A recent study from IIT Kharagpur called “Anthropogenic forcing exacerbating the urban heat islands in India” noted that the relatively warmer temperature in urban areas, compared to suburbs, may contain potential health hazards due to heat waves apart from pollution.

About the study

• The research did study the difference between urban and surrounding rural land surface temperatures, across all seasons in 44 major cities from 2001 to 2017.
• It found evidence of mean daytime temperature of surface urban heat island (UHI Intensity) going up to 2 degrees C for most cities, as analysed from satellite temperature measurements in monsoon and post monsoon periods.
• Other researchers from elsewhere have also noticed similar rise in daytime temperatures in Delhi, Mumbai, Bengaluru, Hyderabad and Chennai.

What is an Urban Heat Island?

• An urban heat island (abbreviated as UHI) is where the temperature in a densely populated city is as much as 2 degrees higher than suburban or rural areas.
• This happens because of the materials used for pavements, roads and roofs, such as concrete, asphalt (tar) and bricks, which are opaque, do not transmit light, but have higher heat capacity and thermal conductivity than rural areas, which have more open space, trees and grass.
• Trees and plants are characterised by their ‘evapotranspiration’— a combination of words wherein evaporation involves the movement of water to the surrounding air, and transpiration refers to the movement of water within a plant and a subsequent lot of water through the stomata (pores found on the leaf surface) in its leaves.
• Grass, plants and trees in the suburbs and rural areas do this. The lack of such evapotranspiration in the city leads to the city experiencing higher temperature than its surroundings.

Latent impacts

• UHI s also decrease air quality in the cities, thanks to pollution generated by industrial and automobile exhaust, higher extent of particulate matter and greater amounts of dust than in rural areas.
• Due to this higher temperature in urban areas, the UHI increases the colonization of species that like warm temperatures, such as lizards and geckos.
• Insects such as ants are more abundant here than in rural areas; these are referred to as ectotherms.
• In addition, cities tend to experience heat waves which affect human and animal health, leading to heat cramps, sleep deprivation and increased mortality rates.
• UHIs also impact nearby water bodies, as warmer water (thanks to the pavements, rooftops and so on) is transferred from the city to drains in sewers, and released into nearby lakes and creeks, thus impairing their water quality.

Control of UHIs and mitigation

• Industrialization and economic development are vital to the country, but the control of UHIs and their fallouts are equally vital. Towards this, several methods are being, and can be, tried.
• One of them is to use greener rooftops, using light-coloured concrete (using limestone aggregates along with asphalt (or tar) making the road surface greyish or even pinkish (as some places in the US have done); these are 50% better than black, since they absorb less heat and reflect more sunlight.
• Likewise, we should paint rooftops green, and install solar panels there amidst a green background.
• The other is to plant as many trees and plants as possible

Why plant more trees?

Relevant to the present context are:

• they combat climate change; clean the surrounding air by absorbing pollutant gases (NXOy, O3, NH3, SO2, and others) and trapping particulates on their leaves and bark;
• they cool the city and the streets; conserve energy (cutting air-conditioning costs by 50%); save water and help prevent water pollution; help prevent soil erosion; protect people and children from UV light;
• they offer economic opportunities; bring diverse group of people together; encourage civic pride by giving neighborhoods a new identity; mask concrete walls, thus muffling sounds from streets and highways, and eye-soothing canopy of green; and the more a business district has trees, more business follows.

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