What makes a city?

The number of people living in cities is growing rapidly. According to the UN, over two-thirds of the global population will live in urban areas by 2050. For cities, that means meeting a booming demand for infrastructure and services - like affordable housing, water, sanitation, employment, schooling, health facilities and transport.

Unprecedented urbanization has also led to ever-expanding slums in some cities, like the Khayelitsha township in Cape Town. Millions of people across Africa, Asia and South America live in shanty towns, where they may not have access to clean water, sanitation, transport or jobs.

While urbanization can lead to overcrowding, it can also bring new opportunities. In a bid to tackle a housing shortage for low income workers in Santa Monica near Los Angeles, the city started building environmentally friendly affordable housing near the center. Colorado Court, equipped with solar panels and a natural gas-powered turbine, was one of the first energy-neutral buildings in the US.

Cities are traditionally built close to rivers or underground waterways to ensure a fresh water supply. But finding a way to deliver water to growing urban populations that is cheap, reliable and sustainable is already proving difficult. Several cities in India, for example, experience frequent shortages and have to rely on water brought in from elsewhere.

Particularly in developing countries, urbanization has added to pressure on already stretched resources. Poor people who've moved from rural areas may struggle to afford food and other basic supplies. In Kampala, Uganda, urban farming projects on tiny city plots are helping families grow their own food and save money. Sometimes they can also supplement their income by selling surplus produce.

A well-connected transport system is vital for residents to get to work or school. But an increasing city population inevitably means there'll be more people on the roads, creating traffic headaches for locals as well as urban planners. Copenhagen, which is aiming to be carbon neutral by 2025, has a network of cycle paths to encourage people to leave their cars at home and take the bike instead.

The Colombian capital, Bogota, has tried to tackle its congestion problems with the TransMilenio Bus Rapid Transit System, launched in 2000. The buses are used by more than 2 million people a day in the city, although the system has copped criticism for being overcrowded. Next, the city aims to replace its diesel fleet with hybrid and electric buses.

Dealing with waste is another important task for any city. Swedish cities burn the trash they produce, converting it into energy. That means only 1 percent of household rubbish ends up in landfill. In the US, San Francisco has banned plastic bags and is aiming to produce zero waste by 2020. Other cities, in the UK for example, have used waste to power public transport.

Cities are a major source of air pollution, but creating a clean and safe environment for residents is also an important aspect of a city's livability. Mexico City is known for its high smog levels. Earlier this year, its air quality crisis came to a head when alarming levels of pollution forced authorities to order cars off the roads and people to stay indoors.

Earlier this year, Mexico City re-implemented an initiative called "Hoy No Circula" ("Don't Drive Today") to ban residents with cars from driving one day a week. Other efforts to curb emissions include reforestation and greening city transport. And there's also this hospital building, the Torre de Especialidades, which is coated in a facade made from special tiles that can suck up smog.

Cities are responsible for 70 percent of greenhouse gas emissions. A number of cities, including Copenhagen, Vancouver and the Swedish city of Malmo have taken a lead, setting targets to curb their output. Freiburg, in southern Germany, has also taken steps, boosting investment in solar power, green transport and recycling. It aims to be climate neutral and rely 100 percent on renewables by 2050.

Green spaces are another vital part of any city - for recreation as well as to help reduce heat in an urban concrete environment. Singapore, rated number one on the Asian Green City Index, is a densely populated island yet it has almost 50 percent green cover. The Garden by the Bay, above, has steel structures shaped like trees that collect solar energy and harvest rainwater to irrigate the park.