Until the nineteenth and twentieth century, the ancient cities were smaller in size and were supported by the larger rural population. The process of urbanization has stimulated rapidly throughout the world since 1800 and subsequently, the cities have been continuously expanding their boundaries and population. The global urban population, since then, has increased from 13% to 50% at present. Massive work opportunities and modern lifestyle have led to a huge influx of people that accelerated the change of urban landscape horizontally and vertically by narrowing inter-building space and open green spaces. This urban development has resulted to changes in the urban microclimate, particularly of the thermal environment.
Formation of urban heat island
The structure of the city differs from its rural counterpart in terms of the building height, building materials, layout etc.; therefore, the thermal environment of cities quite different than the rural area. The concrete building materials, dark surfaces such as asphalt used for roadways and rooftop tend to have a low albedo, and absorb a higher amount of solar radiation and convert it to thermal energy (Pan, 2015). As a result, surplus heat energy accumulates in the urban environment and is likely to produce more heat than its suburban and rural counterparts. The urban regions, especially congested part become warmer than their suburban or rural surroundings, formulating an island of elevated temperatures in the landscape. This phenomenon of a gradual warming of the urban core in comparison to the adjoining suburban and rural areas is referred to as Urban Heat Island (UHI) (Comarazamy et al., 2010). The temperature difference from rural surroundings to the urban area can be as high as 12°C. During the night, when winds are calm, the temperature difference is highly prominent (Oke and Cleugh, 1987).
Why Cities Experiences heat island?
- Deforestation: Rural and the less undisturbed landscape is mostly covered with vegetation and grasses that cool the lower atmospheric environment through transpiration and providing natural sheds. Deforestation and encroachment of green space reduce the natural cooling effect.
- Encroachment of wetland and open space: The changes of landscape (surface cover) alters the interaction between surface characteristics and solar radiation, therefore resulted in micro-climate. In the rural area, a part of solar energy is consumed for evaporation of water from soil surface and water body. In the city region, the gradual encroachment of wetland, water body, open space etc. by the concrete structure reduces heat loss by minimizing evaporation, thus the cumulative accumulation of heat.
- Radiative heat captured by urban structure: The invasion of the urban structure by replacing the green cover and other natural surface covers not only captures and accumulates heat during day time, but also store radiative energy during night-time making urban core as UHI. The tall urban buildings with narrow interspace between the buildings obstruct longwave radiation during the night and accumulate heat.
- Artificial heat generation: The burning of fossil fuel (used by vehicles), industrial and domestic heating and cooling units release more heat than injected into the urban environment (1 to 3° C). The growth of urban area and increase of economic activity often led to amplify UHI.
Urban Heat Island and air pollution:
Unplanned development, continuous development activities, heavy traffic load often increase the concentration of air pollutants and greenhouse gases that absorb radiation emitted from the surface. A large part of this absorbed energy reradiated downwards to warm the ambient air. On the other hand, UHIs can have an impact on air quality by rising ground-level ozone (O3). Sunlight and temperature exacerbate ozone formation, causing peak ozone levels during the summer season. Therefore, the cities experience a marked upswing in the formation of ozone. Interestingly, heat islands intensify the energy demand for cooling and add more pressure for electricity generation that, in turn, increases the emission of greenhouse gases. Vehicle transportation, construction activities, industrial work emit particulate matter (PM) and precursors to PM formation. Crutzen (2004) suggested that a high concentration of pollutants in the UHI plumes can alter atmospheric chemistry. Researches in different cities around the world have found the significant relation between the air pollution and UHI; for example, the relationship between urban climate and suspended particulate matter (Jonsson et al. 2004), urban heat with enhanced ozone levels (Stone 2005), aerosol concentration and UHI (Li et al. 2007). In the Indian background, however, such investigations are still limited (Pandey et al., 2012).
Impact of UHI and changing air quality on human health
The UHI effect alters the urban micro-climate and increases human exposure to heat, while increasing air pollution causes several health issues, mainly on the respiratory system. Heat island exposure can increase the risk of heatstroke in cities (Kjellstrom et al., 2009) and some common chronic non-communicable diseases (NCDs) such as heat stress, discomforts, cardiovascular disease, respiratory distress, cramps, etc. and can even lead to death. Few researchers have found that peoples who have cognitive health issues like depression, dementia etc. are facing higher risk at higher temperatures. People suffering from overweight and diabetics may have sleep deprivation or cardiovascular syndromes due to extreme heat exposure.
Physiological responses to heat exposure may increase heat mortality by affecting the cardiovascular and respiratory systems. The estimation of the Centres for Disease Control and Prevention between 1979 and 2003 found more than 8,000 premature deaths in the United States caused by excessive heat exposure. For instance, 50% of the total heat-related mortality in the West Midlands during the 2003 heat wave was caused by UHI effect.
The ground-level ozone exposure and exposure to pollution contribute significantly to the respiratory tract irritation, and heart and lung disease-related mortality (WHO 2016). The combined exposure to UHI and air pollutants during hot season intensify the cardiovascular diseases (Kinney, 2008). Anenberg et al. (2011) have estimated the annual global mortality caused by exposure to ozone and PM2.5. They found that 0.7 ± 0.3 million respiratory mortalities were associated with surface exposure to ozone, while air pollutant PM2.5 was responsible for 3.5 ± 0.9 million cardiopulmonary and 220,000 ± 80,000 lung cancer mortalities.
Potential impacts in India
NCDs are historically considered as a problem of developed countries, though the problem is growing rapidly in most parts of the world (Kjellstrom et al., 2009). World Health Organization (WHO) has estimated US$6.2 trillion costs to be expended in India between 2012 and 2030 for the treatment of some NCDs like cardiovascular diseases, cancer, chronic respiratory diseases, diabetes and mental health disorders (WHO, 2016).
The rapid urban growth has not only led to the formation of UHI but also reduce air quality especially caused by motor vehicles. The ‘safe’ Air Quality Index (AQI) is thought to be between 61-90 units, but in metropolitan cities like Delhi, Kolkata it has gone to very poor–to-dangerous levels of about 323 unit.
Urbanisation increases better access to health facilities but the residents of poor areas such as slums and informal settlers suffer from “disproportionately from diseases, injury, premature death and combination of ill-health and poverty entrenches disadvantage over time” (UNFPA 2012). Many of the poor residents even cannot bear the cost of health facilities for which mortality among them is on the rise as they tend to resort to unqualified and unregulated providers such as quack doctors.
Mitigation and future policy
According to the prediction of the Intergovernmental Panel on Climate Change (IPCC), the climate change is likely to exacerbate heat wave frequencies in future which, in turn, will increase the mortality from heat strokes. However, the scientific community needs to pay more attention to in-depth analysis of the cause-effect relationship between heat island, increasing air pollution level, global warming, and the resulted in diseases and mortality. Though industrialisation and urban development are essential for economic growth, the control of UHIs and their fallouts are similarly essential. Urban development through proper planning, policy adaptation, administrative control and vigilance, community awareness, however, could be a better solution to maintain a balance between urban growth and environmental protection. Urban climatologists, architect, engineers etc. are investigating for decades to minimize UHI effect. Measures including greener rooftops, cool roof, white or reflective materials to build urban infrastructure like buildings, roofs, pavements, roads etc. which increases the total albedo are used to minimize heat in the urban environment. UHI effects can also be minimized by implementing green roofs as plants absorb carbon dioxide with an increase in oxygen production. Increasing vegetation cover through the plantation in urban space also decreases the UHI effect. Green parking lots also helps in reducing the urban heat island effects as it uses surfaces other than vegetation and asphalt. Besides building engineering and plantation, several policies should be adopted and implemented strictly to minimize UHI effect and air pollution.
- Strict land-use policy formulation and implementation to restrict illegal encroachment over wetland and open space in the urban and peri-urban area.
- Implementation building bylaws for each urban area considering a national policy.
- Encouraging electric vehicle transportation and controlling motor vehicle pollution through vigilance on Pollution under Control (PUC) certificate.
- Increasing community awareness, transparency in governance, effective traffic management etc.
Reference:
Pan, J., 2015. Analysis of human factors on urban heat island and simulation of urban thermal environment in Lanzhou city, China. Journal of Applied Remote Sensing, 9(1), p.095999.
Comarazamy, D.E., González, J.E., Luvall, J.C., Rickman, D.L. and Mulero, P.J., 2010. A land–atmospheric interaction study in the coastal tropical city of San Juan, Puerto Rico. Earth Interactions, 14(16), pp.1-24.
Oke, T.R. and Cleugh, H.A., 1987. Urban heat storage derived as energy balance residuals. Boundary-Layer Meteorology, 39(3), pp.233-245.
Crutzen, P.J., 2004. New directions: the growing urban heat and pollution” island” effect-impact on chemistry and climate. Atmospheric environment, 38(21), pp.3539-3540.
Jonsson, P., Bennet, C., Eliasson, I. and Lindgren, E.S., 2004. Suspended particulate matter and its relations to the urban climate in Dar es Salaam, Tanzania. Atmospheric Environment, 38(25), pp.4175-4181.
Li, Y.M., Zhang, J.H. and Gu, R.Z., 2004. Research on the Relationship between Urban Greening and the Effect of Urban Heat Island [J]. Journal of Chinese Landscape Architecture, 1, pp.72-75.
Pandey, P., Kumar, D., Prakash, A., Masih, J., Singh, M., Kumar, S., Jain, V.K. and Kumar, K., 2012. A study of urban heat island and its association with particulate matter during winter months over Delhi. Science of the Total Environment, 414, pp.494-507.
Stone Jr, B., 2005. Urban heat and air pollution: An emerging role for planners in the climate change debate. Journal of the American planning association, 71(1), pp.13-25.
Kjellstrom, T., Holmer, I. and Lemke, B., 2009. Workplace heat stress, health and productivity–an increasing challenge for low and middle-income countries during climate change. Global health action, 2(1), p.2047.
Kinney, P.L., O’Neill, M.S., Bell, M.L. and Schwartz, J., 2008. Approaches for estimating effects of climate change on heat-related deaths: challenges and opportunities. Environmental science & policy, 11(1), pp.87-96.
Anenberg, S.C., West, J.J., Horowitz, L.W. and Tong, D.Q., 2011. The global burden of air pollution on mortality: Anenberg et al. respond. Environmental health perspectives, 119(4), pp.158-159.
Kjellstrom, T., Holmer, I. and Lemke, B., 2009. Workplace heat stress, health and productivity–an increasing challenge for low and middle-income countries during climate change. Global health action, 2(1), p.2047.
UNFPA 2012. UNFPA Annual Report 2012. Available at: https://www.unfpa.org/publications/unfpa-annual-report-2012
WHO 2016. WHO annual report on noncommunicable diseases 2016. Available at: http://www.emro.who.int/annual-report/2016/noncommunicable-diseases.html
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