Climate Change: Implications Of Urban Heat Island

Introduction

Migration of population to the cities is increasing rapidly worldwide because the birth rate is increasing, rural people tend to move to city in search of better living conditions, due to conflicts and limited amount of resources in rural areas. Urban population has experienced a substantial increase in the last 30 years and the latest estimate indicates that in 2011 it amounted to more than half of the total world population. The global percentage is projected to exceed 65 % of the total population by 2050. The growth of urban population has increasingly drawn the scientific community’s attention to urban climate and the effects of urbanization at different scales. The relative warmth of the urban areas with respect to the rural surroundings is undoubtedly the most prominent of these effects.

Climate change is being identified as a primary environmental concern across the globe for decades. Reasons for climate change come under two main categories. It could be due to natural disasters and anthropogenic activities. The climate change which is occurring due to natural reasons can be due to floods, earthquakes, volcano eruptions etc. the impact of this category is less compared to the anthropogenic activities since the frequency of occurrence is less.

Introduction to Climate Change

Climate change is one of the main reasons among natural disasters that cause economies of many countries to go down. According to UN Human Development Report (2013) statistics it has caused extreme poverty for over 3.1 billion people worldwide, especially in developing countries. Adaptation of the urban areas to climate change has been significantly decelerated with the combined impacts of global warming and urban heat. UNDESA (2015) has shown that the urban population will be around 70% of the world population by 2050. Simultaneously, the global climate is changing rapidly and as a result, urban areas are becoming the most critical part of the earth’s land surface that needs to be planned from the stage of design IPCC, 2014). There is an increasing need to manage environment and energy systems in a way that would support this.

Measurement and verification of the severity of climate change is generally done by measuring temperatures of air and ocean, melting of snow and glaciers and rise in sea level (IPCC, 2014). From the beginning of industrial era, influence of anthropogenic activities generating greenhouse gases that are directly influencing global warming has been significant. It has increased over decades and has come to an alarming rate by now and researches have shown that there is a complex nexus between built environment and climate change. Scientists have predicted on climate related extreme events to be taken place across the globe which will have influences on us both ecologically and sociologically. Traditionally urban development projects are planned assuming an environment that is unchanging. It is essential to redefine urban planning processes considering the dynamics of climate change especially in terms of urban heat. Even though the trends of climatic changes have been revealed in 5th IPCC assessment report there have not been any significant initiations on changing policies related to climate. Global emissions and associated global warming progresses imposing economic costs on economies of many countries.

Studies have commonly identified the effects of climatic change. Rising of sea level in coastal regions, extreme environmental effects on built environment, effects on health and usage of energy and availability of water are amongst them. IPCC (2007) highlights that the location of the city, area of the city and population density have effects on the vulnerability to climatic changes. Furthermore, under developed countries are highly vulnerable since they have extremely limited capacities to adapt to changes in climate. According to Wilbanks et al. (2007) climate change significance needs to be considered with measurements of frequency of occurrence and intensity of extreme events. Built environment which is identified as highly vulnerable for adverse effects of climate change has a complex inter relation with climate change.

UHI and Climate Change

Even though urbanization benefits human beings by enhancing living standards it has created negative consequences on society, economy and environment as well. Urban heat island (UHI) can be considered as a major one of them. The severity of the impacts of UHIs is increasing day by day with the increment of population moving to cities. According to United Nations (2014) 50% of the world population is living in cities. It is expected that by 2050, 65% of the global population would be living in the cities (Phelan et al., 2015). It is one of the main reasons for high number of researches done on UHI implications. Global changes in climate are mainly influenced by increments in surface air temperature, changes in precipitation patterns with time and depending on location and due to floods and droughts that are extreme events. According to studies done over decades, surface air temperature elevation has been identified as the reason with highest impact.

There are many parameters affecting the size and the characteristics of an urban heat island formed in a particular place. Researches have commonly identified the most important factors including precipitation, wind patterns, synoptic conditions in climate and cloud cover. When analyzing studies done in different cities of the world it is clear that the size of the urban heat islands is less when there are cyclonic climate patterns. Past researches done on the impact of humidity on formation of urban heat islands have shown that relative humidity and magnitude of the UHIs is negatively correlated; i.e. when the humidity is high magnitude of UHI is small. This happens because when the relative humidity is high the evaporation rate is high and it causes to reduce UHI effect. It is due to reduction in air and surface temperature as a result of cooling due to evaporation. Furthermore, when considering precipitation, when the precipitation is high in rural areas cooling rates will be less and it causes to reduce the intensity of urban heat island. On the other hand it is an accepted fact that when the wind speeds are low the magnitude of the urban heat islands is high. Inversely, when wind speeds are high it will modify the rates of cooling to reduce the effects of UHI. For UHI intensity to be at a considerable level the wind level should be in a certain range which is known as critical wind speed. It highly differs from place to place and the general range is from 1-5m/s (Morris et al., 2001). Intensity of the UHI will be high when the sky is clear. Infrared radiation from sun cannot escape when the sky is cloudy and the radiation is reflected back by the cloud cover which causes to break the thermal balance at the surface of the earth (Mohan et al., 2012). Furthermore, many researches have shown that there is a synergetic combination between wind speed and the cloud cover. It has been found that the UHI intensity increases when the wind speed is close to zero and the sky is cloudy. Some of the recent researches have studied the impact of see breeze in coastal cities. When the sea breeze is available it will blow cool air towards the land side and it causes to reduce the intensity of UHI.

When considering modifications in the atmosphere UHI can be considered as the one with highest significance. Currently it is spoken in many different platforms in different contexts, However it has been first identified in London by Luke Howard. Numerous studies have been done on the relationship between climate change and UHI and found that the climate change has a significant impact on formation and increasing the intensity of UHI. In a Chinese research it has been shown that UHI has a contribution of 30% on warm climate. Furthermore studies have shown that changes happen to the climate due to increase of green-house gas emission cause to worson the implications of UHI (Hoffmann et al., 2012). Currently it is affecting many cities across the globe in local level and globally as well. As a result of increasing UHI effects in cities urban areas are more likely to be affected by climate change.

According to Hoffmann and Schluenzen 2013 there are several factors that affect how a particular area is responding to climate change. They are, differences in cloud cover, wind speed and evapotranspiration. There is a significant difference in how urban areas are affected by these factors and how rural areas respond to those. It is basically due to differences in vegetation cover, difference in population density and amount and nature of built environment. Response for the climate change by the urban areas and rural areas is different due to differences in cloud cover, wind speed and evapotranspiration Low wind speeds and cloud covers promotes occurrence of stronger UHI. Because of the UHI effect, urban areas, compared to adjacent rural areas, are likely to be affected differently by climate change (which we define as changes in temperature, precipitation, evapotranspiration, cloudiness, wind speed and other variables as a result of anthropogenic release of greenhouse gases in accordance with the IPCC; IPCC 2014). Recent research has found that the UHI does not remain the same under climate change, and so the current UHI cannot be simply added to climate change projections.

Urban and rural areas may respond differently to climate change due to differences in cloud cover, wind speed, evapotranspiration and anthropogenic heat release. The UHI is strongly affected by wind speed and cloud cover, and stronger UHIs tend to occur under conditions of low wind speed and cloud cover (Oke 1982; Bonan 2008). Any changes to winds and clouds from climate change could therefore alter the frequency of high-intensity UHIs. Evapotranspiration is a key component of the UHI, and increases in soil dryness due to climate change and decreases in rural evapotranspiration could potentially decrease the UHI, as rural areas warm more than urban areas and the urban-rural temperature difference decreases (Oleson 2012). Changes to temperature from climate change will affect the amount of heating and cooling used by urban residents and the associated anthropogenic heat release, which would either increase or decrease the UHI depending on whether the reduction in cooling or increase in heating requirements was larger. Examining climate change without considering urban land use patterns excludes the interaction between climate change and the UHI, and could result in under-estimating future increases in urban temperatures, both mean and extreme values.

Urbanization reduces green space, increases impervious surfaces, and alters albedo and geometry compared to rural surfaces. It is now well established that reduced green space and increased impervious surfaces reduce the amount of evapotranspiration and latent heat flux in urban areas, partitioning more energy into sensible heat. Urban surfaces typically have a lower albedo than rural areas due to building materials and the in-canyon rflection of radiation. The geometry of urban areas also blocks outgoing radiation at night, reducing the rate of nocturnal cooling. Anthropogenic heat release which is the amount of heat released due to human activities such as construction of buildings and traffic contribute to increase the intensity of UHI. This becomes significant as urban areas are highly populated than the rural areas (Allen et al. 2011). Researches done by Pielke et al. (2011) have bring out how usage of land and vegetation cover affects climate in local and global level.

Increase in death toll in certain countries and health complications in cities are a result of combined effects of UHI and temperature extremes. It is a clear example on how effects of UHI are exacerbated by climate change. Koomen and Diogo 2015 have highlight in their research how a heat wave in Amsterdam caused a disaster on humans and all the living beings in the region. In that case there has been a difference of about 8 Celsius between urban areas and adjacent rural areas. Moreover, researchers have shown that there is a correlation between UHI and geometry of the cities. Growth of the cities will increase temperatures in the cities and the level of heat stress on the inhabitants (Arnfield 2003). Researches have shown that UHI does not remain the same with the climate change. Therefore it cannot be simply added to the projections of climate change. Evapotranspiration is one of the key components of UHI. Due to climate change dryness of the soil increases and evapotranspiration in rural areas may decrease. It will cause to reduce the effects of UHI as the urban-rural temperature difference reduces (Oleson 2012). Many studies have been done on the changes of energy usage in cities as a result of temperature variations due to climate change. It can cause to increase or decrease the intensity of UHI depending on the reduction in cooling and increment in heating. Furthermore, it is essential to study the patterns of land usage when examining climate change as it is important to consider the relationship between UHI and climate change. Otherwise it can cause in an under estimation of urban temperature increments in future. According to researches done in the field it can be concluded that combined effect of climate change and rate of urbanization has a huge impact on future temperature patterns and formation of UHI.

Mitigation Strategies

Mitigation and adaptation technologies have been introduced over past few decades to overcome the impacts of global warming. The methods that have been developed come under two broad categories as increasing evapotranspiration and increasing solar reflectance. There are a number of recent reviews on mitigation strategies under above mentioned categories and they are focused on reducing exposure rather than increasing capacity to adapt. Increasing solar reflectance refers to mitigation technologies that are based on reducing absorption of solar radiation. Meterials with higher levels of solar reflectance and higher emission of heat are used in order to achieve this by keeping the surface cool. These are known as cool materials and they include cool roofing and paving materials. Reflective coloring is a comparatively cheaper method of increasing solar reflectance. There is a modern trend in considering mitigation of UHI at the design stage of the city. Increasing the airflow through building materials and structures are done as it is a costly operation to redesign them when a case of UHI appears. Water features and green building concepts are strategically used to mitigate future implications of UHI and climate change.

There is a seasonal variation in energy consumption of the cities as well. There is a significant increment of the energy used for cooling and that is further increased as a result of urban heat islands. Santamouris and Kolokotsa (2016) highlights in their research on the adverse health effects and discomfort caused on people due to high level of discharge of hazardous pollutants such as VOCs from cooling systems. Oke and Cleugh (1987) emphasise how unshaded roofs and pavements absorb higher levels of solar radiation during summer. More than 50% of this radiation energy is transferred back to the air and cause to increase the temperature. When the ambient temperature rises it can result in increased reaction rate of photochemical reactions of atmospheric pollutants and as a result smog is formed and affects human health and comfort. Moreover when the heat is absorbed by the buildings energy demand for cooling increases. Because of that it is essential to move forward with mitigation strategies.

Research on developing roofing materials with higher solar reflectance levels has been started few decades ago. It has been started with readily available materials that can be used as an alternative. Currently cool roofs market consist of single ply membrane roofing, metal, liquid coatings, modified bitumen and building up roofing. These are used mainly for commercial buildings with horizontal roofs with a reduced slope. For residential buildings which have high slopes materials such as wood shakes, clay and concrete tiles and fiberglass asphalt shingles are used. These are available in light and dark shades. Many researches have been done on increment in cost of using above materials and found that, in most of the cases there is no significant difference compared to conventional roofing materials. Because of that cool roofing materials have been included in many national standards in U.S. Reflecting back near infra-red radiation is another important area that has been researches by scholars with the contribution of manufacturers of roofing materials as well. It has been resulted in invention of a new category of cool-colored roofing materials. Around 50% of the solar radiation reaching the ground is near infra-red waves. Even though human eyes have no sensitivity to identify them horizontal surfaces tend to absorb them. The concept of cool-colored mateirals is focused on developing pigments with high reflectivity of near infra-red radiation.

A research that has been done using more than 80 pigments with above mentioned properties has been able to prove the effectiveness of those materials and manufacturers have innovatively introduced them to the market as they are economical. Therefore, cool-colored roofing materials are available in the form of tiles, pigments, shingles of fiberglass and asphalt. Method of coating layers is also used to develop cool-colored materials. In this method when near infra-red radiation is not absorbed or reflected by top layers gets scattered as it goes through the layers. They can be absorbed by under layers (Levinson et al 2007). There have been numerous researches done on developing thermochromic roofing materials which increases solar reflectivity when temperature is increased. One of the main problems in using thermo chromic materials is that they tend to disintegrate in sunlight since most of them are organic. Therefore exposing them to UV radiation outside has been studied extensively. Different types of UV absorbers have been taken into consideration which are capable of photo stabilizing those materials. However, the problem has not fully resolved and further research and development is being done to overcome this issue.

As an alternative to thermochromic materials another class of materials are developed which are in dark colors but are capable of reflecting sun rays directional. These are commonly known as Engineered directionally reflective roofing materials. Reflectivity of these materials differ with the angle of the sun. During summer when the angle of the sun is high the materials have a high reflectivity and during winter when the angle of the sun is less reflectivity is reduced. Due to weathering and aging the properties of reflection can be lost in the materials. Many researches have been done on the reasons for this loss. Berdahl et al. (2002) in their research have found that the main reason for this is soot particles. Asphalt concrete and cement concrete are the main two types of materials used for paving. Applying cool-colored materials, chip seals, grasscrete and permeable pavements are some of the commonly used materials for paving. In the method of grasscrete, grass is grown inside cells of plastic or concrete. In some countries such as Japan and Singapore it is used as a flood control mechanism as well. Regulating urban temperature is the main way how above types of paving help to reduce urban temperature. Scholars have studied on how increasing greenery is used as a method of mitigation of UHI. Greenery in the cities are there in the forms of parks, vertical gardens, roof top gardens and reserves. These types of greenery in cities come under two broad categories; natural and man-made. Green construction became popular during last ten years and currently green facades and roofs have become a common practice in building cities and their infrastructure (Santamouris et al., 2012). Wilmers (1990) has studied the impact of greenery in the city on reducing the temperature gap between city and the rural area and found that the impact is largely positive. Following his study there have been many researches done on this and shown that there is a significant positive impact.

According to a study done in U.S by Moore (2016) it has been found that 100,000 number of trees help to save 1.5 million USD per annum as it reduces electricity demand and saves water. Many scholars including Upmanis (1998) and Spronken-Smith (1994) have done quantitative analysis on the role of greenery in reducing intensity of UHI. They have shown when there is a park, few hundred meters around the park has this positive impact and temperature is regulated. In a study done in England it has been shown that this impact spreads to around 400 meters and in Japan it has been found that the impact spreads to about 1km. Quantification of urban cool island intensity is an important apart of the researches done and it has been found that the average difference between an urban park and its surroundings is around 1.5 Celsius at night. During the day time the difference is around 1 Celsius. This conclusion has been drawn after reviewing many studies.

Designing cities which can regulate impacts of urban heat islands strategically is an area that has drawn attention of architects and environmentalists. In this concept the city is designed in such a way where airflow is manipulated, solar reflective materials are used for infrastructure and morphology of the buildings are designed in a way to support mitigating effects of UHI (Santamouris, 2013). Improving the airflow is mainly done by the layout of the buildings. The widths and height of the streets are selected based on the requirements of minimizing the UHI effect. Increasing the randomness of buildings that are too tall is also a strategy followed to mitigate effects of heating. Furthermore, paving and roofing materials need to be selected based on the above mentioned concept. However, viability of this is questionable in developing countries in which economic pressure come as the governing factor (Kleerekoper et al., 2012).

Introducing streams and lakes to the urban areas is a common strategy used to reduce air temperature in urban areas. Several studies have been conducted in order to find effectiveness of this method. None of the studies has been able to find a direct relationship on the effects of water streams introduced. Among those studies some have made conclusions stating that there is a chance to increase the temperature in the certain times of the day. As the latent heat of water is almost constant variations within the day are hardly noticeable. One of the major drawbacks in this method is that water bodies tend to dry during the dry season in which they are mostly needed. Nevertheless, despite of the drawbacks introducing water structures has numerous benefits. When strategically combined with green infrastructure, water provides unique cooling effects and effectively can be used to mitigate UHI.

Research Gaps

Most of the experimental studies carried out show an important intensification of the UHI phenomenon under synoptic heat wave conditions. It is evident that synergistic effects between the urban heat island phenomenon and global climatic change is still an open scientific topic and further studies are necessary. There have been limited number of studies done on the relationship between UHI and climate change. It causes to restrict accurate predictions on future scenario when it comes to global temperature rise. Zong-Ci, Yong & Jian-Bin (2013) in their research highlights that synergetic effects of UHI and urbanization have been hardly studied and it has created a gap in the understanding of that combination’s impact on UHI.

In parallel, given that the duration of the experimental period differs highly from city to city, and the available data may cover a limited period of time, studies based on mobile traverses and non-standard meteorological stations usually report the measured maximum temperature difference during the experimental period. Studies based on multi-year measurements using standard equipment, usually report either the average annual UHI intensity, the annual average maximum UHI intensity, or the absolute maximum intensity of the phenomenon. The diversity of the experimental methods used and the variety of reported UHI intensities makes inter-study comparisons difficult. It points to a problem with the authenticity of existing measurements, the accuracy and representativeness of results, and the overall validity of the scientific conclusions given. Even though many studies have been done on the mitigation strategies of UHI there is less focus on those strategies depending on the density of the cities. Furthermore, when considering green building concept, a limited knowledge is available on types of plants that can be used for green roofs and walls. It is highly likely to be a result of commercial intervention on the researches because many of the studies that are focusing on mitigation strategies have been done as combined projects with orgnizations that want to commercialize the ideas and gain profits.

Recommendations

Although it is reasonable to place emphasis on the future of cities in developing countries, poverty and energy poverty are expected to rise in developed countries as well. Actually, some hundred millions of people live under energy poverty conditions in the developed world, possessing a very limited “if any” capacity to cover their basic energy needs. Climatic change and urban overheating are expected to place low income populations in these countries under extreme stress and increase their vulnerability.

Even though it is practiced worldwide to address the issues of UHI using different types of mitigation strategies it is highly recommended to use strategic urban planning starting from the design of the city to make sure the costs incurred are minimized and effectiveness increases. There are only limited number of studies that monitor and evaluate the success of implemented mitigation strategies. Therefore, it is recommended to use methods to evaluate and to improve the mitigation strategies. On the other hand, how cities are affected by UHI and selecting suitable mitigation method highly depend on the geographical and socio-economic factors of the particular city. Therefore, it is important to validate suitability of the methods evaluating specific local factors.