological ways forward: