Cities are home to half the world's population, according to the United Nations, with that figure set to rise to almost two-thirds in the next 15 years.

One massive challenge that faces cities across the planet is how to lessen their environmental impact. According to the UN, cities occupy just 2 percent of the land in the world, but account for between 60 and 80 percent of energy consumption and 75 percent of carbon emissions.

One way cities can become greener is by making sure their transport links are efficient, green and easily accessible.

Here, CNBC takes a look at the past winners of the Sustainable Transport Award (STA) which, "recognizes profound leadership, vision, and achievement in sustainable transportation and urban livability."

By Anmar Frangoul, Special to CNBC.com

A city once plagued by violence, Bogota is today in the middle of a renaissance.

Colombia's capital won the inaugural Sustainable Transport Award in 2005 thanks to its bus rapid transit system as well as, "the city's commitment to integrating bicycle infrastructure with mass transit, and redefining and reclaiming public space for its citizens."

This megacity, home to around 10 million people, was recognized for replacing an elevated highway with an 11km stretch of water, greenery and walkways called Cheonggyecheon. It also added "exclusive bus lanes" along 36 miles of streets.

Authorities have set out a vision for 2030 that the South Korean city becomes a place where "people will not need to rely on their cars."

Plans include increasing sidewalk space for pedestrians and the "provision of a public bicycle service free of charge throughout Seoul."

The first 15km of the Metrovia, the bus rapid transit system for Guayaquil, Ecuador's largest city, was opened in 2007.

According to the Sustainable Transport Award, the system helped to provide a, "cleaner, higher quality service and reduced trip times in key travel corridors for city residents."

Other initiatives praised included the regeneration of underused public spaces – including the city's waterfront – to encourage more pedestrians.

Two of Europe's most important and busiest cities, London (pictured) and Paris were joint winners in 2008. London was praised for the implementation of a congestion charge zone, while Paris' bike sharing scheme, the Vélib, was behind its recognition.

The congestion charge was launched by Transport for London (TfL) in 2003. Drivers in London are now charged £11.50 per day if they drive in the central zone between 7 a.m. and 6 p.m. on weekdays.

In Paris over 20,000 bikes in 1,800 stations are available to the public, according to the Vélib website, with the first half-hour of a journey completely free.

New York City took the top prize in 2009 following the implementation of a raft of sustainable measures.

The STA highlighted the transformation of almost 50 acres of road space, traffic lanes and parking spots into bike lanes, pedestrian areas and public plazas.

Authorities have set ambitious targets, including the aim of slashing greenhouse gas emissions by 80 percent by 2050, compared to 2005.

Helping the city achieve this goal is a bike-sharing scheme – Citi Bike – which has put 6,000 extra bikes on the road, which serve up to 40,000 daily trips, according to the Mayor's Office of Sustainability.

The Indian city of Ahmedabad won the 2010 award for what the STA described as "the first full featured BRT (bus rapid transit) in all of India."

Called Janmarg, the system has helped to cut journey times in a city of more than five million people. It carries over 130,000 commuters every day, according to the Institute for Transportation and Development Policy.

Guangzhou's bus rapid transit system is one of the world's biggest, carrying around 850,000 people per day, according to the Institute for Transportation and Development Policy.

Launched the same year as the city's BRT, and also helping to reduce the city's reliance on cars, was a bike sharing scheme, which is used by approximately 20,000 people per day.

Medellin and San Francisco were joint winners of the award in 2012.

Medellin, in Colombia, was praised for its bus system, carpooling program and public bike scheme.

San Francisco, meanwhile, won plaudits for the San Francisco Municipal Transportation Agency's SFPark, which harnesses technology and uses a system of "smart pricing" which, "periodically adjusts meter and garage pricing up and down to match demand," according to the SFPark website.

San Francisco has also set itself the target of 20 percent of trips by bike by 2020.

Mexico City's expansion of its bus system – Metrobus – and public bike scheme, as well as the pedestrianization of streets and launch of a pilot to transform parking – ecoParq – were key to its victory in 2013.

According to authorities, the Metrobus is helping to cut CO2 emissions in the city by 122,000 tons per year.

Argentina's capital won the 2014 award for the extension of its bus rapid transit system, which carries hundreds of thousands of people per day, as well as the introduction of dedicated bus lanes on the world famous Avenida de 9 Julio.

According to authorities, this new corridor has halved journey times for 200,000 commuters daily, helping to cut CO2 emissions by 5,612 tonnes annually.

This year saw an unprecedented three cities, all from Brazil, win the award, "giving credit to the scale and substance of Brazil's achievements in increasing mobility and enhancing quality of life in its major cities," according to the STA.

The cities were commended for a range of programs, from the pedestrianization of streets and introduction of 27km of bike lanes in Belo Horizonte (pictured) to Rio's Transcarioca, a 39km bus system which is used by 270,000 people every day.

Sao Paolo expanded its cycling routes in 2014 and is on track to have 400km of cycle lanes by 2015, according to the STA.

Brazil's most populous city is also home to 320km of dedicated bus lanes, helping increase the average speed of buses by 21 percent, according to the STA.