Traffic jams, smog, litter-strewn streets. City life can be hard at the best of times, and with the United Nations predicting that the number of city dwellers will reach 6.25 billion by 2050, the challenge to develop sustainable, livable and green cities is perhaps greater than ever before.

In 2011, the UN released research which reported that, 'the world's cities [are] responsible for up to 70 percent of harmful greenhouse gases while occupying just two percent of its land'.

Yet some cities are attempting to do things right. Since 2010, the European Commission has awarded the European Green Capital Award to cities showing willingness to use innovation and technology to create green, healthy and energy efficient cities.

Here, CNBC.com takes a look at the award's winners, and finds out what makes them so special.

By Anmar Frangoul, Special to CNBC.com

The "Venice of the North," Stockholm was the first city to be named European Green Capital. The city boasts an abundance of green space – there are roughly 1,000 parks, with 95 percent of inhabitants living fewer than 300 metres from a park or water.

And authorities have set ambitious goals, aiming to become independent of fossil fuels by 2050. In the near future, the city is hoping to have cut CO2 emissions by 45 percent by 2020.

In the field of transport, Stockholm has introduced a congestion charge to reduce traffic in the city, while according to the City of Stockholm, "one third of all new cars sold in Stockholm are clean vehicles."

All buses serving the city center are powered by renewable fuels, with the city home to the world's largest bus fleet fueled by ethanol.

With a population of 1.8 million, Hamburg, situated on the shores of the river Elbe, is the second-biggest city in Germany. It has set itself a target of cutting CO2 emissions by 80 percent by 2050.

Hamburg's authorities recently released details of plans to develop a "Green Network," which would mean that people will no longer need to take to their cars to get around the city.

A system of interconnected paths, the network is to cover 40 percent of the city by 2034. Carbon emissions will be reduced, and cyclists and pedestrians will be able to move from the center to the outskirts without encountering motor vehicles.

According to the European Commission, all of Vitoria-Gasteiz's near 240,000-strong population live 'within 300 meters of open green space', while a recent survey showed that 50 percent of journeys in the Spanish city are made by foot, helping to lower carbon emissions.

The city has a commitment to reducing its energy consumption, and is focusing on the potential of renewables, with a strong emphasis on wind, solar and geothermal energy, as well as biofuels generated from waste.

The French city's public transport system is a seen as a benchmark in cutting carbon emissions. Its tram network is fully electric, while the city has been redeveloped to reduce travel by car and increase the amount of pedestrians on sidewalks and cyclists on roads.

This, in conjunction with other policies, has resulted in a CO2 reduction of 4.77 tons per capita, according to the European Commission.

By 2020 authorities also hope to have built 7,500 "energy efficient" homes on the Île de Nantes, an island located in the city's center.

The Danish capital considers itself one of the planet's most forward-thinking when it comes to clean tech, green solutions and ambition.

Copenhagen aims to become carbon-neutral by 2025. This goal will be aided by, among other things, using biomass instead of coal, the construction of 100 offshore and onshore wind turbines, and the retrofitting of buildings to make them greener.

The city is also aiming for 75 percent of journeys to be by foot, public transport or bike by 2025, and plans to have installed 60,000m2 of solar panels on buildings.

Located in the south west of England, Bristol is an historic port city with a bright future, and is set to be the European Green Capital for 2015.

Often referred to as the greenest city in the U.K., Bristol impressed the European Commission with its commitments to go green.

The city's council recently built two wind turbines it hopes will generate 14,000 Megawatt hours of energy annually, while it has committed up to 300 million euros of investment in renewable energy and energy efficiency, according to the European Commission.

The city's policies saw carbon emissions reduce by 15 percent between 2005 and 2009, according to Bristol City Council, while domestic energy use in the city dropped by 16 percent between 2005 and 2010.