Robert Muir-Wood,
Chief Research Officer, Moody’s RMS
Mitigation is the only route to relieve the heat stress, and countries that are used to the heat benefit from previous experience and may have the infrastructure – from building design to designated cooling centers, and awareness campaigns on what to do, and not do, in the heat
The dictionary definition of a heatwave is ‘…a period during which the weather is much hotter than usual.’ The ‘usual’ can vary widely by latitude.
For Sweden, a heat wave is declared when daily high temperatures exceed 25 degrees Celsius (77 °F) for at least five days in a row. In Greece, a heat wave is declared when there are more than three consecutive days with temperatures at 39 degrees Celsius (102 °F) or higher.
As we move to lower latitudes and hotter climes, for the plains of India, a heat wave is declared if the maximum temperature of two weather stations reaches at least 45 degrees Celsius (113 °F) or more over two consecutive days; a severe heat wave is declared if the maximum temperature reach over 47 degrees Celsius (116.6 °F).
So, how can we classify what has happened in India? During April, May, and now June, many parts of central and northern India experienced either ‘heatwave’ or ‘severe heatwave’ conditions.
Mungeshpur: Record Temperature?
In the northern Indian states of Punjab, Rajasthan, and the capital Delhi, heatwave conditions have been a constant feature through May and June.
According to the India Meteorological Department (IMD), between May 16 and June 18, the average maximum temperature for nearly 40 percent of India was more than 40 degrees Celsius (104 °F).
Average maximum temperatures were more than two degrees Celsius higher for the period across 32 percent of the country compared to the 1981-2010 average for the same period.
The capital city of Delhi had an average maximum temperature of 44.5 degrees Celsius (112.1 °F) over this period, and Delhi recorded 40 consecutive days of temperatures at 40 degrees Celsius or over, only broken on June 22 as the temperature dipped to 39.4 degrees Celsius.
Over multiple days in late May, temperatures in northern India reached over 50 degrees Celsius (122 °F). It was so hot, the heat impacted the weather stations, as in Mungeshpur, a district of northwest Delhi, a potential new record for India was recorded on May 29 at 52.3 degrees Celsius (126 °F). The same district holds Delhi’s heat record, at 49.2 degrees Celsius (120 °F). set in May 2022.
But the IMD later said the record in Mungeshpur was an error and potentially three degrees Celsius less, as the weather station equipment can malfunction in extreme temperatures.
During May alone, more than 200 people had died and some 20,000 had been hospitalized with heat stroke in India. On June 1, 33 election officials died of heatstroke in the state of Uttar Pradesh.
Farm animals have died in the heat, government employees working outdoors had their shifts modified to avoid temperatures over 40 degrees Celsius, and residents chased water tankers in Delhi as water shortages limited supplies. Although humidity was only around 25 percent, due to the extreme heat, wet bulb temperatures exceeded safe levels.
But for cities such as Delhi, its officials swung into action for days on end, providing free water at main transport points, spraying water on roads and from the top of high buildings to bring down the ambient temperature.
Heat Domes
Whether it is Mexico, Bangladesh, Pakistan, or Thailand, this year these countries and more have seen temperatures reach above 50 degrees Celsius, with prolonged daily highs of between 40 and 50 degrees.
In the U.S., stalled areas of high pressure in the upper atmosphere have seen warm air pushed toward the surface and trapped, a phenomenon better known as a heat dome, impacting most states. Since early June, temperatures have regularly peaked between 37.7 degrees Celsius (100 °F) and 43.3 degrees Celsius (110 °F) starting with the Western and Southwest states.
As June progressed, the heat domes evolved and by June 23, heat alerts impacted 115 million people almost from coast to coast, and peak temperatures of 32.2 degrees Celsius (90 °F) and more have returned day after day.
Unusually, states in the Midwest and Northeast have seen temperature records broken; Caribou in Northern Maine hit 35.5 degrees (96 °F) on June 19, matching its all-time high, and a heat index of 39.4 degrees Celsius (103 °F), the highest on record. Heat dome conditions in the U.S. are set to remain into July, with the heat dome making its next move to the south.
For countries in southeast Asia they look to the relief provided by monsoons in the months ahead. Although later than normal, India’s vital monsoons have started, but even with monsoons, heat combined with humid conditions can bring severe risk as wet bulb temperatures climb.
Overall, worldwide average temperatures in 2023 reached record levels, with some months achieving more than half a degree centigrade above the previous monthly high. Rising temperatures associated with climate change have combined with a vast outburst of heat from the El Niño event that raised temperatures across the equatorial Pacific Ocean.
Higher averages mean more, longer, and stronger heat waves.
Importance of Mitigation
Mitigation is the only route to relieve the heat stress, and countries that are used to the heat benefit from previous experience and may have the infrastructure – from building design to designated cooling centers, and awareness campaigns on what to do, and not do, in the heat.
But at such severe heat levels, especially when wet bulb temperatures that accommodate humidity and heat, get close to human body temperatures – humans experience debilitating exhaustion above 30 degrees Celsius.
The risk of death rises through wet bulb temperatures of 35 degrees Celsius, as the body’s system to dissipate heat through sweat no longer functions. Without the provision of rapid cooling relief, healthcare systems can become overwhelmed.
The young, elderly, and those with pre-existing conditions need protection in extreme wet bulb temperatures. Outdoor workers cannot work through the middle of the day and lose productivity.
With the poorest without power or water, keeping cool is challenging. Air conditioning brings relief but is not universally available and excess use of electricity leads to brownouts, which can mean in extreme conditions cooling becomes unavailable.
We are now conditioned to see temperature records regularly broken. While short-lived heat waves of a day or two can be managed, the long and severe heat conditions recently experienced in India, bring extreme daily temperatures over 40 degrees Celsius for weeks. This can disrupt many sectors including agriculture, healthcare, distribution, education, and manufacturing.
The Reserve Bank of India (RBI) has set out to quantify the impact of heat stress on the economy, issuing a report in 2023. In the next 30 years, even moderate projections show the length of heat waves increasing by a factor of five.
The most severe projections indicate ‘heat-related deaths 25 times higher than 1990’ (CMCC 2021). Those longer heatwaves will destroy rice and grain crops, too ‘…costing farmers as much as 15 percent in lost income by 2050.’
Quoting World Bank statistics from 2022, the RBI report states that India could account for 34 million of the projected 80 million global job losses from heat stress-associated productivity decline by 2030.
In the near future, the report outlines that up to 4.5 percent of India’s GDP could be at risk by 2030 owing to lost labor hours from extreme heat and humidity conditions.
What was exceptional is now becoming commonplace. And still, the records will continue to be broken. No longer short-duration waves, we should retire the term ‘heatwave’ and maybe replace it with ‘heat-dome’ recognizing how heat links with persistent anticyclones, trapping the dangerous temperatures.