Globally, there’s a rapidly growing trend towards urbanization and modernization. High-density human settlements in cities along with industrialization, construction of high buildings, vehicular emissions and numerous other human activities tend to create a high-temperature environment in the cities along with air pollution. This phenomenon is called urban heat island (UHI) which can be defined as relatively high temperatures in cities as compared to surrounding rural areas as urban form and materials store and trap heat.
According to the Environmental Protection Agency (2015), the annual air temperature of a city with 1 million people can be 1.
8–5.4 degrees Fahrenheit (1–3 degrees Celsius) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C). Elevated day and night time temperatures create an unpleasant and extreme environment to live and work in especially in congested urban areas. It is also unhealthy as it causes discomfort, heat stress, breathing difficulties and increased heat-wave mortality rates. Hence the need for strategies and planning to counter this issue in an effective and environment-friendly way.
The development of green infrastructure in urban areas can lead to the alleviation of urban high temperatures and its impact significantly. Green Infrastructure can be defined as a network of green spaces in or near urban areas that conserve the natural environment and provide associated benefits to the human population. Green infrastructure is a multiperspective approach with various benefits including improvement of air quality, increasing urban biodiversity, conservation of storm water, etc. however, in this paper, it is particularly seen as a heat mitigation strategy.
The development of green infrastructure attempts to reduce the impact of high temperatures and heat waves in metropolitan areas. This can be accomplished through green roofs, Green belts and plantation along sidewalks and in open and vacant spaces in a well-planned and practicable manner.
This paper highlights the potential and feasibility of developing green infrastructure to ameliorate the high temperatures and creating a friendly and comfortable environment in city areas as little is known about its efficiency to reduce temperatures when applied to cities. This paper aims to explore the following two questions:
This paper provides a brief descriptive analysis assessing the effectiveness of green infrastructure by reviewing different sources of literature and drawing a conclusion about whether green infrastructure successfully helps reduce the urban temperatures.
The urban heat island effect is the result of regional and global climate change. Increasing urbanization has led to the clearing of large areas of vegetation and being replaced by high buildings creating skyscrapers, and housing settlements. Building materials usually have high heat storage capacity and therefore store heat during day time and release it during night times, this along with incoming solar radiation during the daytime, vehicular emissions and various human activities contribute to heat in urban areas. Densely constructed urban areas present a specific geometry; with high buildings and lesser open spaces, the flow of heat is restricted both horizontally and vertically and thus, the heat is trapped. The trapped heat presents an unpleasant and unlivable condition to work and live in. While usage of air conditioners tends to lessen and protect from the UHI effect indoors, In the long run, heat from exhausts of air conditioners tends to enhance the UHI and create even hotter conditions outdoors to work in, eventually leading to increased power consumption as the UHI effect increases. Under these conditions, Green Infrastructure tends to be the most accurate and fruitful strategy. The concept of green infrastructure circles around the growing of plants and trees. Plants and trees help in temperature reduction. Multiple types of green infrastructures are found and each has different potential towards mitigation of urban heat.
These include the planting of trees, green facades and green roofs. Trees can be widely planted along the street walks, in the green belts, in parking areas and any vacant public spaces. Among all types of green infrastructures, trees seem to be the most effective in reducing the high temperatures A large body of recent research has shown that trees and other vegetation can help reduce the UHI effect through two major natural mechanisms: these include temperature reduction through shading of urban surfaces from solar radiation; and evapotranspiration which has a cooling and humidifying effect on the air (Akbari, Pomerantz, and Taha, 2001)
Plants absorb solar energy from the sun that is used to drive the process of photosynthesis. Extra heat gain from the sun is later dissipated through the processes of conduction, convection and transpiration. Transpiration is the most dominant of these which is characterized by the conversion of water to vapors by absorbing the latent heat and thus leaving a cooling effect on plants and the surrounding. Evapotranspiration refers to the combined cooling effect produced due to loss of latent heat both from plants and the surface of soil and buildings. While both shading and evapotranspiration help in temperature reduction. The shading effect of trees and plants seems to be the dominant one. Research shows that tree canopies are able to reduce the temperatures of the surfaces they shade by as much as 25°C (Akbari, Kurn, Bretz, & Hanford, 1997). T
ree cover blocks the incoming solar radiations thus reducing the transmittance and shading that area. In this way, they not only reduce surface temperatures but also limit solar absorption by building materials and other construction. Thus the tree canopies not only provide shade, but they also reduce the reflection of heat energy from buildings and ground surfaces e-g. Roads. Green facades and Green roofs work by the principle of evapotranspiration only but they also serve as a physical barrier; by shading the buildings and therefore reducing the amount of heat energy reflected and absorbed by building walls. Green roof temperatures can be 30–40°F lower than those of conventional roofs and can reduce city-wide ambient temperatures by up to 5°F (Santamouris, 2014). Along with mitigating the urban heat island, these can help in improving the thermal energy performance of the buildings and hence reducing the demand for energy supply for cooling systems. Middel, Chhetri, and Quay (2015) suggested that increasing a city’s tree canopy cover from 10 to 25% could potentially offset climate change. Thus, the green infrastructure has a notable potential towards mitigating the UHI. Despite, Its benefits, the development of green infrastructure has been slow in the majority of the cities around the world.
There have been numerous factors hindering its adoption. Its implementation faces many challenges, including involving various stakeholders, overcoming institutional and technical barriers, resolving conflicts of using high-demand open space and subsoil, and the lack of strategic information for its realization (Connop, Vandergert, Eisenberg, Collier, Nash, Clough, and Newport,2016). Many nations in the world have a lack of resources including financial, human etc. and many nations do not regard the investment in the environment important enough to focus on. As a consequence, financial barriers along with lack of funding and unwillingness contribute towards uncertainty and risk towards developing green infrastructure. Moreover, a fixed and limited amount of money and resources dedicated to green infrastructure development for today and the future restricts its development on a large and extensive scale. In many areas, the green infrastructure is still considered a mere concept rather than being implemented in reality to counter this issue, thus a lack of awareness and knowledge about the importance of climate change and protective strategies limit the undertaking of green infrastructure initiatives.
The main confusion among planners and policymakers regarding what actually constitutes Green Infrastructure can thwart the present and future actions on climate change adaptation. Moreover, the mixed system of land ownership by public and private owners creates a barrier and hinders the implementation of a single policy to act and hence indirectly provide resistance to green infrastructure development. Due to high initial costs of implication and maintenance of green infrastructure, it looks like a cost-prohibitive strategy to invest in. Potential private developers often refuse to invest in local green infrastructure projects when there is a high and unaffordable level of risk involved due to property disputes and the lack of success returns on investment. (Ferguson 2017). Another common barrier is a restricted and fixed pattern of thinking and decision making largely focusing on the ideas and strategies that have immediate and short time positive outcomes thus leading towards rigid planning and policies of the government and related administrative bodies. Failure to develop a strategic and efficient plan to develop green infrastructure in a way to get the maximum benefit with minimal input puts its real potential at a stake. With a limited focus and allotment of money and technology, the development of green infrastructure in the majority of areas is quite slow in comparison to the urgent need to develop it as a scheme to counter a major environmental issue in metropolitan areas.
With limited interest, lack of strategic planning and technical skills, conflict of land use to develop and maintain the green infrastructure, the scope of Green Infrastructure, in reality, remains limited and unexplored. Discussion: Various challenges including resources, technology and policies seem to contribute towards a relatively small and slow rate of development and implementation of green infrastructure in city areas. Its importance is largely ignored. However, with the swiftly changing climate, day by day both globally and at local levels, it is really important to consider the numerous impacts it will have on us. While we can’t completely stop and prohibit the causes and factors behind this changing climate, we can draw out plans and work on strategies that are reasonable and sustainable both in their implementation and their outcomes and impact. With increasing UHI effects it will become difficult to provide a healthy and comfortable environment for the citizens. Green Infrastructure is a real-time strategy with effective and notable results as we have seen in detail if developed in a proper and well-organized way. A greater focus on the environment and understanding the true picture of this strategy can help us reduce and offset the impacts of one serious environmental issue in cities.
Managing and mitigating heat stress in high density residential urban areas will become increasingly important keeping in view the negative effects of UHI under today’s climate change. Various evidences from literature in this paper prove that green infrastructure has a large potential to reduce the urban heat island leading to better personal and environmental health conditions in city areas. An appropriate percentage cover of various Green Infrastructure types including trees, green roofs and green facades is required to notably lessen the air and surface temperatures. Meanwhile there are numerous barriers associated when it comes to implement it in reality. Further research can be based on urban planning which may include studying its overall effectiveness at city scale when developed only in specific heat spots in a city.