Rise in Mumbai's average temperature due to increase in built-up area: Study

The hustling-bustling financial capital of Mumbai has witnessed a 2 degrees Celsius rise in average temperature across 27 years, leading to multiple environmental problems and an increase in heat.

Mumbai has lost 81 per cent of its open land (barren spaces without any vegetation), 40 per cent green cover (forests and scrublands), and approximately 30 per cent of its water bodies (lakes, ponds, floodplains) between 1991 and 2018, while the built-up area (areas developed upon) has risen by 66 per cent during the same period, according to a recent study.

Researchers from Faculty of Natural Sciences, Jamia Millia Islamia University in New Delhi, Osmania University in Hyderabad and Aligarh Muslim University, Uttar Pradesh through their study – “Urban Heat Island Dynamics in Response to Land-Use/Land-Cover Change in the Coastal City of Mumbai” - published in the peer-reviewed Springer Journal of the Indian Society of Remote Sensing found that with this pace of urbanisation and landscape transformation, it is expected that the Urban Heat Island intensity will further increase in the city.

“The extreme heat that one experiences when strolling through any urban landscape is caused by the Urban Heat Island Effect, a micro-climatic phenomenon. This is due to a number of causes, the most prominent of which being the usage of materials such as concrete,” explained Prof. Atiqur Rahman from the Department of Geography, Faculty of Natural Sciences, Jamia Millia Islamia.

Using satellite imagery (USA-NASA Landsat datasets freely available), the authors studied an area of 603 square kilometres of Mumbai region (both city and suburbs) to understand the land-use and land-cover changes, difference in maximum, minimum and average temperatures (for Urban Heat Island intensity), land surface temperatures, changes in vegetation cover versus urban built-up density between 1991 and 2018. The findings were represented in the form of high-resolution maps and attempted to enhance urban planning and policy decisions by mitigating the Urban Heat Island effect.

Shahfahad, Senior Research Fellow at Department of Geography, Jamia Millia Islamia University and lead author of the study said, “We found that rapid uncontrolled urbanisation over the past 5-6 decades attracted a large population by providing better economic opportunities. As a result, natural land use patterns such as vegetation cover, scrublands, wetlands, and open lands have all been transformed on a large scale into the city’s built-up surfaces.”

The study noted that during the perios between 1991-2018, Mumbai city lost nearly 40 per cent of its green cover including forests (dense vegetation) and scrubland (sparse vegetation) that fell from 287.76 sq. km in 1991 to 193.35 sq. km in 2018.

The area of open land fell by more than half from 80.57 sq. km in 1991 to 33.7 sq. km in 2018. Mumbai also saw reduction in water bodies from 27.19 sq. km to 20.31 sq. km during the same period.

Meanwhile the conversion of open lands, green cover and water bodies to buildable land showed an increase from 173.09 sq. Km in 1991, which almost doubled and reached 346.02 sq. km in 2018.

Transformation of these areas significantly altered the dynamics of land surface temperature and urban heat island intensity. “We observed that the average temperature in 1991 was 34.08 degrees Celsius. It rose to 36.28 degree Celsius in 2018 (a 2.2-degree Celsius increase) under the heat island zones (vulnerable areas) thus exposing people to higher heat risk,” said Shahfahad.

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