The India Meteorological Department has forecast an unusually high number of heatwave days from April to June, 2024. Credit: S. Priyadarshini

Searing heat gripped India earlier than usual for the third summer in a row, with vast swathes stewing in humid conditions. As climate change makes extreme heat stretches more common, longer, and more severe, scientists warn of impacts that could put more than a billion lives at risk.

With national elections underway, the heat wave has affected people more acutely than in 2023, the warmest year on record. The India Meteorological Department has forecast an unusually high number of heatwave days from April to June.

There is no common global index on what constitutes a heat wave. According to the India Meteorological Department, a heatwave in India means temperatures exceeding 40°C in low-lying terrains, or 30°C in the mountains. The agency has launched a Heat Index, which shows how it feels like when humidity is factored in.

While severe heatwaves cause more immediate deaths, the milder ones1 claim more lives over time as they are more common, says Tirthankar Banerjee at the Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi.

“We may need to lower our (heat) warning thresholds,” says Banerjee.

Banerjee and co-authors mapped the health risks of heat waves in India by looking at excess mortality in ten cities in different climate zones between 2008 and 2019. They considered wind speed, solar radiation, temperature, and humidity to develop a more helpful early warning system. “Together they contribute to thermal comfort, a measure of how the body experiences these atmospheric conditions,” Banerjee says.

Tweaking regional heat action plans (HAP) based on dry and moist heat stress could provide some relief, says meteorologist, Rajib Chattopadhyay, at the Pune-based Indian Institute of Tropical Meteorology. His study2 showed a 30% increase in moist heat strength between 1980 and 2020. In the past 70 years, the areas under heat stress have grown by 30% to 40%.

Chattopadhyay and colleagues studied the mechanisms of moist and dry heatwaves. They found that dry heat has worsened over most parts of India. Climate change-triggered moisture, especially in south-east India’s coasts, is making it feel even hotter in the last 40 years, says IMD’s D.S. Pai, a study co-author.

Predicting heatwaves can help make better climate-adaptive heat action plans. “So far, IMD warnings have helped schools decide when to shut down during the sweltering hot months. Instead, we can tailor warnings to indicate specific times in the day to stay outdoors or indoors based on heat stress,” says Chattopadhyay.

Improving heat advisories

Heat advisories that advocate ‘staying indoors’ may not be helpful for families that experience a higher temperature within their homes than outside during the day, according to a 2022 study3 that used indoor temperature loggers in rural and urban centres in India, Pakistan, Bangladesh.

In these South Asian countries, people in dense, low-income neighbourhoods, with no open green spaces, remain unsheltered from heat even at night, which is why more sophisticated heat indices4 or thermal comfort forecasts, not just temperature thresholds, should guide HAPs.

In a decade since the launch of India’s first HAP in Ahmedabad city in Gujarat in 2013 — which saved more than 1000 lives5 nearly 40 such strategies have been created across states, cities and civic bodies.

Ahmedabad has now included night time temperatures in its HAP as they have risen in recent years, says Dileep Mavalankar, a former director at the Indian Institute of Public Health Gandhinagar who co-created the Ahmedabad HAP. “In India, we don’t count deaths from heat. Having that data will help with better cut-offs for early warnings,” he says.

Heatwaves don’t work in isolation

Intense humid heatwaves also bring heavy rainfall because of clustering of extreme climate systems, says hydrologist, Poulomi Ganguli at the Indian Institute of Technology Kharagpur, highlighting the importance of stable building designs. “Humid heat is considered a trigger for strong short rain bursts, which makes a difference to roof designs,” Ganguli says.

In north-central India, climate change-linked shifts in air currents high in the atmosphere made heatwaves more persistent and longer from the late 1990s, says climate attribution expert, Arpita Mondal, at the Indian Institute of Technology, Bombay. “What is very clear is that global warming is playing a role in the background. Understanding these mechanisms helps us tweak weather prediction models to improve predictions,” she adds.

Mondal and colleagues are now teasing apart the relationship between air pollution and heat stress.

How heatwaves form over India

A chain reaction of events6 in the atmosphere and oceans sets of heat waves over north-central and east coast of India. Persistent hot, dry conditions over the wheat-growing north-central region, like the event in 2015 that claimed more than 2500 lives, are linked to high-pressure systems that become stationary over north of India for an extended period. This ‘blocking’ disrupts the usual flow of weather patterns, with slow-traveling planetary waves called Rossby waves transferring the heat.

“Like ripples in a pond, the heat makes the air sink and get hotter, causing a heatwave,” says earth scientist Madhavan Rajeevan. Clear skies, lack of rains and dry soil also play a role. Lesser than usual cool breeze from the sea triggers heatwaves over India's eastern coast. “Without the cool breeze, the heat gets trapped,” says Rajeevan.

El Niño, which caused the ocean temperature to rise, and a high-pressure system that blocked moist sea breeze from the Bay of Bengal and Arabian Sea, saw record high April temperatures in many parts of India this year. A heatwave also impacted the southwest coastal state of Kerala, where such events are rare.