March 1, 2021

I, Science

The science magazine of Imperial College

In the first part of our series on green spaces, we look at how the world's megacities are getting ahead in the sustainability game.

cardboard cutout of city on grass backgroundEverybody loves green spaces; they are good for our health, our communities, and the environment. Until recently, they have never been associated with megacities (those with over ten million inhabitants). The UN’s 2014 revision on global populations confirmed almost 30 megacities worldwide. This number continues to rise, with Asia expected to host 28 alone within the next decade. Overshadowed by the pressures of climate change, government think tanks are placing more value on green spaces. Considering two thirds of the world’s population will live in cities by 2050, how will they cope with maintaining, let alone increasing, the already diminishing green space? A challenge awaits us, and it is quickly becoming apparent that it will take more than just creative flair to find ways of packing green space into cities already bursting at the seams.

 New York's 'High Line'
New York’s ‘High Line’

The world’s megacities are paving the way, in some cases literally. Inspired by Paris’ ‘Promenade Plantée’, a tree-lined walkway built in 1933, New York’s feted ‘High Line’ in Lower Manhattan opened to the public in 2009. ‘The High Line’ is the successful brainchild of Joshua David and Robert Hammond, who campaigned for the re-purposing of 1.45 miles of derelict rail track 25 feet above Manhattan into a lush green public walkway. The park has reanimated the district of Chelsea, attracting five million visitors a year and increasing local revenue. It supports over 210 species of plants and functions as a ‘green roof’, reducing both rainwater run-off and solar heat gains during hot periods.

In many ways, New York’s green space integration has been a great success. A major problem cited in initiatives such as these, however, is ‘environmental gentrification’: rising property values in the wake of a large-scale urban greening project. In the case of the ‘High Line’, this phenomenon has led to many small-scale businesses and middle-income residents forced out by increasing property prices.

Our London equivalent is also attracting some controversy. With a lot of public money at stake, Thomas Heatherwick’s ‘London Garden Bridge’, a proposed £175 million pedestrian bridge from London’s Southbank to Temple, has been subject to a number of false starts. The project initially garnered enthusiasm but is now losing political and public support as the suggested social and environmental benefits are called into question.  We are left wondering whether large scale projects such as these are really addressing the problem of green space in the most sustainable and effective way possible.

What, then, is the solution? We know that green spaces bring benefits to our cities: better air quality, more carbon capture, more biodiversity, increased physical and mental health and better social cohesion to name just a few. We live in a city with 35,000 acres of public green space- almost 40% of its surface area – making London one of the greenest cities in the world. This is a good starting point for sure, but the battle is not yet won, and it draws into stark light the urgent need for expansion of green spaces into the world’s other megacities. In the planet’s most populous city, Shanghai, less than 3% of its surface area comprised of green space in 2013.  Are large-scale, one-off models the right way to introduce green spaces to these densely populated cities?

Singapore's 'Park Royal Hotel'
Singapore’s ‘Park Royal Hotel’

Singapore is one of the few countries taking a different path. Its government recently agreed to a comprehensive program promoting ‘rooftop greening’. Green roofs are based on waterproof membranes that capture water for irrigation, allow drainage, support the growing medium and resist root invasion. The Park Royal Hotel and the Nanyang Technological University use green roofs to create open space, insulate the buildings and cool the surrounding area. There are more green roofs in the pipeline. In some cities, including Portland in the US, green roof installation is motivated by financial incentives. In Germany, Switzerland and Austria, those with a suitable pitch on their roof are legally required to grow plant life on them.

In the future, we may look towards both green space integration and green roofing to meet the needs of an increasingly urbanised population. Large-scale projects will continue to attract the attention of both the public and policy makers – but developing local initiatives and small-scale projects within communities will best maintain momentum in our quest for urban greenery.

Sophie Walsh is studying for an MSc in Science Communication

Next week’s Green Cities: Erin Frick explores the benefits of sustaining urban wildlife habitats.

Images: Animation, vanatchanan; ‘High Line’, Sean Pavone; ‘Park Royal’, Ronnie Chua.