A woman waits to fill her vessels with drinking water from a tanker, amid a water crisis due to a heat wave in New Delhi. Credit: Sonu Mehta/Hindustan Times via Getty Images

In March and April 2022 India and Pakistan, experienced one of the worst heat waves ever to hit the region, with a prolonged period of temperatures consistently 3ºC to 8ºC above average, breaking numerous records.

There were about 300 major forest fires across India, a third of them in Uttarakhand. This year, heat waves occurred as early as March in some parts and continued into April impacting eastern and southern India.

The effects of global warming are increasingly apparent as numbers and intensity of heat waves shoot up, breaking temperature records for decades. The total duration of heat waves has increased by about three days in the last 30 years and is expected to go up 12-18 days more by 2060.

In India, a heat wave is declared by the Indian Meteorological Department (IMD) when the daytime temperature is above 40ºC and 4.5ºC above average.

Between March and June, heat waves occur over the heat wave zones of central and northwest India. Other hotspots are the coastal regions of Andhra Pradesh and Odisha. These regions experience an average of two heat waves per season, lasting about 6-8 days. The intensity usually exceeds 44°C in north India, and 40°C on the east coast. However, due to higher humidity, heatwaves on the east coast have a greater impact on human health. In 2015, 2500 people died during a heat wave in the east coast.

The physical mechanisms of heat waves are well researched. They are caused by large-scale anomalies in atmospheric circulation and exacerbated by local effects such as land surface processes that affect soil moisture. Global influences such as the El Niño/Southern Oscillation (ENSO) and the Indian Ocean modulate the frequency and duration of Indian heat waves.

Heat waves are generally more frequent in El Niño years. Heat waves on the east coast are also influenced by La Niña conditions over the Pacific Ocean.

Heat waves gravely affect human health and air quality, increase energy consumption, reduce crop yields, increase water loss and intensify droughts. Urban heat islands cause disruptions in critical infrastructure networks, affect the economy through lower labour productivity, and exacerbate the impacts of other climate-related hazards such as droughts and forest fires.

Heat waves have caused more deaths in India than other natural hazards, with the exception of floods and tropical cyclones1. The mortality rate per event due to heat waves in India is about 25.

They can be predicted 5-7 days in advance but forecasting heat waves on longer time scales (at least up to two weeks) and probability outlook on seasonal time scales is also possible. Efforts are on by India’s ministry of Earth Sciences to develop and install an end-to-end, seamless forecasting system that predicts heat waves on all time scales, from short-term to seasonal.

The most effective way to reduce the negative impacts of a heat wave is to develop a comprehensive response plan that combines individual strategies into an integrated approach, encompassing cultural, institutional, technological and ecosystem-based adaptations. For example, the institutional plan could include weather forecasting, monitoring, and education and awareness. Adequate education can improve the health and safety of urban dwellers during heat waves, especially vulnerable groups such as older adults, children, people working outdoors and low-income communities.

Heat Action Plans (HAPs) by the Central and state governments prescribe preparedness and response measures to minimise the impact of heat waves. A recent report by National Disaster Management Authority (NDMA) illustrated how India reduced mortality due to heat waves by effective and coordinated implementation of these plans. But a recent study found that most HAPs are not tailored to the local context and have an oversimplified view of the hazard.

Almost all HAPs are inadequate when it comes to identifying and addressing vulnerable groups. HAPS are underfunded, they lack transparency and have a weak legal basis.

Another study2 argues that impacts of heatwaves can hinder or reverse India’s progress towards Sustainable Development Goals. India needs to assess impacts of extreme weather events better by combining layers of information with existing climate vulnerability measurement frameworks to ensure climate change events do not hinder non-climate structural SDG interventions.