It’s monsoon season now in India, which means it is possible to look back on the extraordinary succession of heat waves that swept across South Asia starting this spring — and begin, at least, to take stock of their impact.
From late March through the end of June, a period of almost 100 days, high temperatures in Delhi were above 100 degrees Fahrenheit on all but 15 of them, with many days breaking 110 degrees. For large parts of those months, the punishing heat stretched over much of the subcontinent, often blanketing more than a billion people and in certain places crossing 122 degrees. In May, the World Weather Attribution initiative, a research alliance connecting extreme events to global warming in real time, declared that the initial wave had been made 30 times more likely by climate change. And the heat kept coming.
For more than a decade, scientists have talked about an “upper limit” of human survivability, reached when the combination of heat and humidity produces what’s called a “wet-bulb temperature” of 35 degrees Celsius (95 degrees Fahrenheit) for six hours — above that level, it is believed that exposure would kill even young, healthy people. But at lower wet-bulb readings, and even in low-humidity extreme heat, health suffers too, especially among those with other vulnerabilities, exacerbating all kinds of conditions (not to mention damaging infrastructure, driving power outages, and limiting labor productivity). A more recent set of studies even suggested the maximum wet-bulb threshold might be lower for humans — as low as 31 degrees Celsius (88 degrees Fahrenheit) at 100 percent humidity or 38 degrees Celsius (100 degrees Fahrenheit) at 60 percent humidity. For the vulnerable, of course, the effects hit much sooner.
The monthslong heat in South Asia breached these levels at several points. In Jacobabad, Pakistan, a small city of less than 200,000, wet-bulb temperatures briefly hit 91.4 in May. Late last month, with the monsoons bringing humidity with them, the wet-bulb in Delhi, where almost 20 million people live, reportedly reached as high as 92.7. The readings produced a wave of climate panic: Berkeley Earth’s Robert Rohde warned “this heat wave is likely to kill thousands”, and The Washington Post reported “India’s heat waves are testing the limits of human survival”. (I had the same worry, as did others at The Times, The Economist, the World Economic Forum and elsewhere.)
And yet while there have been casualties and brutal impacts, these events do not yet appear to have caused mass death. An early estimate of the death toll was just 90 across India and Pakistan, a fraction of the more than 1,000 killed by the heat dome that struck the Pacific Northwest (including western Canada) last summer, where lower temperatures hit millions for a much shorter period of time. (In Seattle, it was above 100 for only three days, with a peak of 108; in Portland, it reached 116.) It’s an even smaller fraction of the number who died in the 2003 and 2010 heat waves in Europe and Russia, which killed 70,000 and 55,000, respectively. In those heat waves, only a handful of places crossed 104 degrees Fahrenheit. According to Dr. Chandni Singh, a senior researcher at the Indian Institute for Human Settlements and a lead author of the recent Intergovernmental Panel on Climate Change chapter on adaptation, it’s likely that this year’s heat event in India will not even be as lethal as one there in 2015 when high temperatures across the country killed several thousand.
When I spoke to the Indian climate activist Disha Ravi, she described recent floods — which have killed more than 100 across India and Bangladesh, and displaced many millions — as a more pressing climate matter locally and the one that had been preoccupying her much more, recently, than the heat.
What explains the relatively low death toll in India and Pakistan from the recent heat? It is still early to say definitively. That goes double given the limited data about all the impacts of the heat wave — not just on human health, but also on agriculture, labor and economic activity, and more.
But the question is a hugely consequential one: About a fifth of the human population was just subjected to almost unimaginably long-lasting extreme heat, and the ultimate toll, if we can accurately assess it, will tell us quite a lot about how some of us might suffer through the future climate.
If the current pattern continues to hold, with fuller calculations showing a still relatively limited impact, five hypotheses provide at least partial explanations: problems with data; the dryness of the heat wave; local adaptation and response; that the hottest parts of the world may already be relatively acclimated to extreme heat; and that our casual use of “wet-bulb” readings may be misleading about actual mortality risk. (The brutal, long-duration experience also suggests some important caveats.)
First, the official death count is an undercount.
This was the interpretation offered to me by Dr. Friederike Otto, of World Weather Attribution, in May; by Kathy Baughman-McLeod, director of the Atlantic Council’s Arsht-Rockefeller Resilience Center, in June; and by Avikal Somvanshi, of India’s Center for Science and the Environment, in July. In fact, it was the first thing mentioned by everyone I spoke to, in India and Pakistan and elsewhere, when discussing the toll of the heat wave.
Heat is something of an epistemological and epidemiological challenge to doctors and coroners, even in places most conscientious about data collection. That’s because few deaths even under the most extreme temperature conditions present obviously as heat stress; mostly, fatalities accrue when underlying conditions are exacerbated or made worse by the additional strain.
Aditya Valiathan Pillai, an associate fellow at India’s Center for Policy Research, went to emergency rooms “just to see if the fact that nothing was popping up in the media about people dying was true or whether it was sort of a mortality whitewash, the way we saw with Covid”, he said. “And universally, I went to government hospitals and they said, we did see a steady uptake in the number of people showing up, but very few deaths. Then again, all of them raised their hands and said, we don’t really actually know how to count a heat death. And so we’re not entirely sure”.
A full accounting of the toll of any heat wave requires “excess mortality data” — overall accounting of how many people died in a given period above a baseline average — but compiling that can take months, wherever the extreme event hits. Without it, Baughman-McLeod said, “we don’t know how many, because we don’t count properly. The data collection isn’t there — whether in India or in the U.S”.
For his part, Somvanshi estimates that the ultimate toll of these heat waves will be “at least twice than what is officially reported. But based on the available information, it will most probably be much more than twice”. A simple doubling of data still yields a relatively low estimate; getting even to the 2015 toll in India would require something like a 20-fold adjustment. To get to the mortality levels of the European heat waves, a much larger adjustment still.
Second, for mortality, humidity matters enormously.
“Humid heat is more dangerous to human health and well-being than dry heat”, Somvanshi said. Through much of the heat wave, the temperatures were alarmingly high but humidity relatively low — making those truly concerning wet-bulb readings relatively rare and brief, even through the oppressively hot spring.
And when temperatures are high but humidity is low, the burden is more manageable — in part because it is, in certain ways, familiar. In India and Pakistan, the temperatures of the past few months were record-breaking when they happened, for the late spring, but not unheard-of in a typical Indian summer.
That made them, for Somvanshi, “unusual from the climate perspective, but from people’s perspective, in Delhi and even most of northern India, we are used to having 40-plus during the summer. It’s just that for us, it happened a little earlier. So, as people, we were kind of surprised, but not very much concerned or disturbed by it”. But he noted that the traditionally drier parts of the country are increasingly getting more humid, and the traditionally humid parts of the country are getting hotter — and this year’s monsoon’s have not brought all that much relief.
In Delhi, he said, “If you take the combined impact of humidity and heat of temperature, then it’s much worse conditions right now than it was in March and April or even May. And that’s something which doesn’t capture the media attention”.
Third, cultural practices and adaptation are playing a role — though there is still a long way to go.
When projecting heat impacts, both immediate and longer-term, it is easy enough to assume an essentially passive population: You take what you know the future holds and calculate the impact that would have on the world as it is today. The result is an intuitive picture of climate horror that has informed both some scientific projections of the next decades and popular dramatizations like the one that opens Kim Stanley Robinson’s novel “The Ministry for the Future”.
But while visions of quasi-apocalypse often center on India, where climate impacts are projected to be especially intense, that conceptual modeling is both patronizing and “fatalistic”, said Tim Sahay, a Mumbai-born climate policy expert. He pointed me to research showing a shrinking number of deaths due to heat on the subcontinent and a number of interventions that could further reduce vulnerability. “We’re not going to die for your narrative”, he said.
And if your conception of adaptation is just air-conditioning, with all the energy complications that brings, the South Asia heat wave suggests a broader spectrum of response, beginning with simple awareness. “The most obvious thing apart from the heat itself has been the reaction to the heat, which is quite unprecedented”, Pillai said. “It was a media maelstrom, in the sense that every newspaper had at least a couple of op-eds. The TV news picked it up”.
“I think in terms of providing some kind of warning of heat and just public awareness and messaging around that from the government — that’s improved”, Dr. Singh agreed, referring to “people going to specifically low-income settlements and telling them that you need to stay out of the heat and all of that”.
“The public conversation around heat — typically, in India, at least earlier years and even earlier decades — used to be that we are Indians, we know heat, we know how to deal with heat”, she said. “But that kind of bravado — false bravado, I think — has changed this year. The problem is, that only gets you so far. You can cope with it. You can maybe protect yourself, if you have the means to. But if you are living in a house that has no cooling and you have to go outdoors to work, there’s not much that warning can do”.
The available tools are familiar across the subcontinent: schedule-shifting for those who can do so; different diet and drinks, and of course regular hydration; lighter clothing; simple ways of cooling the home with wet sheets. But at the scale of policy intervention, the response is still lacking, said Dr. Singh, who mentioned both short-term interventions, like widespread behavioral and labor changes during episodes of extreme heat, and longer-term, like revisions to building codes and urban planning.
After the 2015 heat waves, India embraced the goal of what are called “heat action plans”, at both the national and local levels. But many of those remain unenforced or undeveloped, depending on the region and municipality. “The country currently has no adaptation or resilience plan for heat right now. There is nothing on paper”, Somvanshi said. “We have a document which talks about emergency response, which is very different than trying to adapt and become resilient”.
Fourth, the world’s hottest countries may be less vulnerable to heat, relatively speaking, than simple heuristics suggest.
In recent decades, the deadliest heat waves have been not in the hottest countries but those less familiar with heat — those 2003 and 2010 heat waves in Europe, for instance, where extreme temperatures met populations unaccustomed to dealing with them. This is probably not a coincidence.
Matthew Huber at Purdue is among the world’s leading authorities on heat stress and heat mortality — it was his 2010 paper, co-authored with Steven Sherwood, that introduced the idea of that upper limit of human survivability. But in thinking about heat vulnerability, he said, it is a mistake to assume that all places are equally at risk in the face of equal temperatures. That’s because much of the equatorial band of the planet is already “acclimatized” — both culturally and, to some degree, physically — to temperatures that would in the northern latitudes produce widespread heat mortality.
Unfortunately, he said, much of the scientific literature on these questions doesn’t deal with acclimatization sufficiently, drawing on research conducted in places without much experience of extreme heat. “The problem is, when you go and look at the database, it’s Northern Europe, Canada”, he said. “In Africa, it’s just South Africa. Nothing in China. Nothing in India, I don’t think. Most of the places that actually experience heat stress aren’t in the heat database. There’s a lot of studies that are using this database and the correlations that come out of it to generalize. And those are almost certainly very wrong, because none of these are people who are acclimated to heat stress”. That means we can’t make easy generalizations about how people in different places with different levels of experience with heat will handle an extreme event — and we probably underestimate resilience in the hottest parts of the world.
And fifth, heat beyond the limit of human survivability may be farther off than headlines have suggested.
The wet-bulb temperatures in South Asia this spring have been called “unsurvivable”. But in a lot of coverage, the complexities and nuances of wet-bulb measures are lost, Huber said. To begin with, there are two ways of calculating those figures, yielding slightly different figures and implying somewhat different warning points.
More significantly, those calculating wet-bulb temperatures using simplified calculators often input daily averages for both heat and humidity, though averages misrepresent the way each changes over the course of the day, in part in response to each other, since variations in heat and humidity typically counteract each other over the course of the day. The result, is wet-bulb figures that seem scarier than conditions on the ground really warrant. Sometimes, the daily maximums are used to make a calculation, which yields an especially extreme and misleading picture: Since heat and humidity counteract each other, it is almost never the case that peaks will be reached simultaneously, and simply accurate to use a calculation based on maximum readings to suggest a continuous wet-bulb level for any period of time.
And while media reports may take a brief wet-bulb peak and compare it with a six-hour threshold, the length of time truly matters. “If it’s just an hour or two or three, the effects on people are not going to be particularly big”, Huber said. “I mean, you can hang out in an oven for 15 minutes without dying”.
Which brings us to the caveats: Death is not the only measure of suffering when it comes to heat, and mortality data, while important, tells only a partial story.
This spring, wheat yields were reduced in India between 10 percent and 15 percent, according to official estimates; in Pakistan, the effect was similar. In certain regions, the impact was as big as 30 percent. Livestock died. Infrastructure suffered, including a bridge in Pakistan that collapsed when heat caused glacial melt that caused a flood. There were widespread blackouts and electricity shortages across the subcontinent, triggered in part by the additional demand from those privileged enough to afford some kind of cooling. School days were shortened and in some places, schools closed.
“We are witnessing something that people cannot cope with, even at the present level of global warming”, said Fahad Saeed of Climate Analytics, who is based in Islamabad, Pakistan.
He mentioned the scrambled weather patterns: One year heat and drought, and the next, flooding. In some years there were even biblical locust swarms, as in the summer of 2020, with 10 billion locusts descending on South Asia and eating up its cropland, 8,000 times more locusts than normal. He mentioned people he’d seen abandon their fast during Ramadan because the practice was too difficult to sustain in periods of such intense heat. And he mentioned a farmer he’d visited recently, who’d decided to not even bother to plant three-quarters of his land, given the weather conditions.
What would happen if the world crossed the 1.5 degree Celsius (2.7 degrees Fahrenheit) global warming threshold, Saeed asked, shaking his head. “Anything beyond that? It would even be more devastating”.
David Wallace-Wells, a writer for Opinion and a columnist for The New York Times Magazine, is the author of The Uninhabitable Earth.