The city’s slum settlements are five-six degrees warmer than their neighbouring housing societies, while 37% of Mumbai’s households with metal roof structures are exposed to a risk of high rise in temperatures or heat risk post monsoon, a study by the geo-analytics team of environment group World Resource Institute (WRI) India across the city during October over the past three years has revealed. The study found the city has an alarming shortage of green cover.
WRI India published three satellite maps –October Heat series – highlighting location-specific risk exposure to citizens. The maps established microclimatic changes and temperature variation within Mumbai, and called for city-specific policies that need to build on by adopting more nature-based solutions to adapt to climate risks.
“Coastal cities do not record very high air temperatures, but the combination of temperature and humidity (heat index) is significantly high, especially for humid months like October, making it high risk for those most exposed. Prolonged exposure can lead to exhaustion and serious health ailments,” said Lubaina Rangwala, senior manager (urban development and resilience), WRI India, adding that October was one of the most severe weather months for Mumbai.
“As the rains retreat, there is high moisture availability combined with a rise in temperature. While the absolute temperatures may not be very high, 90% humidity combined with low wind speed allows the heat index and stress to shoot up in Mumbai. This is why we chose October.”
The first map showed slums in Dharavi recorded 35.9 degrees Celsius (during October over three years), while housing societies in Matunga recorded 30.3 degrees Celsius. A similar phenomenon was recorded across Powai-Vikhroli, Goregaon, and Girgaum-Marine Lines.
“We found that this was a function of the type of building material used, compromised ventilation and green cover, and limited access to open spaces,” said Rangwala.
The analysis revealed that industrial and commercial land use with large built-up footprints such as the airport, industrial estates and malls had a significantly higher surface temperature (33-35 degrees Celsius) than natural areas – forests, lakes, mangroves (27-32 degrees Celsius).
The second map showed the difference in surface temperatures of settlements using concrete versus metal roofs. “Roofs are the most exposed surfaces of a building to solar radiation, and play a key role in determining indoor thermal comfort. Metal roofs are poor insulators and good conductors, resulting in increased indoor temperatures, while concrete transfers heat at a slower rate, resulting in increased night-time temperatures,” said Rangwala.
The ward-wise breakup of roofing material showed S ward (Bhandup, Powai, Kanjurmarg, Vikhroli and Nahur) had the highest share of metal or asbestos roofing material (57%), followed by P-North ward (Malad, Manori, Marve, Aksa, and Madh – 55%) and N ward (Ghatkopar, Vidyavihar and Pant Nagar).
While areas such as Pydhonie, Bhuleshwar, Grant Road, Walkeshwar, Malabar Hill, Breach Candy to Haji Ali all had concrete rooftops.
WRI India’s third map was a correlation of the city’s green spaces and heat exposure and its implication on surface temperature. It showed over 31% of Mumbai’s land area had green cover (open maidans, tree-cover, mangroves and forests, city-parks etc.) but it was concentrated only in some wards. “There is a variation of 29 to 34 degrees Celsius across Mumbai wards, which means areas with highest green cover are five degrees cooler than those with less than 10% vegetation. Urban greening helps improve air quality, increased oxygen levels, lower ambient temperatures, and improve livability standards,” said Rangwala.
R-Central ward (Borivli) had the highest percentage of green cover (75%) and the lowest mean temperature (29-30 degrees Celsius) followed by T-ward (Mulund) with 70% green cover and the lowest mean temperature at 29 degrees Celsius. Conversely, B ward (Masjid Bunder, Mohd. Ali Road, Dongri and Bhendi Bazar) and C ward (Pydhonie and Bhuleshwar) had less than 10% green cover and highest mean temperatures at 34 degrees Celsius.
What WRI India said
“Combining all options under a heat action plan for Mumbai is a necessity, implemented by all stakeholders. Heat risk in cities is a problem of social equity. City governments need to support greater innovation in low-income settlements to increase vegetation, deploy sustainable and heat resilient materials for housing construction,” said Madhav Pai, director, WRI India.
Variations in temperature are bound to be there in slum areas where there is no space for trees vis-à-vis residential zones where more green cover is present, said state environment and tourism minister Aaditya Thackeray. “However, we are working on specific plans for Mumbai and will be factoring in solutions for site-specific heat exposure as part of the Maharashtra state climate action policy. We are meeting with various state departments, and looking at what they are offering towards this policy to ensure city-specific climate resilience in Maharashtra,” he said.
The state’s current climate action plan falls short of addressing the heat stress concerns, say experts. “Without such a plan, the situation is likely to worsen annually now under the current effects of climate change, and unfortunately low-income groups would be at the forefront of the health impacts,” said Ashok Jaswal, former scientist of the India Meteorological Department and lead author of 2017 study on decadal rise in average heat index over 60 years in India.
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