Most people who walk through the tall canyon-like buildings of large cities may never stop to consider the impact of buildings on their direct environment. However, the shape of the urban landscape subtly influences a city’s functionality and the inhabitants’ day to day activities. The combination of trapped sunlight, heat expelled or reflected from buildings, and heat from transport creates significantly higher temperatures in urban centres than in nearby suburban or rural areas, producing what scientists have called Urban Heat Islands (UHI).
A number of factors contribute to UHIs. Urbanization processes have led to the replacement of vegetation and green space with dry, impervious surfaces such as roofs, footpaths, roads and parking lots. The use of materials that have a lower albedo (the fraction of solar energy reflected from the Earth back into space) has resulted in less solar reflection which in turn increases the surface temperatures of buildings, roads and pavements. The production of ‘waste heat’ – an outcome caused by the high density of people partaking in heat burning activities (transportation, factory work, day to day functioning), the construction of developments and the use of commercial and domestic air conditioners – also results in higher air and surface temperatures within a city. In some instances, the daytime air temperature can be measured at around 5-6C higher than surrounding rural areas in the summer months, an effect that also filters into higher night time temperatures. Whilst the urban heat island phenomenon impacts all major cities to some extent. Tropical and humid locations experience the most harmful effects on a year-round basis, due to their constantly high temperatures; however, inhabitants of cities with strong seasonal variations can be unprepared for heatwaves, leading to potentially serious health problems for vulnerable sections of the population.
According to the United States Environmental Protection Agency, some of these key effects of UHIs include:
- Increased energy consumption: Higher temperatures in summer lead to an increase in energy demand for cooling – potentially up to 5-10% of the energy demand in summer is used to compensate for the heat island effect. Brown outs and blackouts have been reported as a result of increased energy demand, whilst the waste heat produced by cooling devices (especially in heavily condensed skyscraper zones) only further exacerbates the heat island effect, causing an inescapable loop.
- Poorer air quality and more greenhouse gases: As a result of increased energy demands, power plants are producing a greater number of air pollutants and greenhouse gas emissions. Elevated air temperatures also lead to an increase in the rate of ground-level ozone formation.
- Negative impact on human health and comfort: Higher temperatures in urban centres, reduced nighttime cooling and higher levels of air pollution can cause human health issues, including general discomfort, respiratory difficulties, heat cramps, exhaustion, not fatal heat stroke and heat related mortality.
- Thermal pollution of water: Surface urban heat islands degrade water quality, mainly via thermal pollution. Pavements and rooftop surfaces transfer excess heat, if higher than 27-50C, to stormwater runoffs, which in turn drain into storm sewers and raise water temperatures as it is released into streams, rivers, ponds and lakes, leading to higher evaporation and contributing to water shortages.
Heat islands are not necessarily fully reversible, especially when taking into consideration factors such as weather patterns, climate, geography and topography. However, communities can implement reduction strategies that aim to minimise heat gain and the subsequent negative effects of such a gain. Urban heat island mitigation strategies include:
Vegetation: Trees, plants and greenery located on or around buildings can dramatically lower surface and air temperatures by providing shade, absorbing radiation and evapotranspiration. Vegetation placed on the sunniest side of buildings ensures the façade and interior of the building remains cooler, reducing the need to use air conditioners and ultimately minimising energy consumption and waste heat. The shading of hard surface spaces such as car parks, roads and pavements with vegetation also assists in deterring heat absorption and as a result lowers the urban heat island effect.
Cool Roofs and Materials: Installing building materials and roofs that reduce solar heat gain or reflect radiation can contribute to mitigating the effect of the urban heat islands. Energy efficient materials and light coloured paints on roofs and facades can both reflect solar radiation and transfer less heat into the interior of the building, as well as into the air space around the building (for example a white roof can reflect 80% of sunlight compared to a grey roof which only reflects 20%). With less heat transfer, the need for cooling also subsides, ensuring lower energy use and more consistent comfort for occupants.
Cool Pavements: In cities constructed in the United States, pavements and roads comprise about 1/3 of urban surfaces, thus absorbing up to 80-95% of solar radiation and in turn warming surface, air and stormwater temperatures. The implementation of ‘cool paving’ assists in reflecting solar energy, enhancing water evaporation and maintaining cooler surfaces as compared to conventional pavements. Cool paving is still a new form of technology and will no doubt advance in the coming years.
Cool Cars: Waste heat produced by cars in urban spaces can also be alleviated through the transitioning from dark to light colours. The use of light colours reflects sunlight from a car’s surface, reducing the need for air conditioning and subsequently the level of emissions released from the car.
Many city governments around the world are beginning to implement such strategies. For example, the Cool Cities Network (part of the broader C40 city networking organisation) and the Global Cool Cities Alliance have been established to share experiences and expertise on cool surface programmes. Such groups have explored both the technical specifications that need to be put in place to improve the performance of materials, and also how this is to be financed, regulated, and promoted.
The steady development of urban centres, particularly in countries which already have high average temperatures, only serves to intensify the effects of the urban heat island, potentially threatening the long term sustainability and thermal comfort levels of urban dwellers. The temptation to rely on air-conditioning to make cities liveable and comfortable leads planners and developers to turn their back on the green spaces that produce cooling effects.