How extreme heat is affecting India

Source: CEEW

For their analysis, the researchers developed a heat risk index (HRI), which assessed heat risk across 734 districts in India. The index is based on 35 indicators, including an increase in frequency of very hot days, population density, percentage of persons with disability, and change in land use and land cover.

Understanding heat risk

Contrary to common perception, heat risk is quite different from heat waves and heat stress. While heatwaves — they do not have any universal definition — usually refer to prolonged periods of abnormally high temperatures in a specific region, heat stress is when the body temperature exceeds 37 degrees Celsius in humans and animals. At this temperature, the body is not able to effectively remove excess heat, which can lead to discomfort, heat cramps, and exhaustion. If the body temperature crosses 40 degrees Celsius, a heat stroke can occur.

Heat risk, on the other hand, is essentially the probability of experiencing heat-related illnesses or death due to exposure to extreme temperatures. It depends on three crucial factors: “the intensity of the heat (and its compounding effects such as humidity); the degree of exposure; and the underlying vulnerabilities of affected communities,” according to the CEEW study.

Factors driving heat risk

RISE IN NUMBER OF VERY WARM NIGHTS: In the years between 2012 and 2022, the number of very warm nights has increased faster than very warm days, the study found. More than 70% of districts witnessed five or more additional very warm nights per summer (March to June) during this period.

“These nights and days are defined as periods when minimum and maximum temperatures rise above… what was normal for 95% of the time in the past,” it said.

Source: CEEW

This is concerning because higher temperatures during the night make it difficult for the body to cool down after intense daytime heat, which can lead to a rise in health risks such as heat strokes and worsening of non-communicable diseases like diabetes and hypertension.

INCREASE IN RELATIVE HUMIDITY IN NORTH INDIA: The study found out that between 2012 and 2022 the relative humidity — the amount of water that is present in the air compared to the greatest amount it would be possible for the air to hold at that temperature — has significantly increased in North India, particularly in Indo-Gangetic Plain.

While North India used to have around 30–40% relative humidity during the baseline period between 1982 and 2011, it increased to 40-50% between 2012 and 2022.

Higher relative humidity is an issue as it exacerbates heat stress on the human body, especially during the peak summer months. This happens because high relative humidity makes it harder for the body to cool down through sweat after the body temperature crosses 37 degrees Celsius. “This can increase the occurrence of several heat-related illnesses at a faster rate,” the CEEW study said.

Source: CEEW

HIGH POPULATION DENSITY & INCREASE IN URBANISATION: Apart from very warm nights, very warm days, and relative humidity, several other factors also influence heat risk. For instance, the study found that districts with high population density such as Mumbai and Delhi face the highest exposure to extreme heat.

Districts, especially tier II and III cities such as Pune, Thoothukudi and Gurugram, which have seen rapid urbanisation in recent years have witnessed hotter nights. That is because of the emergence of concrete infrastructure which absorbs a lot of heat during the day and releases it during the night.

Districts in states such as Andhra Pradesh, Maharashtra, Haryana, Punjab, Chhattisgarh, Bihar, and Uttar Pradesh are highly vulnerable to extreme heat. This is due to the combined impact of high temperatures and socio-economic and health vulnerabilities such as more elderly people and a high prevalence of non-communicable diseases like diabetes, according to the study.

The significance

The year 2024 was the warmest year on record for the world, including India. While the global average annual mean temperature during the year was more than 1.5 degrees Celsius higher than pre-industrial levels (1850-1900 period), the temperature over India in 2024 was about 1.2 degrees Celsius higher than the 1901-1910 average.

Although the temperature rise in India is lower than the increase over the world, the country is already witnessing adverse effects of global warming. For instance, in 2024, India saw its longest recorded heatwave since 2010. There were more than 44,000 cases of heatstroke in the country last year.

The situation has been worsened as India’s heat action plans (HAPs) — early warning systems and preparedness plans for extreme heat events — remain lacking in some ways. For instance, most of the HAPs put forth by multiple Indian cities lack long-term strategies to tackle the growing threat of extreme heat in the country, according to a study, published in March this year, by the Sustainable Futures Collaborative (SFC), a New Delhi-based research organisation. The analysis also said that the cities having such strategies did not implement them effectively.

Such gaps in planning could result in a higher number of heat-related deaths due to more frequent, intense, and prolonged heatwaves in the following years, according to the study.