In a groundbreaking study titled ‘Impact of heatwaves on all-cause mortality in India: A comprehensive multi-city study,’ researchers have unveiled alarming findings regarding the consequences of prolonged, high-intensity heatwaves on mortality rates in major Indian cities. The cities examined include Delhi, Mumbai, Chennai, Hyderabad, Bengaluru, Ahmedabad, Pune, Varanasi, Shimla, and Kolkata.
Key Research Insights:
The study reveals that days characterized by high mean temperatures—exceeding those recorded on 97% of the days in a year—result in a noteworthy 12.2% surge in daily mortality rates when observed for a single day. This percentage escalates to 14.7% over two consecutive days, reaching 17.8% for three days. If these elevated temperatures persist for an uninterrupted five-day span, the daily mortality rate can surge dramatically to 19.4%.
Furthermore, the study underscores a profound impact, with the mortality rate soaring to a staggering 33.3% when extreme heatwave conditions—exceeding temperatures recorded on 99% of days in a year—prevail for five consecutive days. Cities such as Ahmedabad, Chennai, Delhi, Hyderabad, Kolkata, and Varanasi have witnessed such extreme heatwave conditions.
Policy Implications and Recommendations:
Researchers emphasize the urgency of incorporating both definitions and the associated burden of each heatwave into policymaking and decision-making processes. The study highlights a paradoxical situation: longer and more intense heatwaves pose a higher mortality risk but result in a comparatively lower burden of heatwave-related deaths.
Contextualizing the Climate Scenario in India:
According to the India Meteorological Department (IMD), a heatwave is declared when the maximum temperature at a weather station reaches a minimum of 40 degrees Celsius in the plains and 30 degrees Celsius in hilly regions. Additionally, a departure from normal temperatures ranging between 4.5 to 6.4 degrees Celsius is classified as a heatwave, escalating to a severe heatwave when the departure exceeds 6.4 degrees Celsius.
India traditionally experiences heatwaves from March to June, occasionally extending into July. Recent years have witnessed an escalation in the intensity and duration of these heatwaves, with notable occurrences in 2016, 2018, 2019, and 2023, as per IMD data.
A 2023 analysis by scientists from the Department of Science and Technology’s Mahamana Centre of Excellence in Climate Change Research (MCECCR) projects a substantial four-to-seven-fold increase in heatwave frequency, with potential mid to long-term implications ranging from five to tenfold.
Over the period from 2008 to 2019, the study identified 168 heatwaves, averaging three per year across all cities. Hyderabad and Pune recorded the highest average number of heatwaves, while Chennai and Varanasi reported the lowest. Delhi and Shimla exhibited the highest ‘intensity’ or percentage increase in temperature above the daily mean temperature average, with intensities of 3.0% and 3.8%, respectively.
These findings necessitate proactive measures to address rising temperatures, ensuring both public health and environmental sustainability.
Note: The study was published in the peer-reviewed Environmental International journal.
