Why are cities caught in the middle of a heat crisis

The Goddard Institute for Space Studies (GISS) declared July 2023 as the hottest month ever recorded since 1880. As global temperatures rise, the sweltering conditions in urban regions are poised to become more severe. Heatwaves are becoming a prominent extreme weather phenomenon in many Indian cities. But heat affects cities adversely. This is because of the absorption and re-emission of solar heat by buildings, roads, and paved surfaces, which exceeds that of natural landscapes. This phenomenon gives rise to localised hotspots known as Urban Heat Islands (UHIs). 

What are UHIs?

Cities and towns experience higher temperatures due to several factors that produce urban heat islands. During the day, densely built neighbourhoods, towns and cities absorb more solar radiation than rural regions due to their high heat-conductive surfaces, such as asphalt. The absence of green spaces and vegetation in urban settings, such as parks, prevents the temperature from decreasing as an outcome of evaporation from plants. Furthermore, as there are fewer trees, there is less shade, which only heats the ground further. Large structures in urban centres further the effect of UHI, as they trap heat and keep out wind, preventing air currents from cooling the environment. The air temperature also rises because of atmospheric pollution due to industries, vehicles, and HVAC systems, which trap solar radiation and prevent heat from dissipating. As a result, UHIs can elevate daytime temperatures by as much as 7.2°F (4°C) and nighttime temperatures by 4.5°F (2.5°C) compared to surrounding areas.

Increased heat has cross-cutting effects across sectors. First, it negatively impacts public health:  causing respiratory problems, heat strokes, dehydration, cardiovascular problems, and exacerbating other health issues. Over 252 deaths were reported in the first half of 2023 in India, as reported by the Ministry of Health and Family Welfare. The 2015 heatwave in India reported over 2081 deaths due to heatwave related issues, as per the EnviStats report 2022, due to heat/ sun stroke. This data only captured direct deaths, while many others died due to heat exacerbating other health issues.  Second, it has a significant impact on the energy systems and the economy. The energy demands of the city increase during periods of increased heat. Heat also affects labour productivity and according to estimates from the International Labour Organisation, heat stress would cause a 2.2% reduction in working hours worldwide by 2030. Third, UHIs further worsen the quality of air, with increased emissions of carbon dioxide and other pollutants, aggravating the greenhouse effect. They have an impact on rainfall patterns and also influence the creation of clouds and fog. Moreover, UHIs may also influence the timing of plant growing seasons in temperate regions. Fourth, heat stress affects low-income and marginalised populations more due to factors like population density, lack of access to air conditioning, and scarcity of green urban areas.

What are wet bulb temperatures?

Along with UHIs, coastal cities like Mumbai, Mangalore, Cuttack, Chennai, and Visakhapatnam face unique challenges as they contend with wet bulb temperatures, which consider humidity alongside temperature. Wet bulb temperature is a meteorological parameter that combines the effects of temperature and humidity, providing an indication of the cooling potential through the process of evaporative cooling. It is an essential metric for assessing heat stress and understanding the environmental conditions that can affect human health. The human body relies on the evaporation of sweat to cool down and this process becomes less effective as humidity increases. Therefore, the wet bulb temperature provides insights into the body's ability to dissipate heat under specific atmospheric conditions. This combination of temperature and humidity (heat index) is significantly high, especially for humid months like October, making it a high risk for those most exposed. Climate experts are increasingly concerned that if human body temperatures rise over the 35°C "wet bulb" threshold, there may be a higher chance of fatalities, as it is estimated that beyond 35°C, a human body can no longer cool itself through perspiration.

Urban Planning as a Potential Solution

Planning cities to build resilience to increasing heat waves poses potential solutions. The UHI effect can be reduced in several ways - by altering construction rules, land zoning, and planning restrictions to improve ventilation and green areas. Strategic urban planning plays a pivotal role in mitigating the heat challenges faced by cities.

1. Increased tree cover

One practical solution involves strategic tree planting, where trees are strategically placed throughout urban areas to provide essential shade, reducing overall temperatures, and combating the UHI effect. Increased green cover regulates the micro-climate of a city by offering shade and lowering the temperature of the surrounding area. The promotion of community gardens and green spaces contributes not only to cooling but also to the well-being of urban residents.

2. Green roofs and walls

Green roofs and walls contribute significantly to cooling by providing insulation, reducing heat absorption, and improving air quality. Green roofs and walls are created by growing a vegetative layer on the surface - planted entirely or partially with plants and a growth media atop a waterproofing membrane. Due to large rooftop cover in urban areas, green roofs have a large potential to mitigate the effects of UHI. In addition to providing shade, green roofs also lower air temperatures both on the roof's surface and in the surrounding atmosphere. In urban areas or other constructed settings with little green cover, using green roofs and walls might help reduce the effects of UHI. Compared to conventional roofs, green roofs can lower building energy usage by lowering peak power demand. Through evapotranspiration, green roofs absorb heat from the atmosphere and serve as building insulators, lowering the energy required for heating and cooling. Advocating for or mandating green infrastructure on buildings can enhance the urban environment.

3. Cool pavements 

Peak summer temperatures for conventional pavement materials can reach 48–67°C, as they absorb heat and also transfer excess heat to the air above them, increasing surrounding temperatures. Due to the large area covered by pavements in urban areas, they become an important element to consider in heat island mitigation. Cool pavements, designed to reflect sunlight and allow water penetration, help combat surface-level heat absorption.  Incorporating reflective asphalt-based coatings, to cover roads and pavements is an effective method to lessen the negative effects of UHI. Cool pavements with white paints also help create a cooler atmosphere. Further, pavements that use evaporation as a method to reduce heat absorption use innovative materials and technologies like porous materials and channels for surface water.

4. Water Features

Incorporating water features, such as fountains and ponds not only aesthetically enhances urban spaces but also has a cooling effect through evaporation. 

5. Increased shade

Shade structures, like pergolas and awnings, strategically placed in public spaces and markets, offer relief from direct sunlight, making outdoor areas more comfortable. Mixed land-use planning, integrating residential, commercial, and recreational areas, disperses heat concentration, fostering more balanced urban microclimates.

Low-Income Settlements: Light and Ventilation

Heat does not affect everyone equally. Socially excluded and vulnerable groups are the most affected by heat. The urban poor are especially vulnerable due to the UHI effect, as they have access to inadequate clean water, sanitation, and hygiene, leading to increased health problems. In a study published by WRI India for Mumbai, the city’s slum settlements are five-six degrees warmer than their neighbouring housing societies. The type of building material used, compromised ventilation and green cover, and limited access to open spaces in slum settlements and low-income housing neighbourhoods increase heat-stress. Many residential buildings in settlements are forced to use metal roofs because of cost constraints Materials like tin are poor insulators and result in increased heat getting trapped indoors. Prolonged exposure to heat may lead to exhaustion and serious health ailments in low-income communities, affecting livelihoods and increasing precarity and risk to other challenges.

Addressing heat challenges in low-income settlements requires a nuanced approach, starting with community-led urban planning. Engaging residents in the planning process ensures that solutions are tailored to their specific needs, promoting a sense of ownership, and sustainability. Designing ventilation pathways, such as narrow streets and openings in buildings, encourages natural airflow, improving air circulation within settlements. The use of reflective materials for roofs and surfaces minimises heat absorption, with light-coloured paints and coatings reflecting sunlight and reducing indoor temperatures.

Compact urban design principles, avoiding excessive concrete and asphalt while incorporating green spaces, can contribute to reducing the temperature in low-income settlements. Affordable and sustainable solar-powered fans can be installed in communal areas or individual homes to enhance ventilation. Elevating structures slightly above ground level can also help facilitate airflow, reducing the impact of heat absorbed by the ground. Finally, establishing cooling stations during extreme heat events, although not a permanent solution, can provide people with a safe space equipped with fans, water, and shade during extreme heat events.

Urban heat risk is a social justice issue. Cities must encourage more creativity in low-income neighbourhoods to boost greenery and use heat-resistant and sustainable building materials. Affordable greening initiatives, such as community gardens or vertical gardens, allow residents to actively participate in creating cooler and healthier living spaces. These practical solutions, implemented collaboratively by government bodies, urban planners, community leaders, and residents, work towards creating more resilient, equitable, and comfortable living environments for all city residents.