Intense heatwaves increase mortality rate, says new study on 10 Indian cities

In India, heatwaves typically occur from March to June, and in some rare cases, even extend till July

New Delhi: High intensity heatwave days for long durations can significantly increase the mortality rates in Indian cities, a new study has shown.

A woman cools off at Fontana della Barcaccia at the Spanish Steps in Rome during a heatwave across Italy in July, 2023. (Reuters)

A study titled ‘Impact of heatwaves on all-cause mortality in India: A comprehensive multi-city study’ analysed 10 cities in India—Delhi, Mumbai, Chennai, Hyderabad, Bengaluru, Ahmedabad, Pune, Varanasi, Shimla and Kolkata—to assess how high intensity heatwaves that continues consecutively for a long duration can directly impact mortality in cities. The study conducted by several experts was published earlier this month in the peer-reviewed Environmental International journal.

The study found that high mean temperatures—referred to as temperatures that are higher than 97% days in a year—when recorded for a day are associated with a 12.2% increase in daily mortality. However, when it continues for two consecutive days the daily mortality goes up 14.7%; for three days it becomes 17.8% and when such high temperatures continue for a continuous duration of five days, the daily mortality can increase up to 19.4%.

The mortality rate can go up to 33.3% when extreme heatwave conditions are recorded—referred to as temperatures that are higher than 99% days in a year—for five consecutive days, the study highlighted. Such conditions, the study noted, were observed in Ahmedabad, Chennai, Delhi, Hyderabad, Kolkata and Varanasi.

“We found strong evidence of heatwave impacts on daily mortality. Longer and more intense heatwaves were linked to an increased mortality risk, however, resulted in a lower burden of heatwave-related deaths. Both definitions and the burden associated with each heatwave definition should be incorporated into planning and decision-making processes for policymakers,” the study read.

According to the India Meteorological Department (IMD), it is called a heatwave when maximum temperature of a weather station reaches at least 40 degrees Celsius in the plains and at least 30 degrees Celsius in hilly regions. It is also classified as a heatwave when the maximum temperature departure from normal is 4.5 degrees Celsius to 6.4 degrees Celsius and severe heatwave when the temperature departure from normal is above 6.4 degrees Celsius.

In India, heatwaves typically occur from March to June, and in some rare cases, even extend till July. Over the last decade, the intensity and duration of heatwaves were found to have increased. In recent years extreme heatwaves were recorded in 2016, 2018, 2019 and 2023, data from IMD show.

A 2023 analysis by scientists from Department of Science and Technology’s Mahamana Centre of Excellence in Climate Change Research (MCECCR) also projects a four-to-seven-fold increase in heatwave frequency, with a potential of going up to five to 10 folds in mid and long-term period.

The study identified 168 heatwaves between 2008 and 2019, highlighting that on an average there were three heatwaves per year across all cities.

“Hyderabad and Pune had the highest average number of heatwaves per year at 4.0 and 4.3, respectively, while Chennai and Varanasi had the lowest average at 2.2 each. Furthermore, Delhi and Shimla exhibited the highest ‘intensity’ or percentage increase in temperature above the average of the daily mean temperature, with intensities of 3.0% and 3.8% respectively,” the study read.