Thanks to their huge cultural and commercial significance, what takes place in cities has an impact on the wider world.
According to the United Nations, cities may occupy only two percent of our planet's land, but nearly half the world's population lives in them and they are responsible for 75 percent of global carbon emissions and 60-80 percent of energy consumption.
Greening a city with trees and parks is one way to mitigate its environmental impact and improve people's quality of life. In Singapore, Gardens by the Bay is one example of how green space can co-exist with densely packed urban communities.
Fully opened in 2012, the project was years in the making.
"Gardens by The Bay started out in 2006 when we carried out a master plan competition, and thereafter physical construction commenced in 2007," Andy Kwek, Director of Conservatory Operations, told CNBC's Sustainable Energy.
"Within the span of five years, we did a lot of studies together with our consultants to put in a lot of sustainable strategies that would be community friendly as well as sustainable for (an) extended period of time," Kwek added.
Singapore, which has just marked the 50th anniversary of its independence, has set itself ambitious targets when it comes to sustainability.
The Sustainable Singapore Blueprint 2015 has set a range of goals for 2030, including 90 percent of houses being within a 10 minute walk of a park, 700 kilometers of cycle paths and a national recycling rate of 70 percent.
A key feature of the gardens are its Supertrees. Described as 'vertical gardens', they stand between 25 and 50 meters tall. According to Gardens by the Bay, 11 of the trees have been "embedded with environmentally sustainable functions like photovoltaic cells to harvest solar energy."
"There are 18 Supertrees in total in the gardens, and they have basically two functions," Kwek said.
"The first function is to create an aesthetic for the gardens itself, replicating the dominating trees of the rainforest. And the second function of it would be able to hold sustainable strategies that we adopt within the gardens."
In addition to features such as the photovoltaic cells, the Supertrees are also home to more than 162,000 plants, "comprising more than 200 species and varieties of bromeliads, orchids, ferns and tropical flowering climbers."