Cities all over the world are now getting warmer than before. In India, metropolises like New Delhi, Mumbai, Chennai, Bengaluru and Kolkata experience hotter summers even during night time. Guwahati is also experiencing the same nowadays and the problem is deteriorating day by day. This is happening due to the phenomenon called �Urban Heat Island Effect�.
The United Nations notes with caution that by 2050, 66 per cent of the world�s population will live only in the cities following rapid expansion of human settlements, accompanied by mass concreting of open grasslands, thus changing the natural landscape totally.
Concrete absorbs heat very quickly and so it gets much hotter than air and heats up the environment even further. At night time, concrete releases heat at a very slow rate, thereby making both days and nights hotter by about 3 to 12 degrees centigrade. Nights remain hot for a longer period. Environmentalists are examining this phenomenon all over the world.
Such speedy growth of volume of concrete, obviously, has many adverse effects on the environment. The previously existing vegetation and tree cover kept the surroundings cool enough for a comfortable living.
Trees and grass receive water from the soil and release a portion of it to the atmosphere as vapour, thereby keeping air cool. Besides, trees give shade to the soil and prevent absorption of heat from the Sun. Now, concrete pavement and asphalt that are continuously replacing grasslands receive more heat and become much hotter than the ambient temperature. Paved surfaces in urban settlements consist of up to 75 per cent of impervious surface that does not allow water to percolate, while natural surfaces, only 10 per cent impervious, allow sufficient water to seep into the soil. Open grasslands absorb water, retain water and by the process of evaportranspiration keeps the ground cool enough.
Bengaluru, once known as a city of gardens, is now experiencing severe summers because of the decrease of vegetation cover. A study conducted by the IISC reveals that there has been a whopping 632 per cent rise in settlement areas during the period from 1973 to 2009, during which the decline of vegetation has been 76 per cent and that of wetlands is 79 per cent.
In Guwahati, there has been an increase of 23 per cent high-density settlement and a decrease of 26 per cent dense vegetation from the year 2000 to 2009. The decrease of open space has been 16 per cent. The congested high-rise buildings block air flow. There is a need to leave space for �wind corridors� between buildings. In high-density settlement areas of Guwahati, temperature difference of even 6 degrees centigrade has been recorded. The World Health Organisation (WHO) recommends an open space of 9 square meter per person. It is a pity to observe that in some of cities, the open space available per person is less than one square meter! Rules, laws and by-laws must be strictly followed in the cities, if necessary, with amendments.
As explained above, though the difference of temperature during daytime is less, at night the difference is more compared to the surrounding rural areas in Indian cities, with New Delhi experiencing the maximum difference. In the same city, depending on settlement, green cover and other factors, different isolated small islands are observed. In the city of Kolkata, tree cover has fallen to 7.3 per cent from 23.4 per cent in 20 years time and build-up areas increased by 190 per cent during the same period. It is feared that vegetation cover will further be reduced to around 3.37 per cent in the year 2030. The same story holds good for all the cities in India.
Under the Smart City project, massive concretisation will take place. In the process, more flyovers, bridges, roads with asphalt will be built, thereby reducing grasslands.
Lawns in front of individual houses, public parks, trees along the roads not only increase the aesthetic beauty but also keep the surroundings cool. Similarly, ponds, tanks, water bodies, marshy land, etc., keep the environment cool.
For Bangalore, the TERI recommended 1 square kilometre of water body for every 7 square kilometre of areas to be expanded newly.
Parking lots in the cities should have enough trees and the floor materials used in building parking lots should be of high quality albedo materials.
The adverse effects of urban heat islands are many like discomfort in living, health hazards due to high temperature, etc. Experts are of the opinion that mortality rate increases by 9 per cent for every increase in temperature by 0.3 degree centigrade.
In the USA, every year 1,000 people die because of heat related problems and death rate has increased exponentially due to urban heat island effects. Cardiovascular problems, respiratory diseases, heat cramps, etc., have also increased due to rising temperature.
People from low income groups and senior citizens are the worst sufferers. Fish population and other aquatic life are also getting affected because of rising temperature. The other factors responsible for rise in temperature are vehicular emission that absorbs much heat from the Sun, large-scale use of air conditioners in urban areas, etc.
City of Los Angeles in the USA spends 100 million dollars annually only for tackling the urban heat island effect.
To control the effect of heat islands, buildings should be constructed in an environment-friendly manner by choosing environment-friendly building materials.
Other options are heat resistant roof surfaces, roof top gardening, etc. Use of albedo materials with more than 0.6 ratio will reflect much of the heat back to atmosphere and bring down surface temperature drastically.
For construction of pavements and roads, albedo materials should be used, which will keep the surface cool. The practice of maintaining lawn must be encouraged by the city authorities. Concreting the court yard must be avoided. Water bodies must be preserved. Large-scale plantation of trees must be encouraged.
Already Guwahati is experiencing urban heat island effect and the city has been found to be 2 to 3 degrees warmer than the nearby rural areas. So, the government must take the problem seriously and initiate steps accordingly to deal with the same.