Environmental Implications of Urban Sprawl: The Case of Urban Heat Islands

In an era of rapid urbanisation and industrialisation, the discussion around climate change is becoming ever more pertinent to the design of cities. Urban populations continue to rise and with that, cities are expanding in a phenomenon known as urban sprawl. As city living becomes increasingly popular and more expensive, low-cost, single-use developments are becoming more favourable, often leading to uncontrolled urban growth. This can lead to severe environmental consequences as urban temperatures rise and waste emissions continue to pollute the atmosphere. The intensification of urban temperatures is a phenomenon known as urban heat islands, and is a harmful by-product of urban sprawl. Understanding the relationship between the spatial organisation, energy use and CO2 emissions of cities is essential in order to implement effective mitigation and adaptation techniques to reduce the adverse effects of urban sprawl on the environment.

Urban sprawl is a direct contributor to the manifestation of UHI. Definitions of urban sprawl can vary and largely depending on context and perspective. Most commonly it is referred to as the low-density expansion of large urban areas. As cities grow, they can excessively encroach surrounding agricultural land (Brueckner 2000) and consequently have serious effects on the environment, such as water and air pollution, loss of farmland and ecological effects on local biodiversity (Gordon & Richardson 2000). UHI can a by-product of urban sprawl and is commonly defined as the temperature difference between a city and its immediate surrounding rural areas (Martin-Vide et al 2015). It must be recognised that urban sprawl also has important economic, political and social implications, however this discussion is around its environmental impacts that can be visualised through the immediate effects of UHI.

Urban sprawl can have climatic consequences regardless of climate zone, population size and population growth. Studies have been done comparing compact and sprawled metropolitan regions, exploring the relationship between urban development and extreme heat events in the U.S. It was found that in sprawling cities the rate of extreme heat events was double the rate than those in compact regions (Stone et al 2010, Gaffen & Ross 1998). Similarly, Malaysia during 1999 – 2007 experienced a period of urban sprawl showing an increase in built-up areas (109%) and a decrease in barren land (78%) and forested land (17%). Built up areas showed an increase in land surface temperature by almost 1°C (Squires 2002). The excessive expansion of a city is associated with low connectivity, low-density developments leading to a higher reliance on vehicles. Patterns that all correlate with enhanced surface temperatures and accentuated UHI when compared to compact regions that normally promote mix-use neighbourhoods, the preservation of green spaces and also favour the use of alternative modes of transport (Squires 2002). Sprawling cities with poor design and land-use management will only contribute more to the effects of heat islands.

The spatial organisation of a city plays a critical role in the management of its energy and thus, has a direct effect on the intensity of UHI. Cities are dynamic, energy is constantly travelling in many forms and physical states (Chrysoulakis & Grimmond 2016). Through anthropogenic heat fluxes, vehicles, humans and buildings release energy, the latter contributing to 80% of urban heat emissions (Iamarino et al 2012). Energy in the form of heat, as well as CO2 waste emissions from vehicles and industrial processes can become trapped due to poor building configuration (Oke 1982) thus creating greenhouse-like conditions and significantly increasing outdoor urban temperatures. The adaptation of green spaces and a larger tree canopy area in a city can help dissipate heat energy and CO2 emissions through evapotranspiration (Memon et al. 2007). At a local scale, UHI can have serious health and liveability implications, adversely contributing greatly to higher urban temperatures and the frequency of extreme heat events at a global scale (Changnon et al 1996). Undoubtedly, the incorporation of mitigation and adaptation techniques is critical in the future of urban design for the preservation of the environment as well as the wellbeing of its occupants.

Zurich, Switzerland is a tangible example of how cities can grow in a sustainable and resilient manner. Human population is expected to exceed 7 billion by 2050 (United Nations 2010) and cities are expected to grow. With this, it is integral that governments and designers intend to address issues that are detrimental to the environment. Long term mitigation methods such as the use of compact, high-density design models, the increased use of public transport, bicycles and walking can make a city less vehicle-dependent. Similarly, the use of reflective and cool materials, preservation of green spaces and incorporation of green roofs can act to reduce surface temperatures, increase outdoor thermal comfort and wellbeing (Younger et al. 2008). The positive effects of mitigation initiatives can be seen in the recent history of Zurich’s infrastructure legislations. Since 1942, sensitive regional areas have been protected and the 1959 Construction Act distinguished building zones from non-building zones. Furthermore, in 1979 land-use restrictions were introduced to limit settlement expansion and avoid sprawl (Nazarnia et al. 2016). More recently, they have motioned for effective waste and recycling methods, as well as big investments in public transport and the inclusion of green roofs in all new housing developments. Currently, as part of the 202020 initiatives, the aim is to cut fossil fuel usage by 20% by 2020, (Rérat et al 2010). Zurich is at the forefront of sustainable urban design, and governments worldwide should look to implement similar models to manage urban sprawl and reduce its harmful environmental effects.

Ultimately, as humanity moves towards an urbanised future, it is critical that the design of cities and the preservation of the environment grow in a symbiotic manner. Anthropogenic heat fluxes, low albedo surface materials and low vegetation in cities all contribute to the intensity of urban heat islands, a harmful by-product of urban sprawl. Through efficient, long-term solutions, governments and designers alike have the opportunity to reduce the effects of urban sprawl both locally and globally.