Anthropogenic forcing exacerbating the urban heat islands in India.

Urban heat island (UHI) phenomena is among the major consequences of the alteration of earth's surface due to human activities. The relatively warmer temperatures in urban areas compared to suburban areas (i.e. UHI) has potential health hazards, such as mortality due to high temperatures and heat waves. In addition, UHI situation demands more energy (e.g. fans and air-conditioners) that would trigger greenhouse gas emissions. Studies on UHI intensity help to assess its impact on urban population, city planning, and urban health planning. This is particularly important for a country like India, where 32% people (~7% of total world population), live in urban areas. We conducted a detailed study on surface UHI intensity (SUHII), which is the difference between urban and surrounding rural land surface temperatures, across all seasons in 44 major cities of India, which shows that mean daytime SUHII is positive (up to 2 °C) for most cities, as analysed from satellite temperature measurements for the period 2000-2017, in contrast to previous studies. However, although statistically insignificant, most cities show a positive trend in SUHII for monsoon and post-monsoon periods, but negative for winter and summer seasons. The increasing night-time SUHII in all seasons for most cities suggest increasing trend in temperature in cities due to the impact of the rapid urbanisation, and thus, suggesting the influence of anthropogenic forcing on SUHII. This is also supported by the analysis of aerosols, night lights, precipitation and vegetation in the study regions. Therefore, this study shall aid planning and management of urban areas by giving insights about the effects of nature and intensity of development, land cover and land use mix and the structure of cities on SUHII.