Population growth, urbanization, and climate change are the major trends shaping the future of cities. The UN predicts that the world's population will grow from 7.7 billion in 2019 to 8.5 billion in 2030, 9.7 billion in 2050, and 10.9 billion by 2100. Over 55 % of the world’s population already reside in urban areas and the UN expects us to add another 2.5 billion to the world’s urban population by 2050. And, according to UN Habitat, cities consume 78% of the world's energy and produce more than 60% of greenhouse gas emissions, while accounting for less than 2% of the Earth's surface. For a sustainable future we must find a solution that allows our cities to grow, remain functional, and significantly reduce their impact on the environment.
“As [population and urbanization] figures continue to climb and ever more people flock to metropolitan areas in the hope of a better life, the big question is: how do we fit everyone in?” ask the University of Edinburgh’s Ruth Saint and Francesco Pomponi in a review of their recent study. “It is the job of city developers and urban planners to figure out how to build or adapt urban environments to accommodate the living and working needs of this rapidly expanding population.”
The key question on the minds of city-focused politicians, planners, and developers should be, what type of urban growth is most sustainable? The 2021 study ‘Decoupling density from tallness in analysing the life cycle greenhouse gas emissions of cities’, led by Associate Professor of Sustainability Science Francesco Pomponi, posed two questions; how whole life-cycle carbon changes based on the building height and density, and how this can be applied to different urban planning scenarios.
“There is a popular belief that taller, more densely packed skyscrapers are the way forward, because they optimise the use of space and house more people per square metre and limit urban sprawl,” said Pomponi and Saint. “But given the global commitments to emissions-reduction targets and mitigating climate change, is this the most sustainable solution from a carbon-reduction perspective?”
The team developed four different urban scenarios based on data from real buildings: high-density, high-rise (HDHR); low-density, high-rise (LDHR); high-density, low-rise (HDLR); and low-density, low-rise (LDLR). Based on data from real buildings, they split the building stock into five main categories: non-domestic low-rise (NDLR); non-domestic high-rise (NDHR); domestic low-rise (DLR); domestic high-rise (DHR); and terraced/house. Then incorporated data on building height, number of storeys, physical footprint, facade material, and neighbouring constraints, as well as average street width, average distance between buildings, and proximity to green spaces.
Critically, the study applied whole life-cycle carbon metrics to assess the sustainability of these various building height and density scenarios. While still emerging as a sustainability tool, whole life-cycle carbon includes operational carbon and the embodied energy used during the extraction, production, transport, and manufacture of raw materials, as well as the carbon consumed during the maintenance, refurbishment, demolition, or replacement of the building. This hidden “embodied energy” can represent 11%-33% of carbon cost for green building projects such as Passive House designs, to as much as 74%-100% for near-zero energy buildings that have only really focused on operational carbon, in line with regulation.
“This aspect is often overlooked, especially in building design, where operational efficiency is always to the fore,” said Saint and Pomponi. “The argument for cutting carbon at the design stage has been made by numerous researchers, and it is gaining traction with leading international organisations such as the World Green Building Council. But it’s still something that is largely disregarded, mainly because embodied impact assessment is voluntary, and there is no legislation concerning its inclusion. It must be advocated for if we are to reach our 2050 emissions targets.”
When planning for a sustainable future embodied carbon becomes invaluable, because while our obsession with operational carbon tells us that our modern smart skyscrapers are leading the building efficiency race, embodied carbon reveals a very different story. As the height of buildings reduce the materials used to build them changes, tall buildings typically rely on large amounts of carbon-intensive concrete, while low-rise buildings tend to use lower-carbon brick, cement, and gypsum. The study showed that concrete currently accounts for 80% of the total carbon cost of building materials (brick 10%, cement 7%, gypsum 3%) underlining the scale of the problem. Using less concrete is more sustainable, but using less concrete means fewer high-rise buildings.
“Our findings show that high-density low-rise cities, such as Paris, are more environmentally friendly than high-density high-rise cities, such as New York. Looking at the fixed population scenarios, when moving from a high-density low-rise to a high-density high-rise urban environment, the average increase in whole life-cycle carbon emissions is 142%,” the research team reported.
Low-rise buildings are greener than high-rise buildings, whilst high density will be required to reduce total land area, making HDLR urban growth the most sustainable. This conclusion means the US will find itself in the worst urban sustainability scenario with mostly HDHR city centers and sprawling LDLR suburbs. Asia-Pacific cities tend to be a mix of HDHR and HDLR, giving them moderate sustainability. European cities are typically HDLR, the most sustainable category, which should drive them to curb modern high-rise trends and embrace their already sustainable urban planning culture.
“It’s time for urban planners to start embedding this new understanding of the whole carbon life-cycle of a building, balancing the impact of urban density and height while accommodating expanding populations,” Pomponi concluded. “To achieve urban sustainability the world will need more Parises and fewer Manhattans.”
This short and compact conclusion does not account for the inefficiency of Europe’s aging buildings, nor the higher rate of smart tech adoption in tall buildings, nor renewable energy generation variables, nor the transport demands of lower-rise cities. However, as we strive to find the most efficient and sustainable approaches to overcome climate change and build a more sustainable future, we must aim for the most carbon efficient way to build our buildings and cities — high-density, low-rise, renewably-powered, and smart.