Two decades of nighttime surface urban heat island intensity analysis over nine major populated cities of India and implications for heat stress

Warmer global climate and urban heat islands (UHIs) interact, by exacerbating heatwaves and increasing the extreme heat days in cities. The implications of added heat stress in urban environments due to intensifying surface UHIs (SUHIs) is of utmost concern. Seasonal, annual and decadal nighttime SUHI intensities (SUHIIs), from 2001 to 2020, for nine major populated cities of India are analyzed. This includes five megacities- Delhi, Mumbai, Kolkata, Bangalore, and Chennai, and four incipient megacities- Hyderabad, Ahmedabad, Surat, and Pune. The key role of increasing urbanization (pre- and post-2010) in expansion and intensification of nighttime SUHIs in India is highlighted. For all cities either pre-monsoon (MAM) or winter (December-February; DJF) seasons show the strongest SUHII development. During the 2001-2010, and the 2011-2020 decade, a nighttime SUHII maxima of respectively (i) 2.1 degrees C and 2.5 degrees C for Delhi, (ii) 1.3 degrees C and 1.5 degrees C for Mumbai, (iii) 1.3 degrees C and 1.5 degrees C for Kolkata, (iv) 0.6 degrees C and 1.0 degrees C Bangalore, (v) 1.7 degrees C and 1.9 degrees C for Chennai, (vi) 1.8 degrees C and 2.3 degrees C for Hyderabad, (vii) 2.8 degrees C and 3.1 degrees C for Ahmedabad, (viii) 1.9 degrees C and 2.4 degrees C for Surat, and (ix) 0.8 degrees C and 1.3 degrees C for Pune is noted. Further, all incipient megacities showed a mean annual growth rate of nighttime SUHII of over 0.007 degrees C/year, substantially greater than in the megacities. High SUHII magnitudes, greater growth rates of SUHII, and huge populations, severely compounds the vulnerability of Indian cities to excessive heat exposure risk, especially during MAM heatwaves. Lastly, the implications of nighttime SUHII findings from the present study, on the increase in heat stress, the loss of labor productivity and the rise in heat-related mortality rate is emphasized. The study recommends implementation of city-specific action plans to mitigate the heat stressed urban environment. Targeted use of cooling strategies in localized hotspots within the urban areas where high intensity SUHIs are likely to form is also suggested.