Key message 2 – Urban density enables reduction in per capita emissions from infrastructure and services.
The viability of cities as climate solutions is in part due to the benefits of proximity—many people in one place makes the distribution of goods and services much easier and more cost-effective, with interconnected environmental benefits. While there is variation, higher population densities are correlated with lower per capita GHG emissions (Gurney et al. 2022). Within that, city centres contribute less GHG emissions per person than low-density suburbs (Jones and Kammen 2014). Estimates suggest that compact urban development can reduce emissions by almost 25% compared to current trends on urban sprawl (Creutzig et al. 2015).
Poorly managed urban sprawl locks cities into high levels of energy consumption and emissions. For example, a study of 44 cities found that those with higher density facilitated the increased use of public and non-motorised transport options. This, in turn, impacted behaviours of citizens and reduced private car ownership (Kenworthy and Newman 2015). With more shared transport, emissions and, consequently, pollution is lowered through the reduction of particle matter in the air. This has been evidenced globally, from Lagos, Nigeria (Otunola et al. 2019) to Leipzig, Germany (Bauernschuster et al. 2017). This also leads to economic benefits—for example, reducing losses to GDP from the cost of congestion, currently at 4% in Kampala (Baertsch 2020), 5% in Jakarta, 8% in São Paulo, and up to 15% in Beijing (New Climate Economy 2018).
Density also means building higher, which is good for material use and related GHG emissions. For example, mid-rise buildings in India use 30% less material per square feet of construction compared to single family homes (Nagpure et al. 2018). It also leaves more room available for green space, both within the city and at the urban edge. If well-planned, this can ensure environmentally sensitive areas are protected, and natural capital preserved. This, in turn contributes to important ecosystem services such as reducing urban heat islands, improved storm water drainage, and flood plain management (see Delbridge and Harman (2022) for further discussion). Relatedly, density enables the sharing of private and public goods and services with other citizens, including waste management, water and sanitation, and district heating.
All of these sustainability benefits are also achieved at a lower cost to government. Evidence shows that higher population density leads to more cost-effective public goods and services, with economies of scale aiding in their delivery (IPCC 2022). For example, when comparing large cities to rural areas, installation of private household water costs approximately 10% per person in cities; power (renewable and grid) approximately 20%; and roads 50%—or half per person (Foster and Briceño-Garmendia 2009).
Higher density also divides the cost of land in cities among more people, lowering the cost of safe and resilient housing (Collier et al. 2017). This, together with clearly defined and enforced land-use regulations, may aid in the reduction of unplanned settlements built on land exposed to environmental pressures, such as floods and landslides. Proactive urban planning and appropriate land-use regulation is therefore vital for a low-emission and resilient city.