Hubs of culture, politics and finance, the cities many of us call home can, at times, be hard to live in.
The challenges of an urban environment often include overcrowding, a high cost of living and air pollution. The latter is a serious issue that can affect us all: according to the World Health Organization, it's estimated that air pollution kills 7 million people each year, with 9 out of 10 people breathing air which contains "high levels of pollutants."
One solution to help tackle the problem of air pollution could be increasing the number of trees and green spaces within urban areas, according to experts. As well as being aesthetically pleasing – the sight of branches covered in blossom can lift even the gloomiest of moods — trees can offer a range of benefits.
The Food and Agriculture Organization (FAO) of the UN, for instance, has stated that one tree can absorb as much as 150 kilograms of carbon dioxide annually. It has also described "large urban trees" as being "excellent filters for urban pollutants and fine particulates."
A number of cities are now making concerted efforts to improve green spaces and boost the number of trees on their streets and in parks.
Take the Australian city of Melbourne. Authorities there say the city's "urban forest" amounts to more than 75,000 trees and they want to boost canopy cover from 22% to 40% by the year 2040, planting over 3,000 trees annually to achieve their goal. Other cities have similar targets: authorities in Milan, for instance, recently announced a target of planting 3 million trees by the year 2030.
"A city like Melbourne realized that trees are an essential way of keeping your city liveable and also adapting it to climate change," Cecil Konijnendijk, from the University of British Columbia's Faculty of Forestry, told CNBC's Sustainable Energy. "So they really integrated urban forestry into their development."
Konijnendijk went on to list Singapore as another place where trees were "taken very seriously." The south-east Asian city-state is aiming to plant 1 million trees over a 10-year period.
In addition to absorbing carbon dioxide — an important step in battling climate change — trees can also provide other benefits that may not be immediately obvious.
"The main thing that trees do actually, they help cool the city. So by cooling the city we know there is less need, for example, for air conditioning costs," Konijnendijk added.
Indeed, according to the FAO, when it comes to cooling urban areas, strategically planting trees can cool air by between 2 and 8 degrees Celsius (35.6 to 46.4 Fahrenheit).
While the aim to plant huge amounts of trees might be an admirable one, some parts of the world face challenges when it comes to greening urban spaces.
"They don't have the planning capacity, they don't have the money, the capacity to really work on this," he added.
"So we see, for example in Africa and other parts of the world, maybe Latin America, cities are growing very rapidly. India is adding another 200-300 million urban dwellers… How do you get your cities ready for that? And how do you integrate trees in that? That's a big challenge."