NEW DELHI: A recent study by the Centre for Science and Environment (CSE) highlights a concerning trend in urban heat stress across major Indian cities, with Delhi experiencing significant thermal discomfort due to a deadly combination of air temperature, land surface temperature, and relative humidity.
This analysis, which CSE has been conducting since 2001, covered metropolitan areas including Delhi, Mumbai, Chennai, Kolkata, Bengaluru, and Hyderabad. The study reveals that the unprecedented heatwave affecting these cities is exacerbating the urban heat island effect. In Delhi, the heat stress is intensified not just by rising temperatures but also by high humidity levels. The analysis, focusing on day-and-night-time temperature trends, humidity levels, seasonal variations, land surface temperature, and built-up areas, found that while Delhi’s average temperatures in March and April were lower compared to previous years, the humidity levels were significantly higher.
“Delhi’s summertime has registered a 0.6 degree Celsius lower decadal average ambient air temperature, but relative humidity has increased by 8% between 2001-10 and 2014-23. High humidity is responsible for adding to heat stress… An average 3.3°C of heat stress is being added,” the study found.
The report also highlighted the diurnal cooling trend, noting a 9% decrease in the cooling of land surface temperature between day and night. This phenomenon contributes to a stronger urban heat island effect at night, where peri-urban areas cool down by 12.2 degrees Celsius, compared to only 8.5 degrees in the city core. Moreover, the study established a direct correlation between the increase in built-up areas and the rise in urban heat stress. The built-up area in Delhi increased from 31.4% in 2003 to 38.2% in 2022. Despite an increase in green cover from 32.6% to 44.2% over the same period, this greening has not mitigated the night-time temperatures or the escalating heat index.
Interestingly, the study also pointed out that the monsoon season has become more thermally uncomfortable. The average heat index during the monsoon is now 9.4 degrees Celsius higher than during the pre-monsoon period. The study defines summer as the period from March to August, subdividing it into pre-monsoon (March-May) and monsoon (June-August). According to the CSE, cities in warm-humid and moderate climate zones have seen temperature increases, while those in composite and hot-dry climate zones witnessed a decline. Overall, the ambient air temperature across the studied cities changed by less than 0.5°C between 2001-10 and 2014-23.
This comprehensive analysis underscores the urgent need for urban planning and infrastructure development that prioritises climate resilience. Addressing the dual challenges of rising temperatures and increasing humidity is critical to mitigating heat stress in Delhi and other major cities across India. Times View: Municipal authorities must prioritise protecting trees and increasing green cover as central to urban planning efforts. Trees play a crucial role in combating climate change, mitigating extreme weather conditions, and improving air quality. Conducting a tree census and implementing urban greening initiatives are essential steps towards safeguarding our cities against escalating heat stress.
