In Delhi, cool roofs, pavements can reverse artificial warming
rises during the monsoon months from June to September. The long-term mean of the monsoon heat index between 1951 and 2010 was 47.6 degrees Celsius. Prolonged exposure to this kind of temperatures can cause cramps, exhaustion and heat stroke.
Another study released six years ago by the Centre for Atmospheric Sciences of IITDelhi, concluded that since 1990, there has been a consistent increase in the night temperatures in the national capital and not so much in the day temperatures, leading to an overall warming.
Both the studies blamed rapid urbanisation and modification of land surfaces as one of the reasons for prolonged hot summer in India’s mega-cities. More specifically for Delhi, concrete structures such as roads, pavements and rooftops absorbed heat during the day, forming a dome of warm air over surrounding areas. This heat would not get released after sundown, keeping nighttime temperatures also high.
The study also found that between 1968 and 1985, Safdarjung in Central-South Delhi had higher night time temperatures because it was more built up than Palam. But after the year 2000, when Palam saw excessive urbanisation, the two stations had almost the same annual mean minimum temperatures. Today, Palam is 2-3 degrees hotter than the rest of the city usually.
At 98%, Delhi’s level of urbanisation is the highest in India. Tightly-packed homes are built with low-ceiling, wafer-thin walls and poor ventilation that make them dank in winters and furnace-like in summers. The trees and plants choke because their roots have been sealed with concrete. New neighbourhoods have barely retained any large trees.
This mindless appetite for growth, greed for land and love for the brick and mortar has turned Delhi into a heat island. This is aggravated by machine heat generated by vehicles, airconditioners, generators and so on.
An analysis of 1,692 cities, published in Nature Climate Change last week, showed that the total economic costs of overheating of cities in this century could be 2.6 times higher when heat island effects are taken into account than when they are not. This effect is also expected to add two degrees to global warming estimates for the most populated cities by 2050.
Is there a way to reverse the trend? The study prescribes local interventions such as installing cool pavements and roofs. A cool roof does to homes what light clothing does to a human body. Reflective paints and tiles, sheets covering for the roof are some modern techniques. Many will remember the traditional blinds made of khus, which cooled Indian homes for centuries.
It is the same traditional wisdom behind cool pavements — minimise the use of asphalt and concrete, and build permeable ones instead. It is best to just leave them unpaved for the grass and trees to grow freely so it also allows stormwater to enter the ground more effectively every monsoon.
As for fighting the summer heat, changing 20% of a city’s roofs and half of its pavements to ‘cool’ forms — the study estimates — can save up to 12 times the installation and maintenance cost, and reduce temperatures by about 0.8 degrees. Can there be a better cost-benefit analysis?