Across the world, cities are expanding rapidly in a race for space – with no clear finish line in sight. This is particularly true for African cities characterised by an ever-increasing, low-density urban footprint and growth shapes that are distinctly outwards-facing and often informal.
With the urgent need for global cities to achieve net zero, this urban sprawl presents a major challenge to decarbonisation. Driven by short-term goals, the rapid development of low-cost locations in suburban peripheries has paved the way for inefficient city layouts. These types of urban configurations are increasing embodied carbon levels in the built environment, while distancing households from more sustainable transport options and locking inhabitants into carbon intensive lifestyles.
To inform decision makers of their impact on city form and citizen lifestyles, Arup collaborated with the Green Building Council South Africa – and Divercity Urban Property Fund – on a research study that quantifies not only the carbon impacts of this kind of development but also the lock in impact on the inhabitants’ lifestyles. Using Johannesburg as a study area, this research provides an evidence base for the theory that more compact cities are not only less carbon intensive, but also offer improved socio-economic outcomes for their people.
Modelling the carbon lifecycles of buildings
To achieve the study’s research aims, our team used a detailed model to analyse the whole life carbon of affordable housing in four key areas – two located within Johannesburg’s urban core and two situated on its outer boundaries. The model accommodated all stages of a building’s lifecycle, including material procurement, construction, operational use, maintenance, and end of life.
Gathering the data needed for this model proved to be challenging as publicly available data was limited. To address this gap, Divercity Urban Property Fund – a private property developer in South Africa – provided reference data from its portfolio of rental properties. We then validated this data using information gained from our extensive network of building experts in the region. The study’s approach was also tested through engagement with external stakeholders, such as city officials, academics, and urban development research institutions.
Uncovering the carbon cost of urban inefficiencies
From the whole life carbon analysis of Johannesburg’s buildings, we identified that location plays an important role in their carbon impact. Across all the different building typologies that we assessed, we found that those in peripheral locations had a higher embodied carbon footprint. The peripheral buildings also had slightly higher operational carbon impacts owing to their larger unit sizes.
These results were consistent when looking at the construction and end of life stages of the city’s buildings. For these phases, the study revealed that peripheral buildings had higher carbon footprints due to their increased transportation distances.
Drawing on these findings, our study projected that the cumulative emissions gap between developing in the urban periphery, as opposed to building within the urban core, would be 224MtCO2e by 2050 – which is almost 10 times the annual total emissions of Johannesburg in 2016 (21 MtCO2e). As a result, our research not only shows that locating buildings in central urban areas will reduce their carbon intensity, but also sends a clear message that Johannesburg could be locked into a high carbon future if its current trend of urban sprawl continues.
The role of lifestyles in city-wide carbon consumption
To provide a holistic view of the carbon impacts of housing, we supplemented the lifecycle analysis of Johannesburg’s buildings with research that explored its inhabitants’ transport-related carbon impacts. We developed family personas to represent the contrasting lifestyles of the people living in our study’s four key locations. This persona mapping was completed by examining what these individuals do – and where they go – in their day-to-day lives, with a particular focus on what transport types they prefer and how far they would be willing to walk to use mass transit. With this information, our team was able to more accurately understand which transport method each persona was likely to prioritise.
While people in Johannesburg’s city centre can often enjoy short, walkable journeys to their workplaces, those living in suburban areas spend up to 30% of their income on commuting – significantly more than their centrally-located counterparts. Recognising this, our study indicates that implementing more mass transit infrastructure in these areas would not only reduce the city’s carbon consumption, but can also significantly improve the lives of low-income families.
With its flat and continuously expanding urban layout, the city's built environment is working against the net zero aims of its planners by encouraging carbon-intensive transport methods. Our study recommended that retrofit projects which develop tall buildings in Johannesburg’s central areas should be incentivised as part of the city’s decarbonisation plans.
Decarbonising cities on a global scale
While the carbon research project focused on Johannesburg, the same challenges that our research identified can be seen across the world. If left unanswered, this urban development pattern will become a major barrier to the decarbonisation of our global cities. By turning the study’s recommendations into action, we can help cities to contain the swell of their outer boundaries and create diversified spaces, enabling us to create more sustainable urban spaces for future generations.