The Urban Heat Island effect is a major climatological phenomenon that impacts cities and communities all over the world. As a result, there are many mitigation methods and strategies that have been devised in order to combat its effects.
Mitigation of Urban Heat Islands is a highly researched topic and the methods/techniques developed have the objective of balancing the thermal budget of cities. This is achieved by increasing thermal losses and limiting the corresponding gains.
Techniques that aim to increase urban albedo and expand the green spaces in urban areas are the most important of the techniques being proposed. There are many mitigation methods and practices that have been explored and tested in an effort to mitigate the urban heat island effect and to minimize its impact. Some of these methods include cool/reflective/pervious pavements, green spaces such as parks, green roofs, and buildings, and thermochromic materials among others.
These methods focus on either cooling the area or reflecting the radiation. The implementation of high-albedo surfaces to increase the reflection of incoming solar radiation, and the planting of urban trees are inexpensive methods that can have a positive impact on the reduction of summer temperatures.
These methods have both direct (immediate benefits) and indirect (benefits seen with widespread deployment) effects. For example, the direct effect of planting urban trees or fitting buildings with reflective materials is to reduce the cooling strain/requirements on the building and to reduce the energy balance around the urban areas where the trees are planted. The indirect effect of these measures is the alteration/modification of the energy balance of the entire city producing widespread changes in climate throughout the city.
Green spaces
The use of green spaces, roofs and buildings are effective methods of mitigating the urban heat island effect. The presence of green spaces in urban areas helps to minimise the effects of urban warming, such as increased photochemical pollution, and does this through the creation of a cooling effect and the provision of a fresh air supply. In addition, vegetation contributes to the mitigation and adaptation of urban areas to the negative consequences expected due to a warming climate by a reduction in energy consumption due to lower temperatures, as well as increased CO2 uptake by trees.
Vegetation works in two major ways to mitigate urban heat islands: through shading and evapotranspiration. Shading reduces the amount of incoming radiation absorbed and stored by otherwise heat absorbing surfaces such as pavements. Evapotranspiration reduces the amount of sensible heat as it transforms energy into latent heat. Trees play a major role in this function and the most suitable, and probable, place to plant them is in parks. Because of these effects, greenery plays a significant role in the alleviation and mitigation of the urban heat island phenomenon by reducing urban temperatures and increasing humidity. It has, therefore, become an important mitigation tool and is regarded as natural alternatives and resources for combating urban heat in city planning.
Urban parks
Urban parks are green spaces located in urbanised areas and typically have lower surface and air temperatures resulting in the formation of a park cool island. Urban parks help to reduce surface temperatures, cool surfaces, and reduce energy consumption. They can also reduce air temperature simply by intercepting incoming solar radiation preventing, not only the heating of the surface, but the heating of the air as well.
Urban parks also have added benefits such as increased spaces for leisure and exercise which contribute to improving the wellbeing and health of the city’s residents, helps reduce air pollution which also improves health, and acts as urban biodiversity hotspots.
Trees
The planting of urban trees is also an effective method of Urban Heat Island mitigation. Trees have both direct and indirect effects: the shading of buildings (direct) and ambient cooling (indirect). The shade produced by trees intercepts incoming solar radiation before it warms a building while the evapotranspiration of the trees cools the air (ambient cooling).
Trees also have the added benefits of decreasing wind speed under their canopy, shielding buildings from cold winter breezes, reducing building air conditioning, and lowering air temperature as well as improving urban air quality in urban areas. Additionally, the species of tree/plant present in the park, plays a significant role in the effectiveness of the cooling effect of the park (how effective it is at decreasing the temperatures of the surrounding areas associated with the urban heat island effect).
Impact on pollution
Urban pollution affects the composition of the atmosphere in many ways. It provides an abundance of condensation nuclei, modifies the thermal properties of the atmosphere, and cuts down the passage of sunlight into cities. This pollution can come in the form of aerosols and gases with each having different chemicals and sources associated with them. The urban heat island effect elevates the rates emission of greenhouse gases and air pollutants. These emissions result in an increase, and exacerbation of, the urban heat island effect creating a positive feedback loop.
Increased air pollution and emissions are dangerous to human health and cause an increase in the urban heat island phenomenon. Parks, therefore, are a significant tool in reducing the amount of air pollution present in cities which helps to mitigate the urban heat island effect and has the added benefit of improving the health of the city’s residents in the process.
Final thoughts
The urban heat island effect is an almost ubiquitous phenomenon and results in cities being significantly warmer than their surrounding rural areas. This phenomenon is widespread and affects a large number of people. There are, however, ways to mitigate its effects and reduce the heat in cities. The best of these methods includes planting trees, implementing the use of urban parks, and using technologies that increases the albedo of cities such as green roofs and reflective/pervious pavements.
Cities around the world have been implementing methods to combat this phenomenon and will continue to come up with new and innovative methods to tackle it in the future.
