Kalyan Chatterjee
Temperatures in Indian cities have been rising to record levels over the past couple of decades and the blame can’t be put either on global warming or climate change. The culprit is a phenomenon that has been called athe ‘Urban Heat Island’ (UHI) effect, which is caused by the rapid rise in cement and concrete buildings and disappearance of the green cover, all of which is associated with urbanisation.
Heat islands occur when a developed area experiences higher temperature than nearby rural areas or when areas experience higher temperatures within a city, according to the US Environment Protection Agency (EPA). Buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas, where these structures are highly concentrated and greenery is limited, become ‘islands’ of higher temperatures relative to outlying areas. Daytime temperatures in urban areas are about 1–7°F higher than temperatures in outlying areas and nighttime temperatures are about 2–5°F higher.
Rising Temperatures and India’s Urban Heat Crisis
Summer temperatures in Hyderabad, for example have risen by as much as 1.8 ° C between 2000 and 2019, according to the US National Oceanic and Atmospheric Agency (NOAA). This finding has been quoted by GreenTree Global, a company that works in the field of mitigating heat absorption by buildings in urban areas. AQI, an air quality monitoring platform has identified five major Indian cities including Delhi and Mumbai which recorded much above normal temperatures during the summer of 2024. This upward trend continues, as evidenced by the numerous heat waves in 2024, with temperatures reaching between 44°C and 50°C across the country. According to the Indian Meteorological Department (IMD), 2024 was the warmest year since nationwide records began in 1901.
When Cities Become Heat Traps
According to AQI, this UHI effect has been on the rise in recent decades with rapid urbanization. Tall buildings, expanding road networks, diminishing green spaces, and dense human activity have altered how heat is absorbed, retained, and released. Cities, built largely with heat-absorbing materials such as asphalt, concrete, and metal, trap solar radiation throughout the day. As evening sets in, this stored heat is slowly released, making urban environments significantly warmer, especially during the night.
Natural surfaces like soil, vegetation, and water bodies help regulate temperature by reflecting sunlight and facilitating evapotranspiration. When these are replaced with buildings and roads, the ability of the environment to cool itself is greatly diminished. In addition, the high concentration of vehicles, air conditioners, industries, and human activity releases waste heat that contributes further to this warming.
One recent research paper pointed out that UHI prompt increased energy consumption and health-related issues. Buildings and infrastructure lead to environmental degradation such as reduced precipitation, decreased soil infiltration capacity, increased runoff, decreased water table, and changes in surface albedo, which further imbalance the earth’s radiation. This exacerbates the UHI effect in densely populated places.
According to Sahidul Islam and a team of researchers from Pune, cities worldwide are marked by both horizontal and vertical expansion, to accommodate the needs of increasing populations. This results in an upsurge in absorption of solar radiation primarily due to artificial dark surfaces and building materials that are characterised by low albedo, higher thermal capacity, and conductivity compared to open rural areas. Albedo is the reflective capacity of a surface – lower values show low reflection of radiation and therefore greater absorption. Because of this, temperature is elevated over the cities.
The Urban Heat Crisis Is Already Here
The growing intensity of UHI effect was evident in five cities that witnessed extreme heat events during peak summer months in 2024. The cities were identified by AQI as Delhi, Ahmedabad, Hyderabad, Mumbai, and Pune. At 47°C Delhi recorded one of its highest ever temperatures in May 2024, and was rated at 57 on the climate severity index. This index was designed by Environment Canada to rate a locality’s climate according to human comfort and well-being and ranged from 1 to 100, with 1 representing least and 100 the most uncomfortable. AQI said that while each city had its own geographical and climatological factors, all were facing UHI effect due to unchecked urban growth.
Ahmedabad recorded a high of 45.4°C in May 2024, but the minimum was 31.2°C suggesting that the city had not cooled down sufficiently during the night ‘a classic symptom of the UHI effect’. Pune, on the other hand was once known for its mild summer. But in April 2024, the mercury touched a high of 40.7°C. AQI blamed it on the expansion of Pune’s IT sector that resulted in real estate booms with green spaces being rapidly converted into commercial and residential blocks. It stood at a high 52 on the climate severity index. Though Mumbai saw a high of only 37°C but the low stood at only 28°C showing that the land-sea breeze was unable to cool it down. The UHI effect in Mumbai is exacerbated by the city’s vertical growth, congested neighbourhoods, and declining mangrove ecosystems.
But there are few signs of the urban push slowing down. Data from the United Nations and India’s Economic Survey 2023–24, indicate that the country’s urban population is expected to exceed 40 percent by 2030, rising from approximately 35–36 percent in 2022. This transition will result in more than 600 million people residing in urban centres.
By 2050, the urban population is expected to reach 814 million or 50 per cent of the total population, as per Building Market Brief – India, 2025.
India is expected to have more than seven megacities with a population of over 10 million, and 68 cities with a population of more than one million, by 2030. With this backdrop it is interesting to note that 70 per cent of the structures that will exist in 2050 are yet to be constructed, in India.
The Mortality Burden of Urban Heat
The concentration of buildings in urban India has already exhibited its costs in terms of increasing heat islands. As thousands of more buildings for residences and commercial establishment come up, the UHI effect problem will multiply many times over. Further creating heat stress for people, as most of the employment will be available in cities. As it is, heat waves have been taking toll on human lives and health, but this is expected to increase many times over in times to come as research studies show.
The human cost of extreme heat is already evident. A study examining the May 2010 heat wave in Ahmedabad, when temperatures soared to 46.8°C, found that mortality rates were 43 percent higher than those recorded during the same period in the preceding and subsequent years. The increase translated into an estimated 1,344 excess heat-related deaths.
A separate multicity study covering a decade of heat-wave events across 10 Indian cities estimated an average of 1,116 heat-related deaths annually. The cities included Ahmedabad, Bengaluru, Chennai, Delhi, Hyderabad, Kolkata, Mumbai, Pune, Shimla and Varanasi, underscoring the widespread and growing public health risks posed by rising temperatures across the country.
Reducing UHI effect: A Blueprint for Cooler Cities
The answer to UHI effect is the cooling action plan that has been initiated in India with the help of the UNEP Cool Coalition. At the urban level, passive cooling can reduce ‘Urban Heat Island’ effect and provide much needed outdoor thermal comfort for the most vulnerable. This can be achieved through increased green and blue infrastructure, reduced hard heat-absorbing surfaces, increased reflectivity of roofs and surfaces, shading and urban layouts that allow cool breezes to flow through the city.
