Heat-waves of the Indian subcontinent

The Indian picture

India’s prolonged and intense heatwaves are a direct impact of rapidly expanding industrialization and urbanization. As a consequence, the Urban Land Cover Change is getting disturbed, simultaneously increasing both Land Surface Temperature and air temperatures which in turn is affecting the surface climate. There was an increase of about 0.7 °C between 1901 and 2018, and if the current pace of pollution continues, there will be a positive shift in the global average temperature by almost 5 °C by the end of this century.
With 2016 and 2020 being the hottest years in the last century, our country is facing new temperature-breaking records. Unstable climatic conditions and associated anthropogenic activities can adversely affect intensity, frequency and duration of heat waves. Even Indian states like Himachal Pradesh and Kerala, which have not seen heat waves in the past, are frequently exhibiting extreme temperatures nowadays. Deaths due to heat wave are also frequent (with >1300 were reported in Ahmedabad in 2010). Karnataka is becoming increasingly hot and is predicted to be 2.0 °C hotter by 2030, making the region more vulnerable to severe heat extremes, posing a higher risk to the vulnerable population.

The reasons for differential heatwaves in India

India is one of the most vulnerable nations to be affected by heatwaves, largely due to its tropical and subtropical climatic conditions. Heatwaves are defined based on temperature thresholds and deviations from average temperatures by the Indian Meteorological Department (IMD). The unique geographical scenario of the country sees multiple ranges of maximum temperatures to be declared as heatwaves. For example, in the plains, a heatwave is declared when maximum temperatures reach 40°C or higher, while the threshold is set at 30°C in mountainous regions. If the temperature rises by 5°C–6°C above normal in areas with an average maximum temperature of 40°C or lower, it is considered to be a heatwave by IMD; a severe heatwave is classified as an increase of 7°C or more. For regions where normal temperatures exceed 40°C, a 4°C–5°C rise constitutes a heatwave, and a 6°C or higher deviation marks a severe heatwave. None the less, temperatures of 45°C or more can also trigger a heatwave declaration irrespective of regional averages.

The ‘new normal’ temperature rise

India experiences heatwaves from mid-March through June, sometimes extending into July. According to IMD data, India suffered a hottest decade between 2001–2010, with an average temperature deviation of 0.4°C compared to previous decades. The years 2015 and 2019 experienced some of the long-lasting heatwaves, with temperatures over 40°C persisting for several weeks. With extreme temperatures arriving earlier than usual for the consecutive years, heatwaves in our country are becoming common place. As climate change intensifies, extreme heatwaves are expected to increase in duration and severity, impacting over a billion lives. To help the public better understand the effects of extreme heat, the IMD recently introduced a Heat Index, accounting for both temperature and humidity.
While severe heatwaves can cause immediate fatalities, milder ones often claim more lives over time due to their frequency, notes an environmental scientist from the Institute of Environment and Sustainable Development. Emphasis was given on the potential need to lower heat warning thresholds to protect public health better. By examining factors like wind speed, solar radiation, temperature, and humidity, early heat warning systems can be improved, accounting for "thermal comfort", a measure of how the body perceives atmospheric conditions. A meteorologist from the Indian Institute of Tropical Meteorology in Pune suggests that tailoring regional heat action plans (HAP) to dry and moist heat stress variations could offer relief.