The summer of 2022 has been particularly harsh all over India, with unrelenting heat spells signalling evolving climatic systems. With March being declared as the hottest in the India Meteorological Department’s 122-year recorded history and April being the third hottest since 1901, the news about the heatwave has captured wide attention.

The increase in heat, however, is not a one-off event, especially in urban areas due to a phenomenon known as the Urban Heat Island Effect, which is “the relative warmth of a city compared to its surrounding rural areas”.

With 56.2% of the world’s population living in urban areas as of 2020, all that the cities comprise – materials used and their ability to absorb or reflect heat, designs of urban spaces and geometry that allow or block the passage of heat, presence or lack of ecological infrastructure such as trees, parks, wetlands and water bodies as well as sources of anthropogenic heat – play a significant role in the heating of urban spaces.

Based on how cities evolve, each neighbourhood has its own mix of Urban Heat Island-contributing factors in different proportions which leads to a variance in temperatures not only between the city and its surrounding areas but within a city as well.

In order to illustrate this phenomenon, we gathered data from different neighbourhoods in Mumbai and Delhi, including their recorded minimum and maximum surface temperatures at a given time, on a regular summer day in 2021.

The minimum and maximum surface temperatures of three Delhi neighbourhoods with varying socio-economic and physical characteristics were taken around noon on May 3, 2021. The range of temperatures in a given neighbourhood is stacked in intervals of 2 degrees Celsius and represented in the illustration. Illustration by Alisha Vasudev/Mongabay. Data sourced by Technology for Wildlifefrom Landsat 8 ST_B10 band, processed using Google Earth Engine.

In Delhi, temperatures were recorded in Nehru Place, Govindpuri and Greater Kailash II on May 3, 2021. “Govindpuri is typically a very low-income area,” says Ronita Bardhan, Associate Professor of Sustainability in Built Environment, University of Cambridge.There [in Govindpuri], temperatures reaching 45 degrees Celsius is fatal, whereas, if you see Greater Kailash II (a higher-income neighbourhood), its temperature reaching 45 degrees Celsius is not as fatal as it would be in Govindpuri,” she says.

She further explains that in Greater Kailash II, people have the means to cool their homes using air conditioners. However, the heat refuse from the mechanical ventilation contributes to artificially raising the outdoor temperature, creating hotspots which burden the urban poor, such as the residents of Govindpuri. She also says that if everyone proceeded to turn on their air conditioners to cool the indoor environment, there would yet be an elevation in the temperature in the high-income neighbourhood with tree-lined streets and several parks.

Decreasing green spaces

With vegetation and water bodies known to regulate the temperature within cities, a decrease in green and blue cover can be contributing factors to the Urban Heat Island Effect. Amir Bazaz, Associate Dean, School of Environment and Sustainability at the Indian Institute for Human Settlements, sheds light on the loss of natural cover in urban areas.

“Jobs are in the cities,” he explained. “People move to the cities, and therefore rural incomes have not increased substantially. There is also an aspiration change between generations – people are increasingly preferring to come to the cities to generate more income, instead of pursuing agriculture.”

“Cities are melting pots of opportunity or places where minds meet and innovation happens,” he said. “That is how people increase their incomes.”

“So, if people rush to cities, where will we build our houses?” he questioned, pointing out that the built environments expand to take over green and blue spaces.

For Mumbai, the two localities chosen (Maharashtra Nature Park and Bandra Kurla Complex) are in close proximity to each other, on either bank of the Mithi river but with extremely varying characteristics. Maharashtra Nature Park is a human-made forest, developed on a former dumping ground and is surrounded by mangrove forest on the Mithi river, whereas Bandra Kurla Complex is a bustling commercial and government complex, built on low-lying land that lacked proper surface drainage along the Mithi river.

The minimum and maximum surface temperatures of two Mumbai neighbourhoods, with different physical characteristics, were recorded at 11 am on April 15, 2021. Maharashtra Nature Park is a green patch with significant tree cover while Bandra Kurla Complex is a concrete, business hub with low green cover. While analysing the temperature ranges in different parts of each neighbourhoods, it was found that about 84% of the area of Maharashtra Nature Park recorded below 43 degrees Celsius while 61% of the area of Bandra Kurla Complex recorded above 43 degrees Celsius surface temp. This illustration shows which temperature range (in intervals of three degrees) is recorded in what percentage of the area in each of these neighbourhoods. Illustration by Alisha Vasudev/Mongabay. Data sourced by Technology for Wildlife from Landsat 8 ST_B10 band, processed using Google Earth Engine.

According to the United Nation’s Intergovernmental Panel on Climate Change, the urban heat island effect is further amplified in cities that lack vegetation and water bodies, both of which can strongly contribute to local cooling. The Intergovernmental Panel on Climate Change’s Special Report: Global Warming of 1.5 degrees Celsius further predicts with high confidence that “urban heat islands often amplify the impacts of heatwaves”.

Bardhan explains this development-linked impact on urban heat islands, saying, “We must first accept that urbanisation will densify our cities and it’s inevitable.”

“We cannot just fight it and it is wrong to fight it because otherwise, how does development happen?” Bardhan said. “We can intelligently layout our buildings, we can intelligently design our neighbourhoods in such a way that we create escape routes for heat.”

With building materials accounting for 10% of emissions while 50% of emissions stem from operational energy, Bardhan makes a case for an optimal mix of both materials and design to help reduce the heat burden in our cities.

“The physics is not only to restrict the heat coming from outside to the inside but also create spaces indoors or create ventilation strategies indoors so that indoor heat is released,” she concludes.

This article first appeared on Mongabay.