When the weather gets hot enough to kill

From South Asia to the US, from Africa to Europe, intense heat is pushing the human body beyond the bounds of physiological tolerance.
United States special presidential envoy for climate John Kerry launches the Climate Action and Finance Mobilisation Dialogue (CAFMD) under India-US Climate and Clean Energy Agenda 2030 Partnership, in New Delhi, India, September 13, 2021. REUTERS/Anushree Fadnavis

On an April day in 1905, the scientist J.S. Haldane descended hundreds of feet into a Cornish tin mine to find out if he could cook himself to death.

Amateur researchers had long known that humans have an extraordinary ability to withstand dry heat. One 18th century experimenter found he could tolerate temperatures up to 115 degrees Celsius (240 Fahrenheit), hot enough to cook steaks. But the moist, saturated air in the Dolcoath mine, dug through hot rock deep below the water table, seemed to change things. Though the temperature never climbed above 31.5C, Haldane’s body temperature and pulse rose with each minute, hitting feverish levels before he ascended after three hours. “It becomes impracticable for ordinary persons to stay for long periods” when the humid temperature rises above 31C, he wrote.
That finding hasn’t significantly changed over the years since — but our atmosphere has.
As the climate warms, conditions once experienced only in saunas and deep mineshafts are rapidly becoming the open-air reality for hundreds of millions of people, who have no escape to air conditioning or cooler climes. After a few hours with humid heat above 35C — a measure known as the wet-bulb temperature — even healthy people with unlimited shade and water will die of heatstroke. For those carrying out physical labor, the threshold is closer to Haldane’s 31C, or even lower.
Brajabandhu Sahu knows the physical signs all too well. A street vendor selling foods like dosa, idli and uttapam on the corner of two busy roads in Bhubaneswar, the capital of the eastern Indian state of Odisha, he’s surrounded at times by what feels like a wall of fire from which he cannot escape. When the day is at its hottest, his head spins, his heart races, his skin blisters and the waves of nausea are constant. The moisture-laden winds that blow in from the Bay of Bengal put citizens in this region at particular risk.
India’s humidity rises before the heat recedes in the buildup to the monsoon in mid-June. It’s at this critical intersection that the danger to human health is greatest. Conditions in Bhubaneswar reached 37 degrees and close to 80% humidity in the week Bloomberg visited last month. If both maximums occur at the same time, that’s equivalent to a wet-bulb temperature of nearly 34C.
In the small outpatient department of the brightly painted Mangalpur Community Health Center — in a farming district that lies halfway between the capital and the coastal city of Puri — medical staff have established a special heatstroke room with one main aim: to quickly bring a patient’s body temperature back down to a safe level. The doctor on duty, Suman Pradhan, sees two or three cases every day, mostly farmers who’ve spent too long in the field, some arriving unconscious or disoriented.
The conditions inside people’s homes are often also hazardous, she says, because there is nowhere they can go to cool down. Even though there’s such a high incidence, the population remains mostly unaware of the risk. “They don’t understand the danger of heat — you can reach a really critical stage where the body won’t respond and the surface temperature won’t reduce. We are seeing that incidence increase year by year,” she says.
India is not alone. An unusually early and intense heatwave spread up from North Africa through Europe this summer, pushing temperatures in some parts of Spain and France more than 10°C higher than the seasonal average and breaking many monthly records. By mid-June, nearly one-third of the US population was under a heat advisory, with some areas reporting temperatures of over 100F.
By mid-June, nearly one-third of the US population was under a heat advisory
As the climate warms, each year gets us closer to the tipping point where large swathes of the planet are exposed to dangerous humid-heat temperatures. The trouble is, we have very little information about just how close we’re getting, because of the poor quality and availability of data. Unlike dry-heat and humidity daily maximums, hour-by-hour wet-bulb measurements are rarely part of the standard set of information produced by national meteorological bureaus.
Until very recently, scientists were equally in the dark about how rapidly the risks were rising. Scenarios where wet-bulb heatwaves could cause mass deaths were a subject for careful, peer-reviewed long-range climate forecasts, or for science fiction (the 2020 novel The Ministry for the Future, by Kim Stanley Robinson, begins with such a disaster killing about 20 million in India in a week).
There’s an “apparent ceiling” on humid heat, according to one 2010 study, because when levels get too high they induce storms, cooling the atmosphere down again. A 2003 incident in the Saudi Arabian oil port of Dhahran, when the wet-bulb tipped 35C, is listed by Guinness World Records. Wet-bulb temperatures approaching the fatal 35C threshold “almost never occur in the current climate,” according to one 2018 paper warning about the rising risks of such events toward the end of this century.
Three years ago, Colin Raymond, then a doctoral student at Columbia University, noticed a problem with those assumptions. Existing studies of the phenomenon all combined weather observations with a theoretical model of the climate to spit out a map of estimated conditions, similar to a weather forecast. None had looked at the terabytes of actual weather station data that have already been collected.
What he found after obtaining those numbers was alarming. Far from being a possible outcome of a future climate-wracked world, humid heat above 35C was already happening — from the Persian Gulf and the Red Sea to Pakistan, India, Australia, Venezuela, and each coast of Mexico. Temperatures above Haldane’s 31C threshold were turning up in dozens of places across tropical South and Southeast Asia, China, West Africa, southern Europe and the Americas, extending to the suburbs of New York and Naples, Italy.
Some peer reviewers refused to believe Raymond’s results. “There’s no way these values could be right,” Radley Horton, a professor at Columbia who supervised and co-authored the study, recalled one comment saying. After multiple rounds of review, however, the study was published in May 2020 in the journal Science Advances, titled “The emergence of heat and humidity too severe for human tolerance.”
Computer models have trouble picking up heat extremes because of their low resolution. If you have a small area of exceptional heat in the midst of a somewhat cooler region — such as a city surrounded by farmland, or beside the ocean — a low-resolution computer model will miss the conditions being experienced by people on the ground outdoors. They’re even less able to work out what things are like inside stuffy buildings. Theories of how the atmosphere behaves in heatwaves are also based on general principles and historical observations, which may not reflect local conditions in specific areas, especially as the climate itself changes. By looking only at the most easily-interrogated datasets, scientists had missed what was happening outside in the world.
“Wow. This is already here,” Horton remembers himself thinking when he saw the figures. “It was there in plain view,” said Raymond.
India’s heatwaves have been extreme — the nation just experienced the hottest March since record keeping started there 122 years ago. New Delhi recorded a maximum temperature of 42 degrees (107 Fahrenheit) and above on at least 26 days this summer season, reaching 49 degrees in some parts of the capital. Temperatures soared way above normal throughout the country. In the Himalayas, the snow melted early, while the wheat crop was scorched and the monsoon rainfall ran 32% below normal in much of the country last month. The heatwave was 30 times more likely to happen than it would have been in the absence of climate change, scientists found in May.
Mahesh Palawat has been tracking India’s dangerous weather for years. The vice-president of meteorology and climate change at Skymet Weather says New Delhi reached the wet-bulb threshold several times between May 24 and June 1, while in Odisha it was even worse: The state hit the danger zone from May 16-17, May 22-23 and June 5-12, Palawat notes.
The India Bureau of Meteorology has no publicly available information on wet-bulb temperatures, and its heatwave warnings do not mention the particular danger that comes with the combination of high heat and humidity. Bloomberg Opinion contacted the bureau’s head office in New Delhi, its Odisha branch and its data center and was unable to obtain wet-bulb data.
In 1905, Haldane wrote that there was “a great lack of information” about wet-bulb temperatures. The situation persists to this day. That’s a critical information gap, because this measure is far more important to understanding the ability of our bodies to withstand heatwaves. In a twist of fate, his son J.B.S. Haldane moved to India in 1957, took citizenship and headed the government’s Genetics and Biometry Laboratory in Orissa, as the state of Odisha was then known. He died in Bhubaneswar in 1964.
As any athlete knows, sweating is how our bodies keep cool. It’s such an effective strategy that early humans may well have evolved to take advantage of it. The hairless ape, with abundant sweat glands exposed to the open air, could chase its prey over the African savannah to the point where its quarry collapsed from heat exhaustion. That strategy has its limits, however. When humidity and heat are too high, the air becomes too saturated for even our sweat to evaporate. Unable to cool down, we rapidly succumb to heatstroke and die after about six hours’ exposure.
Able to smother entire societies, heatwaves could become a far more damaging phenomenon in the 21st century than more familiar cataclysms such as cyclones and floods. “Heat is such a silent disaster,” says Chandni Singh, a climate researcher for the Indian Institute of Human Settlements in Bengaluru. “I don’t think we’ve really internalized how it will break down multiple systems, how that will ripple through the economy.”
By mid-June, nearly one-third of the US population was under a heat advisory
A major heatwave just a few degrees higher than those seen in India over the past two months might not just kill tens, even hundreds of thousands — it could also overload electricity grids, cause power plants to shut down, make harvests fail, and hamper the ability of affected economies to grow. May’s heat caused India to cut its forecast for annual wheat production by about 6 million metric tons, putting further pressure on a global trade already disrupted by the war in Ukraine.
“A lot of these extreme events have never happened in the past, so they’re really outside the bounds of experience,” says Ethan Coffel, an assistant professor at Syracuse University and lead author of the 2018 study.
More troubling than the mere fact of the temperatures is where they’re occurring. Many of the worst-affected areas are in South Asia, where over a billion people live and less than 10% have access to air-conditioning that could be life-saving in a serious heatwave. Many of them work in agriculture or informal jobs where there’s no shelter from extreme daytime heat.
By mid-June, nearly one-third of the US population was under a heat advisory
“You are talking about relatively poor farmers, and there are many of them, concentrated in this area that happens to be a hotspot for how climate change and heat stress is going to evolve,” says Elfatih Eltahir, a professor at the Massachusetts Institute of Technology. Half of households in India and three-quarters in Pakistan still won’t be able to afford air-conditioning by 2050, according to one estimate last year.
Those countries bear very little responsibility for the warming that’s already occurred. India accounts for just 3.7% of historic emissions, a smaller tonnage of carbon dioxide than Germany. Pakistan’s burden is just 0.36% of the total, less than Belgium.
Even so, many of the things they’re doing right now to develop their economies risk making the situation worse. Irrigation lowers dry-air temperatures and enhances agricultural production, but it increases humid heat by providing a greater store of moisture to enter the atmosphere — one reason that the plains between the Indus and Ganges rivers in Pakistan and India are so at risk. Indian states on the coast or with irrigated farms covering more than half their land area — the ones that are likely to be most exposed to humid-heat stress — already have mortality rates that are above average, especially when considered in relation to income levels.
Air conditioners reduce indoor temperatures for those who can afford it, but dump hot air into the surrounding streets. Powering them requires fast-growing volumes of electricity, which in India is provided mainly by fossil fuels.
“When you start burning coal at this rate you are creating problems for the population you are trying to help,” says Eltahir. The demand has already caused rolling power cuts in India in recent months, with 60% of homes facing outages during some of the hottest days this May. There’s also the fact that thermal power plants need to cool themselves down, a challenge that gets harder as the wet-bulb temperature rises.
By mid-June, nearly one-third of the US population was under a heat advisory
A scenario that now seems alarmingly plausible would see months when daytime peak dry temperatures are above or close to 40, while night time lows rarely dip below 30 for weeks at a time — another factor that increases heat risks, by taking away opportunities for our bodies to cool down. Punctuating that pattern would be occasional episodes of extreme humid heat pushing up close to the edge of survivability. Cumulatively, the effect of all these conditions happening together would be far worse than in isolation. By the second half of this century, swaths of the tropics and sub-tropics could be seeing months at a time above the highest recorded wet-bulb temperatures, according to Coffel’s 2018 study.
How many people might be killed by such incidents? It’s hard to be sure, because of the poor quality of vital statistics on birth and death in India. Official figures show 481,000 Covid deaths during 2020 and 2021, but a World Health Organization estimate last month based on excess deaths reckoned 4.7 million might have perished. At present, around 89,000 people are estimated to die every year in India from hot temperatures — perhaps surprisingly, fewer than the 632,000 who die from cold. With 4C of global warming, heat deaths will rise to 1.5 million a year.
So far this summer, there’s been no sign of the sort of mass crisis brought about when the Covid epidemic was at its height in India. But even so, just staying alive takes a community-wide effort. The government, as usual, is mostly absent from this life-and-death equation. The real work is left to citizens themselves. Painting building roofs white is a low-cost way to reduce indoor temperatures if you can’t afford cooling. Large earthenware pots filled with cool water are found on streets across the country, sometimes sheltered in tiny huts and mostly maintained by volunteers. For those who live and work outside, these pots, or maktas, may be their only form of relief from the weather.
Sitanhshu Bhushan Baral, 38, oversees 15 acres of farm land along the Bhubaneswar-Puri highway, growing eggplants on his holdings. “This summer has been tough,” Baral says, as one of his laborers steered a tractor up and down a field, preparing the soil for planting. “We are planning to build a shade house and dig a bore well close by, because the heat is now unbearable.” Like construction workers in the nearby towns, he and his crew stop work at midday and stay inside until 3 p.m., when temperatures start to ease. Since Cyclone Fani hit the state in 2019, the heatwaves have felt more intense, he says, leaving him with nauseating headaches.
The world is currently going through its third consecutive year of La Niña, a climate cycle that tends to bring cooler, wetter conditions to India. There hasn’t been a strong El Niño since 2016, a relatively long stretch of time — but when it inevitably comes, it’s likely to bring record wet-bulb temperatures, as well as drought. India, and the countries that share its humid-heat furnace, won’t be ready.
Singh, the climate researcher, remembers reading Stanley Robinson’s novel last year during winter in her ancestral village of Rasmai in Uttar Pradesh, a vast, poor state with roughly the same population as Brazil. “It was extremely cold, and he was writing about this heatwave where millions die. I told my family: ‘I can’t imagine such a heatwave.’”
What was once inconceivable now seems worryingly plausible. Over the past few months, villagers in Rasmai have been waking at 4 a.m. to avoid working in the hottest part of the day, and bathing their livestock to keep them alive, she said.
“It’s very different seeing it happen, compared to reading it in a paper,” she said. “Science fiction is no longer fiction.”
This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.
David Fickling is a Bloomberg Opinion columnist covering energy and commodities. Ruth Pollard is a Bloomberg Opinion editor.
(This column first appeared on July 7, 2022)


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