This April, more places in India saw mercury over 45 degrees in a decade

Heatwaves in India: New study shows record-breaking heat to become more common. Exposure to extreme heat can cause heat exhaustion or heat stroke

The heat waves in March and April 2022 were record breaking; of the last 12 years for which we accessed data from the India Meteorological Department (IMD), April 2022 had the highest number of weather stations recording more than 45 degrees Celsius (°C) maximum temperature.

Temperatures crossing 45°C is one, although not the only, criteria for declaring a heat wave over a region.

April 2022 also had the most number of times these stations recorded over 45°C. For example, in April 2010, 11 weather stations had crossed 45°C 23 times. One weather station crossing 45°C on one day is counted as one instance. In 2019, 13 weather stations together crossed this heat threshold 37 times in April. In 2022, 25 stations crossed this threshold, as many as 56 times in all. This, while the total number of weather stations in the country (204) has not changed since 2010.

In terms of the number of times a region was declared to be experiencing a heat wave, April 2022 recorded the second-highest (146) heat waves and severe heat waves after 404 in 2010.

Several weather stations broke their all-time maximum temperature records in April 2022 and studies show this will become more common henceforth. A record-breaking heat wave in north-west India and Pakistan is now over 100 times more likely because of climate change, said a study by the United Kingdom's national meteorological service published on May 18, 2022.

Exposure to heat waves can cause health issues such as heat exhaustion and heatstroke, and even deaths, impact worker productivity, and agricultural productivity, and make a country more prone to food insecurity.

Unlike other extreme weather events, heat waves last much longer and have a protracted impact. A cyclone usually impacts a region for a day but heat waves could last for up to 15, we had reported in June 2020.

A summer like no other

If a weather station experiences a maximum temperature of 45°C or above, or the temperature is 4.5°C to 6.4°C above normal, IMD declares it a heat wave. The criteria for a severe heat wave are a maximum temperature of 47°C or above, or temperatures 6.4°C higher than normal, and thus a place may experience a heat wave even though its absolute temperature may be under 45°C. The maximum temperature has to be at least 40°C for the plains, 30°C for hilly regions and 37°C for coastal stations for the IMD to declare a heat wave.

Many parts of the country had high daytime temperatures in March itself and India's average maximum temperature in that month was the highest in 122 years, at 33.1°C, breaking the previous record of 33.09°C in March 2010.

The trend continued in April when average maximum temperatures in northwest India and central India were also the highest ever in 122 years. Even the country as a whole recorded the third-highest average maximum temperature this year. The highest was in 2010, and the second-highest in 2016.

In April, Rajasthan's Ganganagar crossed 45°C six times, three of which were consecutively on April 28, 29 and 30. Between April 20 and 30, Maharashtra's Chandrapur crossed 45°C five times. The same happened with Jharkhand's Daltonganj between April 18 and 30, data accessed from IMD's Climate Research and Services division show.

The unusual April heat caused some stations to break their all-time maximum temperature records. Some of these include Daltonganj, Allahabad, Jhansi, Lucknow, Dharamsala, Alwar, Jaisalmer and Panchmarhi.

No weather station in India had recorded a temperature of 45°C or above in April 2020 and April 2021. Even between 2011 and 2015, there were very few stations that crossed 45°C.

"In 2020 and 2021, there were zero stations recording 45+ because they were outlier years. These were the only summers when there were two consecutive cyclones, the easterly winds in the Gangetic plains, and western disturbances were very activated," explained R.K. Jenamani, senior scientist at the IMD.

Even hill stations of Dharamsala, Panchmarhi and Madikeri were unusually warm in April 2022. In Maharashtra's Vidarbha, known for its hot weather, maximum temperatures were above normal on all days, other than four. Last month was also the second-hottest April in 72 years for Delhi with an average maximum temperature of 40.2°C, less by 0.2°C from 40.4°C in 2010.

"March experienced the highest number of heat wave incidents this year since 2010 but for April, 2010 still remains highest mainly because eastern parts of the country had experienced such conditions (on a large scale at that time). This year, only northwest and central India experienced more such incidents (making it second-highest)," said Mrutyunjay Mohapatra, director general of the IMD, in a presentation given to the media.

Mohapatra attributed the unusual summer heat this year to deficient rainfall. "East, northeast and south India have got very good rainfall activity which did not allow temperatures to rise whereas northwest and central India had scanty rainfall," he said.

Mohapatra said that one cannot quantify the impact of climate change but it is certainly there and data of the last 50 years show an overall rising trend in maximum temperatures.

Warming climate

The science has been clear about warming temperatures for a while now.

According to a Ministry of Earth Sciences report titled 'Assessment of Climate Change over the Indian Region', the all-India average frequency of summer heat waves will increase to about 2.5 events per season by the mid-21st century (2040–2069), with a further rise to about three events by the end of the 21st century (2070–2099).

"The recent high temperatures (April) in India were made more likely by climate change. Before human activities increased global temperatures, we would have seen the heat that hit India earlier this month around once in 50 years. But now it is a much more common event--we can expect such high temperatures about once in every four years," said Mariam Zachariah, Research Associate at the Grantham Institute at Imperial College London, in a press briefing by Climate Trends, a climate communications organisation. "Until net [greenhouse gas] emissions are halted, it will continue to become even more common."

Zachariah's latest analysis of the heat waves with colleague Friederike Otto had found that the heat that hit India is already much more common as a result of higher global temperatures caused by human activities.

Their analysis was echoed by another study that the UK's meteorological office released on May 18.

The study shows that the natural probability of a heat wave exceeding the average temperature of 2010 (which was the highest combined average April and May temperature since 1900) is once in 312 years. In the current scenario, accounting for climate change, the probability increases to once in every 3.1 years. And by the end of the century, the study, which incorporated climate change projections, showed that this will increase to once every 1.15 years.

Although a new record is thought likely, climate scientists will have to wait until after the end of May, when all the temperature records for April-May will have been collated, to see whether the current heat wave will exceed the levels experienced in 2010, a note on the UK study said.

Exposure to heat waves can cause heat exhaustion, resulting in fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating. It could also cause heat strokes, with symptoms including a body temperature of 40°C or more along with delirium, seizures or coma, a potentially fatal condition. Between 1992 and 2015, heat waves caused 24,223 deaths across the country. India could be undercounting deaths from heat strokes, we had reported in June 2020.

Further, for every 1°C rise in temperature beyond 27°C on a hot day in India, productivity of workers drops by as much as 4%. Increasing temperatures also means that crop turnaround time is impacted, and some crops will mature faster and lose their nutrient content, potentially leading to undernutrition and malnutrition, IndiaSpend had reported in October 2021. Every 1°C rise in temperature increased the probability of severe food insecurity by 1.64% in 2019, we had reported.

"The data available for the coming decades for the Indo-Pak region clearly shows that the number of heat waves, their duration, area covered and intensity are all going to increase," said Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology. "We have heat wave forecasts which help in quick measures but we need long-term policies tailored to specific sectors such as agriculture, labour to help people in those regions worst hit by heat waves."