KOLKATA: The combined effect of climate change and rapid urbanisation has triggered an urban “heat island effect” in Kolkata, with the city’s annual mean land surface temperature rising as much as 4.72°C in the last three decades, causing heat distress to both flora and fauna, claims a recent study by Calcutta University and Vidyasagar University.
The study says that the urban heat island (UHI) effect has intensified, in the last thirty years or so, along with a 33% rise in the city’s built-up area and a 25% decline in vegetation.
A careful monitoring of the correlation between urban development on urban heat islands has also revealed that the annual mean temperature increased 0.157°C per year in the city.
Kolkata’s built-up areas were 22% in 1990, 31.4% in 2000, 44.2% in 2010 and 55.7% in 2020. Ismail Mondal, a climate scientist at Calcutta University and a lead author of a study on Kolkata’s UHIs, explained why there would always be a positive correlation between built-up areas and UHI intensity. Concrete, he said, absorbs more solar radiation, while dissipating little heat at night. Also, asphalt used in laying roads is dark in colour and absorbs more heat. Thus, the sharp increase in the city’s built-up area has accelerated the heat-island effect, he said.
On the other hand, there is a negative correlation between land surface temperature and the normalized difference built-up index (NDVI), which is a measure of the area covered by natural vegetation. As a result, the latter has been severely depleted by urban expansion, Mondal said.
