In a recent study conducted in Pune in Maharashtra, significant intra-city differences in heat were revealed through the analysis of a Landsat image captured on May 2, 2024. Affluent neighbourhoods like Prabhat Road and Fergusson College, with their lush vegetation cover, maintained lower surface temperatures. In contrast, densely built-up and populated areas such as Ravivar Peth and surrounding slums, including Gosavi Vasti in Karve Nagar, experienced alarmingly high surface temperatures, with some reaching up to 69 degrees Celsius.
A closer look at the image also reveals that most of the slums experiencing similar high temperatures are densely packed settlements, have low vegetation cover, and not much view of open sky which helps the radiation to escape from built-up surfaces. This disparity highlights the critical impact of vegetation and urban planning on local air temperatures and spatial vulnerability in urban heat islands.
While the urban population as a whole is vulnerable to heat waves owing to (Urban Heat Islands (UHI), more vulnerable are those urban dwellers who are more exposed to the kind of land use which is trapping heat and causing UHI. For example, construction workers, street vendors, gig workers, other outdoor workers, and even slum dwellers have larger exposure to heat while having no economic capacity to mitigate the impacts experienced. These people also do not have enough access to healthcare for required treatments, and to livelihood security to cover the financial losses incurred. In a 2023 World Bank report, children, informal workers, and residents of informal settlements are found to have high “social vulnerability” to heat.
Recently, between June 16 and June 24, it is reported that more than 600 million people suffered from climate change induced heat waves. Urban Heat Island or UHI is also a buzz word now and sometimes used as an alternative to heat waves in urban areas. UHI and heat waves are different but yet often linked with each other. Urban Heat Island is a phenomenon where the temperatures, typically nighttime (daily minimum) temperatures, are higher in urban areas compared to its rural counterparts due to the presence of dense built-up area and surfaces. On the contrary, heat waves are a period of unusually high (daily highest) temperatures as compared to what is normally expected over a region (Indian Meteorological Department), urban or rural.
Urbanisation changes the natural land cover (such as bare soil, rocks, water, vegetation, etc.) to paved roads and buildings. The heat absorbed by the built-up surfaces during the day is unable to get fully released at night because of lack of open spaces. The thermal appearance of urban areas creates a picture of heat islands surrounded by cooler rural areas, thus the term Urban Heat Island. It is now well established that urbanisation is not only the cause of UHI but makes the urban dwellers more susceptible to impacts of climate change such as heat waves. This has a variety of implications such as increase in indoor and outdoor discomfort, increase in energy consumption for indoor temperature management, and health implications such as fatigue, heat stroke, and deaths. Elderly people and children are more susceptible to heat waves.
In a recent study by the Centre for Science and Environment (CSE), it was found that six cities in India, namely, Delhi, Mumbai, Bangalore, Hyderabad, Chennai, and Kolkata experienced the worst heat waves. Delhi’s maximum temperature reached 48.8°C recently, leaving a feeling of helplessness in the citizens while they were struggling for potable water and breathable air. It should also be noted that all these six cities identified are among the top six most populated metropolitans of India as per the 2011 Census, thereby suggesting Urban Heat Island as a critical factor in increasing heat wave occurrence.
Mitigation and adaptation strategies
There are various mechanisms of better urban planning that can be used to control the UHI or rising surface and consequently air temperatures in urban areas. First of all, reflective colours should be and are being used to paint built-up surfaces in different parts of the world. Houses and building tops are being converted into cool roofs using highly reflective white paints so that they don’t absorb heat. These measures have shown some promise in improving indoor and outdoor cooling.
Image taken from World Bank 2023 report titled Urban Heat in South Asia available here
In India, though some NGOs are promoting cool roofs at their local levels, the government is yet to prioritise this at the policy level and push for its immediate implementation. Also, while India’s policy mandates cool or green roofing in commercial and formal housing to some extent, the policy documents such as the Energy Conservation Building Code 2017 and the National Mission for Sustainable Habitat 2021-2030 have absolutely no mention of informal settlements. This omission is further disastrous for the residents of informal settlements that already experience higher temperatures because of their social as well as spatial vulnerability, as explained earlier. Informal settlements, where 49 percent that is about half of the urban population of India lives, are the ones needing resources for cool roofs the most.
Next, Heat Action Plans (HAPs) have to be strengthened. At the policy level, many districts and cities in India have come up with local and regional heat action plans in recent years with the guidance of the National Disaster Management Authority. There are no separate heat “mitigation” plans in India. The heat action plans primarily focus on preventing deaths due to heat waves. In total, these plans are designed to be reactive and respond when heat waves occur, rather than being proactive to mitigate urban heat more comprehensively in the long run. Hardly any HAP addresses intra-city social and spatial vulnerabilities. For example, among informal workers, only construction workers are paid some attention though the implementation of their restricted work hours during heat waves is very poor. The policy lacks provisions for strict implementation and monitoring. Also, no attention is paid by HAPs to informal settlements. Local vulnerability and need assessments are rarely done. In addition, mostly HAP programs remain poorly funded.
An important measure to control outdoor temperatures is to increase vegetation cover so that direct interaction of solar radiation to exposed surfaces is reduced. Increase in shaded places is also recommended to reduce exposure to heat. This is much needed for outdoor workers and pedestrians. In addition to green solutions, blue solutions, that is increase in water bodies in urban spaces, are also recommended by experts. Also, dense settlements should be avoided and more open spaces should be allowed in urban areas as heat reflection from built-up surfaces increases with sky view factor. The material of building surfaces should enable passive cooling instead of dependence on mechanical cooling systems and energy consumption, and roads should be more permeable to allow infiltration and evaporation of water. However, these measures have to be adopted in policy and planning. Heat mitigation needs to become the focus of urban planning and redevelopment to address the issue more comprehensively. World Bank has suggested a simple framework where all strategies should focus on three solutions – increasing shade, improving solar reflectance, and increasing evapotranspiration (see the figure).
More innovative urban initiatives such as 15-minutes cities, designed to avail all facilities within 15-minutes of convenient public transport, can also reduce use of vehicles and contribute to less demand of wide roads and reduce air pollution. While these pointers are towards the direction in which urban planners can think, more scientific research should be carried out to understand and implement logical steps for making urban areas more resilient.
The authors teach Environmental Studies/Communication in FLAME University